Short-Term Memory for Order Information

Alice F . Healy UNIVERSITY OF COLORADO BOULDER, COLORADO

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Item and Order Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Temporal Sequence and Spatial Location Information . . . . . . . . . 11. Experiment 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Method ................................................. B. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Experiment 2 ..................................... A. Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Discussion .................. IV. Experiment3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Results . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... V. Experiment 4 . . . . . . . . . . . . . A. Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Results .............................................. C. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Independence of Item and Order Information ........... B. Coding Strategies.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Mental Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.

I. A.

ITEM

AND

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Introduction

ORDERINFORMATION

In the development of models for short-term memory, one issue that has received an enormous amount of attention is the relationship between the retention of item information and the retention of order information. Actually, this issue can be divided into two distinct questions about the separation of item and order information. Are order and item information THE PSYCHOLOGY OF LEARNING AND MOTIVATION, VOL. 16

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Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-543316-6

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separately represented in short-term memory? Are order and item information separately lost from short-term memory? Different models yield different answers to these questions, and a single model does not necessarily answer the two questions in the same way. For example, the model by Shiffrin and Cook (1978) includes separate and distinct mechanisms for the representation of item and order informationitem bonds and relational bonds-but the loss of the two types of information is not independent; rather, in one version of their model, the loss of item bonds can cause the loss of relational bonds, and vice versa. Drewnowski’s (1980) attribute model also postulates distinct memory representations for item information and ordinal position information and includes different assumptions for the loss of these two types of information. However, some interaction between item and order loss is included in the model because it assumes that items that have been tagged for ordinal position are lost with one-third the probability of untagged items, although ordinal position information is assumed not to be forgotten. Furthermore, it is assumed that once a given item has been forgotten, its associated ordinal position tag cannot aid recall (Drewnowski, 1980, p. 21 1). Another link between item and order information occurs in the attribute model as well. Drewnowski postulates that information about the auditory (phonemic) properties of the items is used to reconstruct order information. In addition, Murdock (1976) has explored a Markov model which includes separate memory representations for item and order information, but postulates that forgetting order information precedes and increases the probability of forgetting item information (see Murdock, 1976, p. 195). In contrast to these models, which postulate distinct but interdependent mechanisms for the loss of order and item information, the slot theory proposed by Conrad (1965) includes only a single forgetting mechanism, which is responsible for the interdependent loss of item and order information. According to this model, order errors are a by-product of item errors and completely dependent on them. Specifically, the memory codes for the to-be-remembered items are entered into an ordered series of slots, or positions, and retrieved from these slots at the time of recall. Forgetting occurs as a result of a decay process that causes the contents of the slots to become degraded, but does not cause any movement of information from one slot to another. In one sense, the perturbation model of Estes (1972) and Lee and Estes (1977) is the complement of Conrad’s slot model. The perturbation model also postulates only a single forgetting mechanism, but in this case the mechanism involves loss of positional information-perturbations in the timing of the cyclic reactivations of item representations. In addition, according to the recently augmented version of this model (Lee & Estes,

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1981), the perturbation process is assumed to operate on different levels concurrently, thereby causing the independent loss of item and order information. Order errors result from the perturbation process for the encoding of the relative positions of the items within the trial, and item errors result from the perturbation process for the encoding of the relative position within the session of the trial on which the items appeared. This conception is sympathetic to that of Crowder (1979), who argues that there is no fundamental difference between item and order information, since each type of information locates a well-known item in a particular temporal-spatial position. In the case of item information, the location specified is “coarse grained,” referring to the list containing the item; whereas for order information, the location is ‘‘fine grained,” referring to the position of the item relative to other items in the same list. One method I have used to help resolve this issue is to separate experimentally the retention of item and order information and determine whether the same pattern of results is found in the two cases (Healy, 1974). Specifically, I designed two experimental tasks that differed primarily in the amount and type of information provided in advance to subjects about the to-be-remembered material, and hence in the amount and type of information to be held in short-term memory. In one task (Order Only) subjects were told in advance the items that would be shown on a trial; they had to recall only their order. In the second task (Item Only) subjects were told in advance constraints on the order of the items that would be shown, so that they had to recall only item information. The patterns of results were different in these two situations, the most striking difference being in the serial position functions, which were bow shaped in the Order Only Experiment, but not in the Item Only Experiment. These findings suggest that order and item errors are caused by two different processes; these results provide difficulty for Conrad’s ( 1965) model of a single forgetting mechanism. Nevertheless, these results are not able to discriminate among the remaining models for the retention of item and order information, which postulate either different (but interdependent) mechanisms for the representation of item and order information (Drewnowski, 1980; Murdock, 1976; Shiffrin & Cook, 1978) or similar mechanisms for representing item and order information but independent loss of the two representations (Lee & Estes, 1981). One empirical question which should help differentiate these models is whether the retention of item and order information draws on the same processing capacity or pool or resources (cf. Navon & Gopher, 1979). This question is the focus of the present study. In Experiment 1, I attempt to answer this question by determining whether the retention of temporal sequence information is

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independent of the memory load for item information. In the subsequent three experiments, I try to answer this question by investigating whether the retention of spatial location information is affected by the properties of the to-be-remembered items. If temporal sequence and spatial location information are not affected by the amount and quality of the to-beretained item information, the models postulating interdependencies in the loss of order and item information would seem to need some revision. B. TEMPORAL SEQUENCE AND SPATIAL LOCATION INFORMATION

In most situations that involve visual information processing, the temporal sequence in which a set of characters is read corresponds to their spatial arrangement; typically, characters are read from left to right. However, the two types of order information can be unconfounded experimentally, and I have devised such an experimental situation for the short-term retention of letters. (See Healy, 1975, for a detailed discussion of the methods employed, and see Berch, 1979, for a comparison of my procedures to those used by other investigators.) Specifically, the standard task I have adopted is a modification of the distractor paradigm used, for example, by Bjork and Healy (1974), Conrad (1967), and Estes (1973). In earlier experiments with the distractor technique, characters occurred in only one spatial location of the display screen; but in my new technique, characters could occur in four different spatial locations arranged in a horizontal linear array. Four to-be-remembered letters were displayed successively on the screen, each letter occurring in only one of the four spatial locations, the remaining locations being left blank. The letters were not necessarily displayed from left to right, so that their spatial arrangement did not usually correspond to their temporal sequence. Following the presentation of the letters on a given trial and before their recall was a sequence of digits, with the length of the sequence varying from trial to trial and defining the retention interval for the trial. In order to isolate the retention of order information, as in the Order Only Experiment, the same four letters were shown on every trial of a subject’s session and the subject was told what these four letters would be in advance. All that the subject had to recall was the order of the letters-either their temporal sequence or their spatial location, depending upon the recall condition-which was varied between subjects. Further, to isolate the retention of temporal sequence or spatial location information, the letters shown to subjects in the spatial location recall conditions were displayed in a fixed temporal sequence known by the subjects in advance of the trials; and the letters shown to subjects in the temporal sequence recall conditions were displayed in a fixed spatial arrangement.

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The studies using this procedure revealed numerous striking differences between the retention of temporal sequence and spatial location information. In temporal sequence retention there was a preponderance of phonemic confusion errors, especially at the shortest retention interval; the retention function was quite steep; and recall level was dramatically affected by whether subjects responded aloud or silently to each of the interpolated digits. In contrast, in spatial location retention, the percentage of phonemic confusion errors was no greater than chance; the retention function was much flatter; and recall level was greatly affected by whether subjects responded with the name or the spatial location number of each of the interpolated digits, but not by the mode of response (aloud or silent). These and related observations (e.g., Healy, 1975, 1977) led me to suggest that the retention of temporal sequence information was based on phonemic coding, whereas the retention of spatial location information was based on the coding of the temporal-spatial pattern of letter presentations. For example, subjects asked to retain spatial location information would code the information that on a particular trial the first letter occurred in the right-most spatial location and the next three letters occurred successively from left to right, starting in the left-most location. A Markov model specifying the features of this coding process was developed (Healy, 1978) and tested by varying the relationship between the temporal-spatial pattern of the to-be-recalled letters and the temporal-spatial pattern of the digits interpolated during the retention interval. As predicted, the percentage of correctly recalled letters was greatly affected by the interpolated digit pattern in spatial location recall, but not in temporal sequence recall. Why do subjects use different coding strategies to retain the order of letters arranged in a temporal sequence and in a spatial array? The temporal-spatial pattern coding strategy should be available for temporal sequence recall as well as for spatial location recall, but the strategy may not be used if subjects ignore the spatial locations of the stimulus letters, as they may tend to do in temporal sequence recall. In support of this hypothesis, an earlier experiment (Healy, 1977, Experiment 3) demonstrated that subjects did employ pattern coding in temporal sequence recall when a new method of response forced them to attend to the spatial locations of the stimulus letters. Even in this situation, the subjects used phonemic coding in conjunction with the pattern-coding strategy. Since phonemic coding is so prevalent in the retention of temporal sequences (e.g., Healy, 1974; Sperling & Speelman, 1970), it may seem surprising that subjects did not use phonemic coding in spatial location recall. However, phonemic coding may not have been a reasonable coding strategy in the spatial location recall conditions employed in the earlier studies by Healy (1975, 1977, 1978), since the subjects said the same thing on every

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trial of their session because the temporal sequence of letters was fixed. Hence, a record of what was said would not have been helpful for spatial location retention in that situation. Perhaps subjects choose a phonemic coding strategy whenever they can rely on a memory code that reflects what they heard themselves say. Experiment 2 tests the hypothesis that the coding strategy chosen will depend on what the subjects say during the presentation of the stimuli. Alternatively, subjects may not use phonemic coding in spatial location recall conditions because they attend to the visual properties of the letters, rather than their sound. In fact, unreported analyses of the errors in an earlier experiment (Healy, 1978, Experiment 2) indicated more visual confusions than phonemic confusions in spatial location retention, whereas more phonemic than visual confusions were found in temporal sequence retention. This phenomenon is examined more thoroughly in Experiments 3 and 4 in the present article. 11. Experiment 1

When asked to recall the temporal sequence of a short list of letters or words, subjects have been found to recode the information into a phonemic format, even when the to-be-remembered items are presented visually (e.g., Conrad, 1964). How flexible is this coding strategy? Can subjects develop a more efficient recoding scheme when given some information about the to-be-remembered items in advance of presentation? Such information available to the subject prior to a trial has been termed the “schema” by Lee and Estes (1981). Whereas some investigators have argued that subjects can use schema information to improve their guessing at the time of recall of the to-be-remembered items, but not to improve their coding and retention of the to-be-remembered items (Drewnowski, 1980; Healy, 1974); other have implied that subjects can use schema information to improve coding and retention (e.g., Brown, 1958; Crossman, 1961). In an earlier study (Healy, 1974), I compared two experimental conditions in which subjects were to recall the temporal sequence of a list of four consonants: In one (Bjork & Healy, 1974), the subjects knew nothing about which four to-be-remembered letters would be presented on a given trial. In the second (the Order Only Experiment described earlier), the subjects were told which four letters would occur and this information was made salient by the fact that the same four letters occurred on each of the 72 experimental trials in a given subject’s session. I found that the percentages of correct recall responses were higher when the identity of

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the letters was known in advance, but there was no difference in the percentages of correct recall responses between the two experimental conditions when only memory for order information was considered. The percentages of correct responses in the Order Only Experiment were essentially equivalent to the conditional percentages of recalling the correct position of an item, given that the item was correctly recalled in the Bjork and Healy (1974) experiment. With a task similar to the Order Only Experiment but involving consonant-vowel-consonant trigrams, rather than letters, as to-be-remembered items, Drewnowski (1980) also demonstrated that there was no need to assume that the experimenter-supplied item information was actively represented in the subject’s memory during the trial. Drewnowski was able to fit his attribute model for short-term retention with a single set of parameter values to both his order-only condition and an analogous item-only condition, in which information about the order, but not the identity, of the items was given in advance to the subjects. One of the parameters in the attribute model (Pk) represents the probability of losing item information within a .5-sec interval when k items are currently in memory, and a second parameter (P,) represents the probability of storing order information in the form of position tags. Drewnowski did not find the best fit to the data from his order-only condition when Pk = 0, as would have been the case if subjects had been able to retain perfectly the redundant item information. Likewise, Drewnowski did not find a higher value for P , in his item-only condition, even though subjects in that condition were given in advance constraints on the order of the trigrams. Rather, the best fitting values for P , and P , were found to be the same in the two experimental conditions, suggesting that subjects used the same coding strategy in the two cases. Experiment 1 of the present study was designed to provide a direct test of the hypothesis that schema information about the identity of to-beremembered items helps the subject to code or retain order information more efficiently. As in my earlier study (Healy, 1974), an order only condition, in which subjects knew that the same four letters would occur on all experimental trials, was compared to an item + order condition, in which the identity of the four letters shown varied from trial to trial. In addition, a third experimental condition (the item + order - item condition) was constructed, in which the identity of the four letters varied across trials and subjects were told nothing in advance of a trial about which four letters would be shown, but subjects were reminded at the time of test (after coding, retention, and forgetting had already occurred) which four letters had been shown. In the item + order - item condition, as in the order only condition, the information provided by the experi-

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menter could improve the subjects’ guessing, but was given too late to have any effect on coding or retention. Therefore, if there was no advantage for subjects in the order only condition, compared to those in the item + order - item condition, then we could be assured that subjects were not able to use schema information about the identity of items to improve their coding and retention of the order of the items. A.

METHOD

I.

Subjects

Seventy-two male and female undergraduates of Yale University participated as subjects in order to fulfill a course requirement. There were 24 subjects in each of three conditions: item + order, order only, and item order - item. The order only condition included eight subconditions, with 3 subjects in each subcondition.

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2 . Apparatus A Digital Equipment Corporation VT50 terminal, controlled by a PDP-11/40 computer, was used for the visual display of the stimuli. The alphanumeric characters were presented in a single location, 5 lines down and 5 spaces over in the screen, which was 12 lines by 80 spaces. Each trial began with the display of a single hyphen repeated twice in a row. The characters were .4 X .2 cm and were all upper case. The computer was programmed to display each character for approximately 400 msec, with an interstimulus interval of approximately 50 msec, and an intertrial interval of approximately 16 sec, with a warning clicking sound occurring in the intertrial interval after every 8 sec. Timing is approximate because a time-sharing system was employed.

3. Design and Materials Nine different 72-trial experimental sequences were prepared; one was employed in the item + order and item order - item conditions, and the other eight were used in the order only condition. A given subject was shown only one experimental sequence, and 3 subjects were shown each of the sequences for the order only condition. Each experimental trial consisted of four successively presented consonants followed by a retention interval including either 3, 8, or 18 intervening digits. The presentation order of the trials was quasi-random with the constraint that every block of 12 trials have four instances of each of the three retention

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intervals. Only digits appeared during two initial practice trials shown to all subjects. The interpolated digits displayed on each experimental and practice trial were randomly selected from the digits 1 to 9 with the constraint that no digit immediately succeed itself. The same digits occurred in each experimental sequence. Only the consonants differed across sequences. For the eight sequences used in the order only condition, the same four consonants appeared on each trial of a given sequence. The 24 permutations of the four letters each appeared three times, once at each of three retention intervals. The letters were drawn from a population of 8 consonants: BPFSKMHL. The eight sequences of trials contained eight different four-letter subsets of these eight letters: BPKM, FSHL, BPHL, FSKM, BFKH, PSML, BSHM, and FPKL. The sequences were created from a master list by applying a set of mapping rules which ensured, for example, that every instance of the letter B in the first sequence corresponded to an instance of the letter F in the second sequence. Four of the sequences-BPKM, FSHL, BFKH, and PSML-were identical to those used in the Order Only Experiment by Healy (1974). For the sequence used in the item order and item order - item conditions, all eight letters from the experimental population were employed. On a particular trial one of the eight four-letter subsets of letters was shown. Each four-letter subset occurred in 3 of the 24 trials with a given retention interval. The order of the letters shown on a trial was the same as that shown on the corresponding trial of the order only sequence that contained the same subset of letters. These constraints ensured that across sequences, the same letters occurred in each of the three conditions.

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4 . Procedure

Subjects were tested individually in sessions of approximately 1 hour. Each subject was instructed to read aloud the letters and digits as they appeared. At the end of each list of letters and digits, the screen went blank and the subjects were given 16 sec to write the four letters seen on that trial. The subjects wrote their responses on 4 X 6-inch cards which included four boxes arranged in a horizontal linear array. The subjects were to write the first letter seen in the first box, the second in the second box, and so on. They were not required to fill in the boxes in any particular temporal sequence; hence, to use the terminology of Berch (1979), ordinal recall, rather than serial recall, was required. The subjects were forced to fill in all the boxes on the card; they were not allowed to leave a box blank; and they were told to guess if necessary.

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At the start of the experiment, subjects in the order only condition were given full information concerning the subset of four letters that would be seen in varying orders during their session. In addition, as a reminder to the, subjects, the subset of letters used in the session was printed in the upper left-hand corner of each response card. The four letters on the cards were always shown in the same arrangement, depending on the subset: BPKM, FSHL, BPHL, FSKM, BFKH, PSML, BSHM, and FPKL. No information concerning the letters that would be seen was provided to the subjects in the item + order condition either at the start of the experiment or at the time of test. Although subjects in the item + order item condition were also not told anything at the start of the experiment about the letters that would be shown, they were provided with information about the subset of letters that occurred on a given trial at test time. This information was printed in the upper left-hand corner of the response cards in the same format employed in the order only condition. The subjects were told that the four letters were placed on a card in a random order, which did not necessarily correspond to the order in which the letters were shown on the trial. The subjects were told further that their task was to recall the proper order of the letters and to write them in that order in the boxes on the card. Subjects were also told to make sure that for every trial the letters written down as responses corresponded to those in the corner of the card, although the order might differ. In fact, the four letters in a subset were always printed in the same arrangement on the card, as in the order only condition. The response cards used by the subjects were given to them in a stack facing downward. The subjects in all conditions were told not to turn over the response card for a given trial until they had seen all the letters and digits for the trial. This procedure ensured that subjects in the item order - item condition would know the information about the identity of the four letters shown on a trial at the time of test, but not before.

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B. RESULTS

The results are summarized in Table I for the three conditions of the experiment in terms of unconditional percentages of correct responses for all conditions and conditional percentages of correct positions, given correct items for the item + order.condition. The standard error of the values in Table I broken down by retention interval is 2%, as determined by analyses of variance. When only unconditional percentages are considered, subjects in the item + order - item condition performed as well as those in the order only condition; in fact, the overall recall level in the item + order - item condition was slightly better than that in the order

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Short-Term Memory for Order Information

TABLE I TIMECOURSES OF FORGETTING IN CONDITIONS OF EXPERIMENT 1" Number of digits Condition

3

8

18

Totals

Order only Item + order - item Item + order Unconditional Conditional

84 87

68 72

52 49

68 69

83 86

63 71

39 52

62 70

UConditions+rder only, item + order, and item + order - item-in terms of (a) unconditional percentages of correct responses for all conditions and (b) conditional percentages of correct positions given correct items for item + order condition.

only condition. Both of these groups, who had to recall only order information, performed at a level that was superior to that of the item order group, who had to recall information about the identity of the items as well as information about their serial positions. The difference between groups was especially marked at the longer retention intervals. An analysis of variance performed on the unconditional percentages did not yield a significant main effect of condition, F ( 2 , 69) = 2.3, MSe = 1948, p = .104, but the interaction of condition and retention interval was statistically significant, F(4, 138) = 3.9, MS, = 265, p = .005, as was the main effect of retention interval, F(2, 138) = 398.0, MS, = 265, p < .001. In addition, a planned contrast comparing the item + order condition with the other two conditions was statistically reliable, F( 1, 69) = 4.4, MSe = 1948, p = .037, as was the interaction of that contrast with retention interval, F(2, 138) = 4.7, MSe = 265, p = .010. A comparison of the conditional percentages of correct order recall responses, given correct item recall responses for the three conditions, revealed no differences among conditions. (The item information was necessarily correct in the item + order - item and order only conditions, so the conditional percentages are equal to the unconditional percentages in these cases.) This finding indicates that the lower overall recall level of the subjects in the item + order condition cannot be attributed to poorer recall of order information.' An analysis of variance conducted on the

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'It is important to be aware of the possible problem of selective effects when comparing conditional percentages to absolute percentages. The conditional percentages in this case only involve recalled items, not the more difficult items that were forgotten; it may be easier to recall the order of the less difficult items.

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conditional percentages did not yield either a main effect of condition, F ( 2 , 69) < 1, or an interaction of condition and retention interval, F ( 4 , 138) = 1.7, MS, = 250, p = .150, although the main effect of retention interval was significant, F(2, 138) = 350.7, MS, = 250, p < .001. C . DISCUSSION

These results provide dramatic support for the hypothesis that subjects under these conditions do not develop a more efficient method for coding order information in short-term memory, even when they are told in advance which items will occur. Subjects performed no better in the order only condition, in which they knew in advance which four letters would be shown and the same four letters occurred on every trial of their session, than in the item order - item condition, in which different subsets of four letters occurred on different trials and the subjects were reminded only at the time of test which four letters were shown on a particular trial. Subjects in both of these conditions did correctly recall more letters than in the item order condition, in which the subjects were given no information about the to-be-recalled letters in advance or at the time of test. However, the poorer recall level in the item + order condition can be attributed solely to differences in item retention, not to differences in the retention of order information. These results also provide a clear answer to the more general question posed initially concerning the independence of the processes used to retain item and order information. The order only condition differed from the item + order - item condition in terms of the memory load for item information. Nevertheless, the retention levels were essentially the same in the two conditions. Hence, the retention of temporal sequence information seems independent of the memory load for item information. Is the retention of spatial location information also independent of the amount and quality of item information? This question was addressed in Experiment 2 using a very different set of procedures from those used in Experiment 1.

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111.

Experiment 2

To determine whether the coding used to remember the spatial arrangement of a sequence of letters is influenced by the identity of the letters, on each trial of Experiment 2 a single stimulus, the capital letter X, was shown four times successively, each time in a different spatial location of the display screen, and the subject was asked to indicate the temporal

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sequence of spatial locations that contained Xs. The aim was to assess whether the coding strategy used by subjects in this situation would be similar to that employed when subjects are required to remember the spatial locations of a set of different letters, as in earlier studies of spatial location retention (Healy, 1975, 1977, 1978). If subjects rely on a memory code that reflects what they heard themselves say whenever that code contains some information, then subjects’ performance on this task should depend largely on what they say during the presentation of the stimuli. Two different conditions were therefore included in this experiment. In one condition, as in the previous spatial location recall conditions involving letters, the subjects said the same thing on every trial of their session, so that remembering what they said would not be informative. Specifically, in the count condition, subjects counted from 1 to 4 as the four Xs were presented successively. In contrast, in the second condition of this experiment, as in the previous temporal sequence recall conditions with letters, remembering what they said would be a useful strategy for the subjects. Specifically, in the position condition, subjects said aloud the position number of the spatial location where each X occurred. If the primary factor differentiating temporal sequence recall and spatial location recall conditions in the previous studies of letter retention was the usefulness of what the subjects said during stimulus presentation, then the two conditions of the present experiment should show differences similar to those found between temporal sequence recall and spatial location recall. In particular, the count condition should be analogous to the previous spatial location recall conditions involving letters, and the position condition should be analogous to the previous temporal sequence recall conditions. Such a pattern of differences between the present experimental conditions would be extremely impressive because the subjects’ recall task was identical in the two conditions; the conditions differed only in terms of what the subjects said during stimulus presentation. Another possible outcome of this experiment was that the position condition as well as the count condition would resemble the previous spatial location recall conditions with letters. This outcome would be expected if subjects were simply coding the temporal sequence of spatial locations in the previous spatial location recall conditions, as suggested, for example, by Berch (1979). The “baseline” model, based on this assumption, was tested in the earlier study by Healy (1978) but rejected because it could not account for the consistent effect of temporal-spatial pattern on the percentages of correct responses in spatial location recall. For example, simple “straight-across” patterns, in which the letters were presented either entirely from left to right or entirely from right to left,

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were recalled much better than other patterns; but the baseline model could not account for this observation. It is possible that certain sequences of position locations, such as 4321 or 1234 would be easier to retain than other sequences because of their familiarity, so that the notion that subjects rely on the retention of the sequence of position numbers in spatial location recall is worth further consideration. The present experiment was designed to be as analogous as possible to the previous experiment of letter retention by Healy (1978, Experiment 2), so that comparisons of the two experiments would be facilitated. As in the previous experiment, a manipulation of the interpolated digit patterns was included in order to determine whether subjects were coding pattern information about the to-be-remembered stimuli. Four different types of interpolated digit patterns were employed: digit patterns that matched exactly the pattern of Xs (“same” patterns), those that matched the pattern of Xs in pattern class but differed in terms of the other two items of information included in the pattern model of Healy (1978) (“similar” patterns), those that differed from the pattern of Xs in terms of all three items of information included in the pattern model (“different” patterns), and those that had no regular relationship to the pattern of Xs because of repetitions of digit locations within a block of four successive digits (“random” patterns). If the basic unit of memory is the temporal-spatial pattern of X presentations, then recall should be greater for same digit patterns than for the other digit pattern types, and subjects should be likely to confuse the interpolated digit pattern with the pattern of Xs when they are similar. A.

METHOD

1. Subjects

Twenty male and female undergraduates of Yale University participated as subjects in order to fulfill a course requirement. There were 10 subjects in each of two conditions: position and count. 2 . Apparatus

The same apparatus was employed as in Experiment 1, except that four spatial locations of the display screen, rather than just one, were used to present stimuli. The locations were arranged horizontally, with a single space separating adjacent locations. The first (left-most) location was the same as used in Experiment 1. Hyphens were used to represent blank locations. Each trial began with the display of four hyphens repeated twice in a row.

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3. Design and Materials A 192-trial experimental sequence was prepared. A trial consisted of four successively presented instances of the letter X, followed by a retention interval of either 4 or 16 successively presented interpolated digits. Each character was displayed in only one of the four locations on the screen, while the other three locations contained hyphens. The four Xs were presented in different locations, not necessarily from left to right. Each of the 24 possible temporal-spatial patterns of X s was presented eight times, four times at each retention interval, once paired with the same digit pattern, once paired with a similar digit pattern, once paired with a random pattern of digits, and once paired with a digit pattern that differed from the pattern of Xs in terms of all three items of pattern information, as defined in the pattern model proposed by Healy (1978). The similar patterns matched the pattern of Xs shown on the given trial in pattern class but differed from it in the other two items of information. Each pattern of Xs was matched with a single digit pattern that met the criteria for a different pattern and a single digit pattern that met the criteria for a similar pattern; and across all 24 patterns of Xs, each of the 24 digit patterns was employed once as a different pattern and once as a similar pattern. The random patterns were constructed in the following manner: The location of a given digit was quasi-random with the constraint that every group of four successive digits have at least one location repetition, although a given location never occurred twice in immediate succession. Thus, random patterns could not be classified as any of the 24 regular temporal-spatial patterns. For same, different, and similar, but not for random patterns on a long retention interval with 16 interpolated digits, the digit pattern was repeated four times in a row. These constraints on patterns are identical to those employed by Healy (1978, Experiment 2). The presentation order of the trials was quasi-random with the following constraints: In every block of 24 successive trials there were six instances of each of the four digit-pattern types, and across the entire 192trial sequence, each of the 24 patterns of Xs occurred twice with each of the four digit-pattern types, once at each of the two retention intervals. As in the study by Healy (1978, Experiment 2), the intervening digits shown on each trial were selected with the constraints that each block of four digits show a given permutation of the digits 5 , 6, 8, and 9, and no two successive digits be the same. Eight practice trials were shown to the subjects. Eight different temporal-spatial patterns of Xs were used, one with each of the eight combinations of retention interval and digit pattern type.

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4 . Procedure Subjects were tested individually in sessions that lasted approximately 2 hours. Each subject was instructed to say aloud the name of each digit as it appeared and either to say aloud the position number (from 1 to 4) of the location where each X appeared (position condition) or to count aloud the numbers 1 to 4 as each of the four Xs appeared (count condition). At the end of each sequence of Xs and digits, the screen became blank and the subject was given 16 sec to indicate the spatial positions of the four Xs seen on that trial on a 4 X 6-inch card which included four rows of four boxes. Each row of boxes was for a different one of the four Xs, and the four boxes in each row were for the four spatial locations of the display screen. For each of the four Xs, the subject was to place an X in the appropriate box. For example, if the subject decided that the first X occurred in the left-most location, he or she was to place an X in the first row and first column. The subjects were instructed to place one, and only one, X in each of the four rows. At the end of every 48 experimental trials there was a short rest break. B.

RESULTS

The results are summarized in Table I1 in terms of percentages of correct responses as a function of condition (position and count), retention interval (4and 16 interpolated digits), and relation between the to-berecalled pattern of Xs and the interpolated digit pattern. The results from the temporal sequence recall and spatial location recall conditions of the analogous letter recall experiment by Healy (1978, Experiment 2) are also included in Table I1 for comparison purposes. An analysis of variance performed on the combined data from both experiments yielded 2% as an estimate of the standard error of the entries in Table 11. As Table I1 reveals, the count condition of the present experiment bears a strong resemblance to the spatial location recall condition of the earlier letter recall experiment, whereas the position condition is more similar to the earlier temporal sequence recall condition. Specifically, there was no overall difference between the count and position conditions, F( 1 , 18) < 1, but there was an overall decline in recall level as retention interval increased, F(1, 18) = 46.7, MS, = 778, p < .001, and this decline was much larger for the position condition than for the count condition, F(1, 18) = 6.3, MS, = 778, p = .021, just as it was much larger for the temporal sequence recall condition than for the spatial location recall condition in the earlier experiment. In addition, there was an overall advantage for same digit patterns, compared to the other types of interpo-

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TABLE I1 PERCENTAGES OF CORRECT RESPONSES IN EXPERIMENT 2 AND IN LETTER BY HEALY(1978, EXPERIMENT 2) BY CONDITION, RECALLEXPERIMENT AND RELATION BETWEEN THE TO-BE-RECALLED RETENTION INTERVAL, DIGITPATTERN PATTERNAND THE INTERPOLATED Retention interval

16

4

Condition Experiment 2 Position Count Healy (1978) Temporal Spatial

Same Different

Similar

Random

Same Different

Similar

Random

92 88

89 83

89 79

88 82

74 83

64 72

71 71

67 68

84 88

83 79

78 80

86 86

61 84

55 69

57 72

56 70

lated digit patterns, F (3, 54) = 11.3, MSe = 183, p < .001, and the difference between digit pattern types was somewhat greater for the count condition than for the position condition, F (3 , 54) = 2 . 5 , MS, = 183, p = .069, just as the effect of digit pattern was larger in spatial location recall than in temporal sequence recall in the earlier experiment. These initial analyses support the conclusion that what the subject said during the presentation of the stimuli was very influential in determining the recall strategy used. Subjects in the count condition, who said the same thing on every trial, showed a pattern of results similar to that found in earlier spatial location recall conditions, in which subjects also said the same thing on every trial. In contrast, subjects in the position condition, whose utterances during stimulus presentation contained useful information, showed a pattern of results like that in earlier conditions of temporal sequence recall, in which subjects’ utterances also provided useful information. The more detailed analyses that follow give further support for these initial observations.

I.

Serial Position Functions

In previous studies of spatial location retention (e.g., Healy, 1975, 1977), large effects of temporal sequence position were obtained, and these effects were greater than the analogous effects of spatial location position. The temporal sequence position function for spatial location retention included a primacy advantage, but not a recency advantage, although both primacy and recency were characteristic of the function for

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208

TABLE 111 PERCENTAGES OF CORRECT RESPONSES IN EXPERIMENT 2 AND IN LETTER RECALLEXPERIMENT BY HEALY(1978, EXPERIMENT 2) BY CONDITION, AND SPATIAL LOCATION POSITION TEMPORAL SEQUENCE POSITION, Sequence position Condition Experiment 2 Position Count Healy (1978) Temporal Spatial

Location position

1

2

3

4

1

2

3

4

82 82

71 80

71 76

81

15

80 19

79 17

79 71

79 79

76 84

61 79

66 74

71 71

14 80

I0 78

69 71

67 19

temporal sequence retention. Similarly, in the present experiment, the effect of temporal sequence position, F(3, 54) = 12.2, MS, = 70, p < .001, was larger than the effect of spatial location position, F(3, 54) = 3 . 2 , MSe = 37, p = .032, and the temporal sequence position function was somewhat different in the two conditions, F ( 3 , 5 4 ) = 6.6, MS, = 70, p = .001. The position condition, like the earlier temporal sequence recall conditions, showed both strong primacy and recency advantages; whereas the count condition, like the earlier spatial location recall conditions, showed only a strong primacy advantage. Table I11 presents the temporal sequence and spatial location serial position functions for the present experiment in terms of percentages of correct responses by condition. The analogous functions for the letter recall experiment by Healy (1978, Experiment 2) are also provided in Table 111 for comparison purposes. Analyses of variance performed on the combined data from both experiments yielded 1% as an estimate of the standard error of the entries in Table 111.

2. Distance Functions In previous studies of temporal sequence and spatial location retention, the temporal distance functions were quite orderly. These functions reveal the distance in the temporal.sequence between the correct letter and the one that replaced it on the subject’s response card. In general, it has been found that when subjects respond incorrectly, they replace the correct letter with one close to it in the temporal sequence. Another property of the observed temporal distance functions is their symmetry: The functions for serial positions 1 and 2 are mirror images of those for positions 4 and 3 . There is one outstanding exception to this symmetrical pattern. In

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spatial location recall, the number of interchanges involving letters in the last two temporal positions is exceptionally large, and, specifically, is larger than the number of interchanges involving letters in the first two positions. This asymmetry is typically less marked for temporal sequence recall. The temporal distance functions for the present experiment are shown in Table IV, where they are compared to the functions from the letter recall experiment by Healy (1978, Experiment 2). The functions for the present experiment show the features described above, with the count condition being more similar to spatial location recall, and the position condition being more similar to temporal sequence recall. TABLE IV OBSERVED TEMPORAL DISTANCE FUNCTIONS FOR EXPERIMENT 2 AND LETTERRECALLEXPERIMENT BY HEALY(1978, EXPERIMENT 2)“

FOR

~

Condition Healy (1978, Experiment 2)

Experiment 2 Positionb

Position

Count

Temporal

Spatial

82 7 6 5

82 8 5 6

76 9 8 7

84 7 5 3

8 77 8 7

8 80 6 6

9

7 79 7 6

6

5 6 76

1 1

2 2

3 4

1

2 3 4

3 1

4

2

9

3 4

77 7

I 2 3

4 7 9

4

81

67

14 10 8

13

13 66 12

5 7 74 14

4 7 14 75

6 I1 12 71

4 6 13 77 ~

“By condition. bThe position code i, j (where i is the superordinate position label and j is the subordinate position label) refers to instances in which the correct letter being scored was presented in the temporal sequence position i and the subject’s response was the letter presented in the temporal sequence position j.

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3 . Pattern Confusions As in the letter recall experiment by Healy (1978), an analysis was made of the number of times the subject responded in complete accord with the interpolated digit pattern, rather than the to-be-recalled pattern of Xs, shown on a given trial. For this analysis, each trial was scored as a whole, in terms of the pattern with which the subject responded. In the earlier experiment by Healy (1978, Experiment 2), subjects were found to respond with the interpolated digit pattern, rather than the letter pattern, with a relatively high frequency when the interpolated digit pattern was similar to the letter pattern in the spatial location recall condition. The corresponding frequencies of confusions were considerably lower for the temporal sequence recall condition and for digit patterns that were different from the letter patterns, but not similar to them. As in the previous spatial location recall condition, subjects in the count condition of the present experiment responded with the interpolated digit pattern on 32 trials when the digit pattern was similar to the pattern of Xs (out of 163 trials of this type with errors). In contrast, subjects in the count condition responded with the interpolated digit pattern on only 7 trials when the digit pattern was different from the pattern of Xs (out of 154 trials of this type with errors). Once again, the position condition more closely resembled the temporal sequence recall condition than the spatial location recall condition. Subjects in the position condition responded with the interpolated digit pattern on only 8 trials when the digit pattern was similar to the pattern of Xs (out of 138 trials of this type with errors) and on only 6 trials when the digit pattern was different from the pattern of Xs (out of 157 trials of this type with errors). The large frequency of pattern confusions that occurred in the count condition for the similar digit patterns is especially impressive, given that none of the 24 patterns had a correspondence between the spatial location of a character in a given temporal sequence position for the pattern of Xs and for the matched similar digit pattern, although there was such a correspondence between the letter patterns and the matched different digit patterns in one temporal position in 1 1 of the 24 patterns and in two temporal positions in 1 pattern. C.

DISCUSSION

A number of striking differences between the count and position conditions of this experiment were analogous to those observed between the spatial location recall and temporal sequence recall conditons of the earlier experiments involving letter retention. The count and position condi-

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21 1

tions differed only in terms of what the subjects said as the stimuli were being presented: Useful information about the to-be-remembered sequence of spatial locations was contained in the subjects’ vocalizations in the position condition, but not in the count condition. These results suggest that subjects use a phonemic code that reflects what they said during stimulus presentation whenever that code contains useful information; otherwise, if available, subjects code information about the temporal-spatial pattern of stimulus presentations. The strong similarity between the count and spatial location recall conditions also has implications for the more general question of the independence of item and order retention. The two conditions differed markedly in terms of the amount and quality of item information, but this difference did not seem to affect the coding strategy used to retain spatial location information. The pattern information coded by subjects in the count condition of this experiment and in the spatial location recall conditions of earlier studies is not equivalent to the sequence of spatial location position numbers. If pattern coding were equivalent to such position coding, then the subjects in the position condition should have performed similarly to those in the count condition and should have shown strong evidence for pattern coding, but they did not. Rather, the effects of interpolated digit patterns were weaker in the position condition than in the count condition, and the results of the position condition more strongly resembled those of previous temporal sequence recall conditions. Instead of position numbers, the memory units used for pattern coding appear to be items of information referring to aspects of the entire temporal-spatial pattern of stimulus presentations, as specified in the pattern model proposed by Healy (1978).

IV.

Experiment 3

Because of the similarity between the count condition of Experiment 2 and the spatial location recall condition of the earlier letter recall experiment by Healy (1978), it was concluded that the strategy used to retain spatial location information is not affected by the properties of the to-beremembered items. Further support for this hypothesis derives from the fact that previous studies indicated no effects of the phonemic properties of letters that were to be recalled in their spatial arrangement, although phonemic characteristics did influence the recall of letters according to their temporal sequence of presentation (e.g., Healy, 1975, 1977). It is possible, though, that subjects attend to the visual features of the letters,

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rather than the sound of their names, in spatial location retention. Earlier studies involving short-term memory for temporal sequences (e.g., Cimbalo & Laughery, 1967) did not find effects of visual similarity, but such effects might be found for spatial location retention. Just as differences in the phonemic properties of successively presented letters seem to aid in remembering their temporal sequence (cf. Drewnowski, 1980), differences in the visual properties of adjacent letters may aid in remembering their spatial arrangement. In fact, as indicated above, unreported analyses of the letter recall experiment by Healy (1978, Experiment 2) indicated more visual than phonemic confusion errors in the spatial location recall condition. Specifically, subjects were shown the letters BVSX on every trial of that experiment, and they tended to confuse the visually similar letters V and X or B and S in the spatial location recall condition, whereas they confused the phonemically similar letters V and B or X and S in the temporal sequence recall condition. Experiment 3 was aimed in part at determining to what extent subjects are influenced by the visual features of letters when they are asked to recall them in the spatial arrangement in which they were presented. Experiment 3 was also aimed at testing the generality of the previous findings concerning the spatial location retention of letters. There was strong evidence in the previous studies that subjects coded information about the temporal-spatial pattern of letter presentations in spatial location recall conditions (Healy, 1975, 1977, 1978); however, these conditions were constrained in several respects. Perhaps the most critical constraint was the number of to-be-remembered items shown on a particular trial. In every case studied by Healy, the subject had to remember the order of exactly four letters. Because there were four letters, there were 24 possible temporal-spatial patterns of letter presentation. Perhaps if more letters were shown, there would be less of a tendency to use pattern coding because of the larger number of possible temporal-spatial patterns that would result. For example, 120 possible temporal-spatial patterns result from the presentation of five letters; and it may be difficult for the subject to discriminate among so many different patterns. On the other hand, if fewer than four letters were presented to the subject for recall, then there would be fewer possible temporal-spatial patterns, so that pattern coding might be more likely to be used, even in temporal sequence recall. If three letters were to be recalled, then there would be only 6 possible temporal-spatial patterns, and these might be easy for the subject to discriminate. In contrast to earlier studies, in Experiment 3 sequences of three, four, and five to-be-remembered letters were presented to subjects. The pattern model proposed for the spatial location retention of four-letter sequences

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213

(Healy, 1978) cannot be applied directly to sequences of three and five tobe-remembered letters. Nevertheless, the same general principles should apply for all three string lengths. Evidence for pattern coding was gleaned in this experiment by examining the effects on letter retention of the nature of the interpolated digit pattern. The temporal-spatial pattern of the intervening digits interpolated between the to-be-remembered letters and their recall on a given trial was either identical to the temporalspatial pattern of the letters shown on that trial (“same” pattern) or did not bear any regular relationship to the letter pattern (“random” pattern). Fewer letter recall errors on same digit patterns than random digit patterns has been taken as evidence for pattern coding (see Healy, 1978). Other features that have characterized and differentiated temporal sequence and spatial location recall of four-letter sequences were also examined in this study with three, four, or five to-be-remembered letters. In particular, the time course of forgetting, the serial position functions for both temporal sequence positions and spatial location positions, and the temporal distance functions were examined. In previous studies, the time course of forgetting has been found to be considerably steeper for temporal sequence recall than for spatial location recall; the serial position functions for temporal sequence positions have yielded a larger primacy advantage than the functions for spatial location positions for both temporal sequence and spatial location recall; and the temporal distance functions have revealed a disproportionately large number of interchanges involving the last two temporal positions in spatial location recall. A.

METHOD

1. Subjects

Thirty-six young men and women, who were recruited from posters placed on the Yale University campus, participated as subjects and were paid at the rate of $2.50 per hour. There were 18 subjects in each of two conditions: temporal sequence recall and spatial location recall. The subjects were further subdivided into six subconditions of the two conditions with three subjects per subcondition. 2.

Apparatus

The same apparatus was employed as in Experiments 1 and 2, except that five spatial locations of the display screen, rather than four, were used. The first four (left-most) locations were the same as those used in Experiment 2. A subject, depending on the subcondition, saw letters and digits occurring in three, four, or five different spatial locations. When

214

Alice F. Healy

three or four locations were used, they were always the left-most locations. A given location that was not used in a particular subcondition was left blank; a hyphen did not occur in the location, but hyphens were used to represent locations among those employed in a given subcondition that were blank on a particular trial. Each trial began with the display of three, four, or five (depending on the subcondition) hyphens repeated twice in a row. 3 . Design and Materials Twelve different 168-trial experimental sequences were prepared, 1 for each subcondition. Each subject saw only 1 experimental sequence, and each sequence was shown to three subjects. A trial consisted of three, four, or five successively presented capital letters as stimuli, followed by a retention interval of 3, 4, 5, 15, or 16 successively presented interpolated digits. Four sequences, 2 for the temporal sequence recall condition and 2 for the spatial location recall condition, included three to-be-remembered letters; 4 included four letters; and 4 included five letters. Each character was displayed in only one location on the screen. The letters shown on a given trial were presented in different locations, not necessarily from left to right. For the sequences with five to-be-remembered letters (and five different spatial locations), 72 of the 120 possible temporal-spatial patterns of letters were presented on a single trial, paired with a random digit pattern either 3, 4,or 16 digits long, and the remaining 48 of the letter patterns were presented twice, once paired with the digit pattern that was the same as the letter pattern and once paired with a random digit pattern of the same length (either 5 or 15 digits long). The random patterns were constructed in the following manner: The spatial locations of the digits were selected at random from the locations available for a given subcondition with the constraint that no location occurred twice in immediate succession. Likewise, the identity of the digits was selected at random from the numbers 1-9 with the constraint that no digit occur twice in immediate succession. For digit patterns that were the same as the letter patterns, on the longer retention interval, the digit pattern was repeated 3 times in a row. Seven different types of retention intervals occurred, 24 times each. Five of these retention interval types involved random digit patterns, including those with 3, 4, 5, 15, and 16 digits, and five involved digit patterns that were the same as the letter patterns, including those with 5 and 15 digits. The presentation order of the trials was quasi-random with the following constraint: In every block of 14 successive trials there were two retention intervals of each type. The assignment of letter patterns to

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215

retention intervals was quasi-random with the following restriction: The 120 letter patterns were divided into 24 sets of 5 each. The 5 patterns in a set were identical, except for the temporal sequence position of the letter that occurred in the fifth (right-most) spatial location. Thus, for example, the pattern with the sequence of spatial locations 1, 3, 5, 4, 2 was in the same set as that with the sequence 1, 3 , 4 , 2, 5 . Each one of the five types of random digit patterns (3, 4, 5 , 15, and 16 digits) was matched with a different one of the 5 patterns in a set. Those 2 patterns in every set of 5 that were matched with the 5-digit and 15-digit retention intervals were the ones selected to occur twice, once with a random digit pattern and once with the digit pattern that was the same as the letter pattern. The sequences with four to-be-remembered letters were derived from those with five to-be-remembered letters in the following way: The letter pattern on a given trial in the sequence with four letters was identical to that on the corresponding trial in the sequence with five letters, except that the letter in the fifth spatial location was eliminated. For trials with random digit patterns, the identity of the digits on a given trial in the sequences with four letters was the same as that on the corresponding trial in the sequences with five letters. The spatial locations of the digits were the same, except that whenever a digit occurred in the fifth location in the sequences with five letters, a new location for that digit was chosen at random for the sequences with four letters, without violating the constraint that no location occur twice in immediate succession. For trials with same digit patterns, either 4 or 16 interpolated digits occurred (rather than 5 or 15), and on the longer retention interval, the digit pattern was repeated four times in a row (rather than only three). The identity of the digits on a given trial with the same digit and letter patterns in the sequences with four to-be-remembered letters was identical to that on the corresponding trial in the sequences with five to-be-remembered letters, except that the final digit was eliminated from the trials with the shorter retention interval and a new digit was added at the end of the trials with the longer retention interval. The presentation order of the trials in the sequences with four letters corresponded to that in the sequences with five letters. These constraints ensured that each of the 24 possible temporal-spatial patterns of letters occurred 7 times, once at each of the five retention intervals with random digit patterns (3, 4,5 , 15, and 16 digits), and once at each of the two retention intervals with digit patterns that were the same as the letter patterns (4and 16 digits). The sequences with three to-be-remembered letters were derived from those with four letters in a manner analogous to that used to derive the sequences with four letters from those with five. For trials with same digit patterns, either 3 or 15 interpolated digits occurred (rather than 4 or 16),

Alice F. Healy

216

and on the longer retention interval, the digit pattern was repeated 5 times in a row (rather than 4). Every one of the six possible temporal-spatial patterns of letters occurred 28 times, 4 times at each of the five retention intervals with random digit patterns, and 4 times at each of the two retention intervals with digit patterns that were the same as the letter patterns. The same letters appeared on each experimental trial of a given subject’s session. The letters were drawn from a population of five consonants: BVSXH. Note that two pairs of these letters are phonemically similar (BV and SX) and two pairs are visually similar (VX and BS). There were four sequences with each of the following letter combinations: BVX, BVSX, BVSXH. The four sequences with a given letter combination included two sequences in the temporal sequence recall condition and two in the spatial location recall condition, which were identical except for the temporal sequence and spatial arrangement of the letters on a given trial. For the sequences in the temporal sequence recall condition, the spatial arrangement of the letters was held constant throughout the 168 trials, and for the sequences in the spatial location recall condition, the temporal order of the letters was held constant. The constant temporal order of the letters in a sequence for the spatial location recall condition was the same as the constant spatial arrangement of the letters in the corresponding sequence for the temporal sequence recall condition. The constant letter orders were BVX, XVB, BVSX, BSVX, BVSXH, and BSVXH for the six sequence pairs, respectively. Seven practice trials were shown to the subjects before the experimental trials, one of each retention interval type. There were six different sequences of practice trials, one for each combination of string length and recall condition. The constant letter orders were ABC, ABCD, and ABCDE for the three-, four-, and five-letter sequences, respectively. 4.

Procedure

Subjects were tested individually in sessions that lasted approximately 1 hour and 40 min. Each subject was instructed to read aloud the letters and digits as they appeared. At the end of each list of letters and digits, the screen became blank and the subject was to write down the letters shown on the trial in their temporal sequence in the temporal sequence recall condition, or in their spatial arrangement in the spatial location recall condition. The subjects wrote their responses on 4 X 6-inch index cards on which three, four, or five (depending on the subcondition) boxes had been printed in a horizontal linear array. The subjects were not required to fill in the boxes in any particular temporal sequence, but they were forced

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Short-Term Memory for Order Information

to fill in all the boxes on the card and they were told to guess if necessary. Short rest breaks occurred after experimental trials 43, 85, and 127. Subjects were told which three, four, or five letters would be shown to them on every trial and were given the constant temporal sequence or constant spatial arrangement of the letters. B.

RESULTS

The results are summarized in Table V in terms of percentages of correct responses as a function of recall condition (temporal sequence recall or spatial location recall), string length (three, four, or five to-beremembered letters), digit pattern type (same or random), and retention interval (short or long). Only retention intervals used for both same and random digit patterns are included in this summary (3 and 15 digits for the three-letter sequences, 4 and 16 digits for the four-letter sequences, and 5 and 15 digits for the five-letter sequences). An analysis of variance yielded 2% as an estimate of the standard error of the entries in Table V . All three string lengths showed the same general pattern of results, although overall recall level was greatest for three-letter sequences and poorest for five-letter sequences, F(2, 24) = 84.5, MSe = 514, p < .001. Specifically, for all string lengths, retention was better at the short interval than at the long interval, F( 1, 24) = 105.3, MSe = 65, p < .OO 1, and the difference between retention intervals was greater for temporal sequence recall than for spatial location recall, F( 1 , 24) = 5.5, MSe = 65, p = .026. Although the decline in recall level across retention intervals increased with increasing string length, F(2, 24) = 9.1, MS, = 65, TABLE V

FUNCTION OF RECALL CONDITION, STRING LENGTH,DIGITPATTERN TYPE,AND RETENTION INTERVAL (EXPERIMENT 3) PERCENTAGES OF CORRECT RE~PONSESAS A

String length Retention interval Short Same Random Long Same Random

Temporal

3

Spatial

Temporal

95 95

97 96

87 91

95 89

4

5

Spatial

Temporal

Spatial

80 75

95 89

62 57

61 57

67 63

91 83

42 41

48 45

Alice F. Healy

218

p = .001, the three-way interaction between string length, recall condition, and retention interval was not significant, F(2, 24) = 1.0, MS, = 65, p = .378. In addition, for all string lengths, retention was better when the digit pattern matched the to-be-remembered letter pattern than when it was a random pattern, F( 1, 24)= 14.5, MS, = 55, p = .001; the interaction of digit pattern type and string length was not significant, F(2, 24) = 2.6, MS, = 55, p = .090. As in earlier studies (Healy, 1978), the advantage for same digit patterns was somewhat more pronounced in spatial location recall than in temporal sequence recall for all string lengths; but neither the interaction of digit pattern type and recall condition, F ( 1 , 24) = 2.2, MS, = 55, p = .144, nor the three-way interaction of string length, digit pattern type, and recall condition, F(2, 24) < 1, was significant. Despite the overall similarity in the pattern of results for the three string lengths, the evidence for pattern coding in spatial location recall seemed to be greatest for the four-letter sequences. In particular, recall level on spatial location recall was superior to that on temporal sequence recall for all string lengths, F(1, 24) = 8.0, MS, = 514, p = .009, but the difference between recall conditions was most marked for the four-letter sequences, F(2, 24) = 3.9, MS, = 514, p = .034. Perhaps pattern coding was facilitated in the four-letter sequences relative to the other string lengths, because of the intermediate number (24) of possible temporal-spatial patterns in that case. Pattern coding may not be used to the same extent with five-letter sequences because of the very large number of possible patterns (120), and the high recall levels with three-letter sequences may not give room for differences between conditions to be seen. I.

Serial Position Functions

The serial position functions differed across string lengths, as one might expect, since serial position functions naturally depend on the number of serial positions. The serial postion functions are presented in Table VI for both temporal sequence positions and spatial location positions, as a function of string length and recall condition. Only the five retention intervals with random digit patterns (3, 4, 5, 15, and 16 digits) were included in this analysis. Separate analyses of variance were conducted on these data for both temporal sequence and spatial location positions for each string length. The analyses of variance yielded 1%, 1%, and 2% as estimates of the standard errors of the entries in Table VI for string lengths 3, 4, and 5, respectively. In previous comparisons of temporal sequence and spatial location recall (e.g., Healy, 1975), the

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Short-Term Memory for Order Information

TABLE VI PERCENTAGES OF CORRECT RESPONSES AS A FUNCTION OF RECALL CONDITION, STRINGLENGTH,TEMPORAL SEQUENCE POSITION, AND SPATIAL LOCATION POSITION FOR RANDOM DIGITSEQUENCES ONLY(EXPERIMENT 3) String length 3

Condition Temporal Sequence position Location position Spatial Sequence position Location position

4

5

1

2

3

1

2

3

4

1

2

3

4

5

93 94

92 91

93 92

76 73

71 71

69 71

70 70

68 57

46 49

42 48

42 46

56 54

96 92

92 94

92 93

92 91

89 88

87 89

88 90

69 57

52 51

45 52

43 49

56 56

temporal sequence positions exhibited more of a bowed shape, or at least a greater primacy advantage, than the spatial location positions for both temporal sequence recall and spatial location recall. This same general pattern of results held for all of the string lengths in the present experiment, although the serial position functions were generally flattest for the three-letter sequences and least flat for the five-letter sequences. 2.

Phonemic and Visual Confusions

An analysis of phonemic and visual confusion errors was conducted to determine the form of coding used in temporal sequence recall and spatial location recall. In order to make the string-length conditions as analogous as possible, only errors made on the stimulus letters B , V , and X were considered. Separate tabulations of phonemic and visual confusion errors were prepared. For the tabulation of phonemic confusions, only responses to the stimulus letters B and V were included, and a confusion error was scored only if the response to the stimulus B was V , or the response to V was B ; all other error responses were scored as nonconfusions. On the other hand, for the tabulation of visual confusions, only responses to the stimulus letters V and X were included, and a confusion error was scored only if the response to the stimulus V was X or the response to X was V. For example, consider a subject in the 4 stringlength temporal sequence recall condition who saw the simulus letter V in the first temporal sequence position and wrote a B in the first position of the response card. For the analysis of phonemic confusion errors, this

220

Alice F. Healy

subject’s response would be scored as a confusion error; and for the analysis of visual confusion errors, it would be scored as a nonconfusion error. Table VII summarizes the confusion error tabulation by providing the percentages of correct responses, confusion errors, and nonconfusion errors as a function of confusion error type, string length, recall condition, and retention interval. Only the five retention intervals with random digit patterns were included in this tabulation. Since it is difficult to make sense of absolute percentages of confusion errors across conditions with different overall error percentages, conditional percentages of confusion errors, given that an error was made, were computed for each subject. For example, assume the subject deTABLE VII PERCENTAGES OF CORRECT RESPONSES,CONFUSION ERRORS,AND NONCONFUSION ERRORSFOR RANDOMDIGITPATTERNS (EXPERIMENT 3)a,b ~

~

Retention interval

3 String length

3 Temporal Phonemic Visual Spatial Phonemic Visual 4 Temporal Phonemic Visual Spatial Phonemic Visual 5 Temporal Phonemic Visual Spatial Phonemic Visual

4

15

5

16

Cor Conf Nonc Cor Conf Nonc Cor Conf Nonc Cor Conf Nonc Cor Conf Nonc

94 95 97 95

5

95 94

2 5

3 1

92 94

6 2

2 4

91 90

5 6

5 4

88 88

6 6

6 6

1

95 94

2 3

3 2

95 93

2 5

3 2

88 89

7 7

5 5

91 91

5 5

4 4

1

1

3

1

2

4

83 81

16

1 19

76 74

20 3

4 23

75 76

20 2

6

0

22

63 61

17 10

20 29

65 64

16 10

19 26

92 91

2 4

6 5

89 89

5 3

7 7

95 93

1 4

4 2

89 86

2 7

8 7

85 82

4 8

10 10

63

19 6

18 35

58 52

22 8

19 40

67 56

15 7

18 37

44

60

42

18 14

38 44

44 38

16 13

41 49

59 48

10 19

31 33

61 44

20

11

28 35

62 53

8 14

30 34

50 35

22

10

40 43

9

44

35 38

56

18

OInvolving stimulus letters B , V, and X and as a function of string length, recall condition, retention interval, and confusion type. bKey: Cor = correct response, Conf = confusion error, Nonc = nonconfusion error.

22 1

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TABLE VIII MEANCONDITIONAL PERCENTAGES OF PHONEMIC AND VISUAL CONFUSION ERRORSINVOLVING THE STIMULUS LETTERSB, V, AND X (EXPERIMENT 3)" Retention interval String length

3 Temporal Phonemic Visual Spatial Phonemic Visual 4 Temporal Phonemic Visual Spatial Phonemic Visual 5 Temporal Phonemic Visual Spatial Phonemic Visual

3

4

5

15

16

16 13

42 65

69 26

51 46

54 34

32 12

34 52

40 55

50 56

48 52

90 1

77 13

70 12

42 24

39 25

24 27

40 29

19 61

18 39

24 49

50 14

52 16

45 15

32 24

27 24

25 36

26 35

22 26

20 32

21 32

AS a function of string length, recall condition, and retention interval for random digit patterns.

scribed above made 10 errors overall on the stimulus letter V , 6 which involved replacements of V by B , 1 which involved replacing V by X, and 3 which involved replacing V by S. Then the conditional percentage of phonemic confusion errors, given that an error was made on the stimulus letter V , would be 6/10 = 60%, and the corresponding conditional percentage of visual confusion errors would be 1/10 = 10%. The confusion error analysis is summarized in Table VlII in terms of mean conditional percentages of phonemic and visual confusion errors, given that an error was made on an instance of the stimulus letter B or V (for the phonemic confusion errors) or on an instance of the stimulus letter V or X (for the visual confusion errors). The conditional percentages are given as a function of string length, recall condition, and retention interval. Only the five retention intervals with random digit patterns were included in this analy-

222

Alice F. Healy

sis. An analysis of variance yielded 9% as an estimate of the standard error of the entries for Table VIJI. Although errors on the same letters were considered for each of the three string lengths, one difference between string lengths remained in the analysis: The chance conditional percentage of a phonemic or of a visual confusion error was 50% for the three-letter sequences, since one out of every two incorrect letters would constitute a confusion error of a given type; but the chance conditional percentage was 33% for the four-letter sequences and 25% for the fiveletter sequences. These differences in chance values presumably account for a significant main effect of string length found in the analysis of variance performed on these conditional percentages, F(2, 24) = 86.9, MS,= 140, p < .001. As in previous studies, for all three string lengths in the present experiment, the conditional percentages of phonemic confusion errors were higher than those of visual confusion errors (and higher than the chance value) for temporal sequence recall, but a smaller difference in the opposite direction was found for spatial location recall. The main effect of confusion enor type (phonemic or visual) was significant, F(1, 24) = 5.4, MS, = 1210,p = .028, as was the interaction of recall condition and confusion error type, F( 1 , 24) = 37.5, MS, = 1210, p < .001, but neither the two-way interaction of string length and confusion error type, F(2, 24) = 1.2, MS, = 1210, p = .308, nor the three-way interaction of string length, recall condition, and confusion error type, F(2, 24) = 1.9, MS, = 1210, p = .174, were significant. In previous experiments the conditional percentages of phonemic confusion errors in temporal sequence recall declined toward the chance value as retention interval increased. Likewise, for all string lengths in the present experiment, there was a decline in the conditional percentages of phonemic confusion errors in temporal sequence recall, but not in spatial location recall. No comparable decline was found in the conditional percentages of visual confusion errors in temporal sequence recall or spatial location recall: These findings are reflected in a significant interaction of confusion error type and retention interval, F(4, 96) = 3.5, MS, = 490, p = .010, and a significant three-way interaction of confusion error type, retention interval, and recall condition, F(4, 96) = 6.2, MS, = 490, p < .001. Although this pattern of results obtained for each of the string lengths, it was not quite as distinct for the three-letter sequences. Consequently, there was a significant three-way interaction of string length, confusion error type, and retention interval, F(8, 96) = 3.1, MS, = 490, p = .004, but the four-way interaction of string length, confusion error type, retention interval, and recall condition was not significant, F(8, 96) = 1.4, MS, = 490,p = .197.

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223

Whereas a preponderance of phonemic confusion errors was found for temporal sequence recall, more visual than phonemic confusion errors were found for spatial location recall in the analyses described above. To determine whether the preponderance of visual confusion errors in spatial location recall was a general phenomenon or was more specifically attributable to the subset of errors considered in the above analyses (errors involving the letters B , V , and X ) , a more complete analysis included all possible confusion errors: The phonemic confusion errors involved in this analysis were those of B and V or S and X, and the visual confusion errors were those of V and X or B and S. In order to determine whether both pairs of phonemically confusable letters (or both pairs of visually confusable letters) showed the same pattern of results, this analysis made a distinction between the first pair of confusable stimulus letters of a given type and the second pair of letters of that type. The relative positions of the letter pairs were determined with respect to the constant letter order of the sequence. For example, consider a subject in the spatial location recall condition who saw the letters BVSX on every trial, with the temporal sequence always B , then V , then S , then X . The first pair of phonemically similar letters was BV and the second was SX, whereas the first pair of visually similar letters was BS and the second was VX. Note that for the three-letter sequences, there was only one, not two pairs of confusable stimulus letters of a given type, so that responses to the stimulus letter X did not enter into the calculations of the conditional percentages of phonemic confusion errors, and responses to the stimulus letter B did not enter into the calculations of the conditional percentages of visual confusion errors. Likewise, for the five-letter sequences, responses to the stimulus letter H did not enter into the calculations of the conditional percentages of either phonemic or visual confusion errors. The confusion error analysis is summarized in Table IX in terms of mean conditional percentages of phonemic and visual confusion errors, given that an error was made on an instance of the stimulus letter B, V, S, or X, as a function of string length, recall condition, constant letter order (BVX, or XVB for the three-letter sequences, BVSX or BSVX for the fourletter sequences, and BVSXH or BSVXH for the five-letter sequences), and letter pair position (first or second in constant letter order). Only the retention intervals with random digit patterns were included in this analysis. Analyses of variance yielded 12, 5, and 3% as estimates of the standard errors of the entries in Table IX for string lengths 3, 4,and 5, respectively. For all three string lengths, for both constant letter orders of each string length, and for both letter pair positions of each constant letter order, the conditional percentage of phonemic confusion errors was greater than that of visual confusion errors in temporal sequence recall. In

224

Alice F. Healy

TABLE IX MEANCONDITIONAL PERCENTAGES OF PHONEMIC AND VISUALCONFUSION ERRORS(EXPERIMENT 3)“ String length

3 Recall condition Temporal Phonemic First Second Visual First Second Spatial Phonemic First Second Visual First Second

4

5

BVX

XVB

BVSX

BSVX

BVSXH

BSVXH

52

65 -

59 64

69 70

42 45

40 46

39 -

14 14

16 16

17 18

16 19

63 -

31 38

19 24

23 31

23 25

42 -

45 31

31 51

20 31

27 33

-

35 -

19 -

73 -

“As a function of string length, recall condition, letter pair position, and constant letter order for random digit sequences only.

contrast, the pattern of confusion errors for spatial location recall was strongly influenced by string length, constant letter order, and letter pair position. In particular, when the letters next to each other in the constant temporal sequence were visually similar (as in the sequence SSVX), then more visual than phonemic confusions occurred. However, when the letters adjacent to each other in the constant temporal sequence were phonemically similar (as in the sequence SVSX), then more phonemic than visual confusion errors occurred-at least when the adjacent letters were the last two in the constant temporal sequence. These findings presumably do not reflect visual or phonemic similarity per se, but result from the large number of interchanges involving adjacent temporal positions, especially the last two, as was noted for four-letter sequences by Healy ( 1978). 3 . Distance Functions

To determine whether a preponderance of interchanges involved the last two temporal positions in spatial location recall in this experiment, as had been the case in previous studies, an analysis of the temporal distance

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225

functions was performed. Table X shows these distance functions in terms of response percentages which reveal the distance in the temporal sequence between the correct letters and the letters that replaced them on the subject’s response protocol, for each of the recall conditions and string lengths. These functions do indeed show a disproportionately large number of interchanges between positions 2 and 3 in the three-letter sequences, between positions 3 and 4 in the four-letter sequences, and between positions 4 and 5 in the five-letter sequences, especially in spatial location recall. Also, it is interesting to note that for the five-letter sequences, the greatest number of interchanges does not involve the last two positions, but positions 3 and 4. C . DISCUSSION

These results provide support for the generality of the differences between temporal sequence and spatial location retention found in earlier studies by Healy (1975, 1977, 1978), which were restricted to sequences of four to-be-remembered letters. The same general pattern of results was found for all three string lengths used in the present experiment and is consistent with the earlier conclusion that phonemic coding is used in temporal sequence recall and pattern coding in spatial location recall (although pattern coding appears to be most effective for the four-letter sequences). It is particularly interesting that in the present experiment, recall level on spatial location retention was actually superior to that on temporal sequence retention, even at the shorter retention intervals of the four-letter sequences. Thus, despite the fact that pattern coding was an efficient strategy and despite the fact that pattern coding was available for temporal sequence retention, subjects chose the less efficient phonemic coding strategy in this situation. This finding is consistent with the hypothesis proposed above that subjects choose a memory code that reflects what they heard themselves say during stimulus presentation, whenever that code contains some useful information-as it does in temporal sequence recall, but not in spatial location recall. Although the pattern model proposed for the spatial location retention of four-letter sequences (Healy, 1978) cannot be applied directly to sequences of three and five to-be-remembered letters, its general principles can be applied to all three string lengths. According to the pattern model, subjects code three different items of pattern information: the spatial location of the first letter, the pattern class, and the spatial arrangement of the last two letters in the temporal sequence. Presumably, no new principles would be needed to derive a pattern model for three-letter sequences: The primacy advantage found in the spatial location recall condition for the temporal sequence positions is consistent with the hypothesis that an

226

Alice F. Healy

TABLE X OBSERVED TEMPORAL DISTANCE FUNCTIONS AS A FUNCTION OF RECALL CONDITION AND STRING LENGTHFOR RANDOM DIGITPATTERNS ONLY (EXPERIMENT 3) String length

3 Positiona

4

Temporal

Spatial

Temporal

Spatial

Temporal

Spatial

93 4 3

96 1 3

92 4 2

68

-

-

76 9 9 6

1

2

1 2 3 4 5

5

2

9 6 6

69 13 7 6 6

3 89 4 3

13 46 15 17

13 52 15 12

-

1 2

92

3

4

5

4

3 92 5

9 71 10 10

5

3 1

4

2 3 4 5 1 2 3

5

4 5

1 2 3 4 5

3 4 93

-

1 7 92

7 11 69 13

3 3 87 7

n

n

6

in

8 6 14

42 19 14

45

1

7

4

14

17

7 12 20 43 18

6 11 13 15 56

5 8 14 16 56

-

9 12 70

10

6 88

20

42

22 13

“The position code i, j (where i is the superordinate position label and j is the subordinate position label) refers to instances in which the correct letter being scored was presented in the temporal sequence position i and the subject’s response was the letter presented in the temporal sequence position j .

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221

item of pattern information coded by subjects is the spatial location of the first letter in the temporal sequence. Furthermore, the disproportionately large number of interchanges involving the last two sequence positions in spatial location recall (which was large in relation to the number of interchanges involving the first two sequence positions) is consistent with the hypothesis that another item of pattern information coded by subjects, but rapidly lost, is the spatial arrangement of the last two letters in the temporal sequence (see Healy, 1978). In contrast, at least one new item of pattern information would have to be added to a pattern model for the five-letter sequences. Possibly this item of information might reflect the large number of interchanges involving temporal positions 3 and 4, since numerous interchanges that involved intermediate temporal positions were not found in the four-letter sequences.

V.

Experiment 4

In Experiment 3, more visual than phonemic confusion errors were found overall for spatial location recall, but this difference was not found consistently. Furthermore, there were some indications in the data that the visual confusions observed were not reflecting visual similarity per se, but were the natural consequence of temporal-spatial pattern coding. Experiment 4 was designed to provide a clearer answer to the question of whether visual letter coding is employed in spatial location recall. The subset of letters included in this experiment was expanded in comparison to the previous experiments, so that a more definitive answer to the question could be obtained. Three visually confusable pairs of letters were selected: VX, BS, and FP. As in the previous experiments, the determination of which letters were visually confusable was made informally on the basis of the visual characteristics of the stimuli used. Since large effects of visual confusability were found for four-letter sequences in Experiment 3, the present experiment was restricted to sequences of four to-be-remembered letters. The design of the experiment was strictly analogous to that of the earlier experiment by Healy (1978, Experiment 2). The only essential difference in method was the expansion of the set of letters shown to subjects. A.

METHOD

1.

Subjects

Thirty-six male and female Yale University undergraduates participated as subjects in order to fulfill a course requirement. Two of the subjects received 1 hour of course credit and $2.50 for the second hour of

228

Alice F. Healy

participation. There were 18 subjects in each of two conditions: temporal sequence recall and spatial location recall. The subjects were further subdivided into six subconditions of the two conditions, with 3 subjects per subcondition.

2 . Apparatus The same apparatus was employed as in previous experiments. The first four spatial locations of the display screen were used to present stimuli.

3. Design and Materials Twelve different 192-trial experimental sequences were prepared, based on those used by Healy (1978, Experiment 2). Each subject saw only 1 expermental sequence, and each sequence was shown to 3 subjects. Six sequences were for temporal sequence recall and 6 were for spatial location recall. A trial consisted of four successively presented consonants, printed in capital letters, followed by a retention interval of either 4 or 16 successively presented digits. The temporal-spatial pattern of letters on a given trial corresponded to the temporal-spatial pattern of letters shown on the corresponding trial in Experiment 2 of the study by Healy (1978) and in Experiment 2 of the present study. Similarly, the digits shown on a given trial corresponded exactly in identity and location to those shown on the corresponding trial of Experiment 2 by Healy (1978) and Experiment 2 of the present study. The population of consonants employed differed from that used by Healy (1978); it included six letters: BVSXPF. There were four sequences with each of the following letter combinations: BVSX, BPSF, and PVFX. Note that each of these combination includes two pairs of letters that are visually similar (BS, VX, and PF) and two pairs of letters that are phonemically similar (BV, BP, PV, SX, SF, and FX). The four sequences with a given four-letter combination included two sequences in the temporal sequence recall condition and two sequences in the spatial location recall condition, which were identical except for the temporal sequence and spatial arrangement of the letters on a given trial. The constant temporal order of the letters in a sequence for the spatial location recall condition was the same as the constant spatial arrangement of the letters in the corresponding sequence for the temporal sequence recall condition. The constant letter orders were BVSX, BSVX, BPSF, BSPF, PVFX, and PFVX for the six sequence pairs, respectively. Different permutations of the letters ABCD were shown on eight practice trials. Two different sequences of practice trials were used, one for

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229

the temporal sequence recall condition and one for the spatial location recall condition. For both sequences the constant letter order was ABCD. Eight different temporal-spatial patterns were used, one with each of the eight combinations of retention interval and digit pattern type. 4 . Procedure

Subjects were tested individually in sessions that lasted approximately 2 hours. Each subject was instructed to read aloud each consonant and digit as it appeared on the display screen. At the end of each sequence, the subject’s task was to write down the four letters shown on that trial in their temporal sequence in the temporal sequence recall condition and in their spatial arrangement in the spatial location recall condition. The subjects wrote their responses on 3 x 5-inch index cards on which four boxes had been printed in a horizontal linear array. The subjects were not required to fill in the boxes in any particular temporal sequence, but they were forced to fill in all the boxes on the card, and they were told to guess if necessary. At the end of every block of 48 experimental trials, there was a short rest break. Subjects were told which four letters would be shown to them on every trial and were given the constant temporal sequence or constant spatial arrangement of the letters. B.

RESULTS

The results are summarized in Table XI in terms of percentages of correct responses for the two experimental conditions as a function of retention interval and the relation between the to-be-recalled letter pattern and the interpolated digit pattern. An analysis of variance yielded 2% as an estimate of the standard error of the entries of Table XI. The results replicate in all essential details those found in the earlier experiment by Healy (1978, Experiment 2), which differed from the present experiment only in terms of the particular letters shown to subjects. Although overall performance did not differ significantly between the two recall conditions, F(1, 34) = 2.5, MS, = 4641, p = .121, retention was much better at the short retention interval than at the long interval, F(1, 34) = 145.6, MS, = 540, p < .001 , and the effect of retention interval was substantially larger in temporal sequence recall than in spatial location recall, F(1, 34) = 38.9, MS, = 540, p < .001. In addition, subjects made fewer errors on trials in which the interpolated digit pattern was identical to the pattern of to-be-remembered letters, F(3, 102) = 17.9, MS, = 127, p < .001, but this difference was greater at the longer retention interval

Alice F. Healy

230

TABLE XI PERCENTAGES OF CORRECT RESPONSESBY RECALLCONDITION, RETENTION INTERVAL, AND RELATIONBETWEEN THE TO-BE-RECALLED LETTERPATTERN AND THE INTERPOLATED DIGIT PATTERN(EXPERIMENT 4) Retention interval 4

Condition Same Temporal Spatial

81 85

16

Different

Similar

Random

Same

Different

Similar

Random

81 76

82 77

83 80

61 79

55

56 69

56 69

69

than at the shorter interval, F ( 3 , 102) = 2.7, MSe = 183, p = .047, and was greater in the spatial location recall condition than in the temporal sequence recall condition, F ( 3 , 102) = 5.4, MSe = 127, p = .002.

I.

Phonemic and Visual Confusions

An analysis of phonemic and visual confusion errors was conducted, analogous to that performed for Experiment 3 . Phonemic confusion errors consisted of confusions of B and V , B and P , P and V , S and X , S and F , or F and X ; visual confusion errors consisted of confusions of B and S, V and X , or P and F . The tabulation of confusion and nonconfusion errors is summarized in Table XI1 in terms of absolute percentages of correct TABLE XI1 PERCENTAGES OF CORRECT RESPONSES,CONFUSION ERRORS,AND NONCONFUSION ERRORSAS A FUNCTION OF CONFUSION TYPE,RECALL CONDITION AND RETENTION INTERVAL(EXPERIMENT 4) Retention interval 16

4

Recall condition Temporal Phonemic Visual Spatial Phonemic Visual

Correct

Confusion Nonconfusion Correct

Confusion

Nonconfusion

82 82

13 2

5 16

57 57

17 13

26 30

80 80

8 9

13 12

72 72

10 10

18 18

Short-Term Memory for Order Information

23 1

responses, confusion errors, and nonconfusion errors as a function of confusion error type, recall condition, and retention interval. As in the preceding experiment, conditional percentages of confusion errors were also computed and are more readily interpreted. Table XIII presents the mean conditional percentages of phonemic and of visual confusion errors, given that an error was made on a particular letter as a function of recall condition and retention interval. An analysis of variance yielded 2% as an estimate of the standard error of the entries in Table XIII. As previous research had indicated, there were more phonemic than visual confusion errors in temporal sequence recall; but in spatial location recall, visual confusion errors were as prevalent as phonemic confusion errors. These effects were reflected in an overall main effect of confusion error type (phonemic or visual), F(1, 24) = 38.1, MS, = 2240, p < .001, and an interaction of confusion error type and recall condition (temporal sequence or spatial location), F(1, 24) = 37.0, MSe = 2240, p < .001. In each case of visual and of phonemic confusion errors for temporal sequence and spatial location recall, the conditional percentages moved in the direction of the chance level (33%, since one out of every three incorrect letters would constitute a confusion error of a given type) as the retention interval increased from 4 to 16 interpolated digits. Consequently, the main effect of retention interval was significant, F( 1, 24) = 17.8, MS, = 270, p < .001, as was the interaction of retention interval and confusion error type, F(1, 24) = 73.7, MS, = 550, p < .001, and the three-way interaction of retention interval, recall condition, and confusion error type, F(1, 24) = 75.1, MS, = 550, p < .001. Although the constant letter order did not affect temporal sequence TABLE XI11 MEAN CONDITIONAL PERCENTAGES OF PHONEMIC AND VISUALCONFUSION ERRORSBY RETENTIONINTERVAL AND RECALLCONDITION (EXPERIMENT 4) Retention interval Recall condition Temporal Phonemic Visual Spatial Phonemic Visual

4

16

69 11

40 30

37 37

34 34

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Alice F. Healy

recall, it did strongly affect spatial location recall, as in Experiment 3. More phonemic than visual confusion errors were found in spatial location recall when the letters that were phonemically similar were adjacent to each other in the constant temporal sequence (as they were in the sequences with constant order BVSX, BPSF, PVFX) , and more visual than phonemic confusion errors were found in spatial location recall when the letters that were visually similar were adjacent to each other in the constant temporal sequence (BSVX, BSPF, PFVX). This effect was reflected in an interaction of constant letter order and confusion error type, F ( 5 , 24) = 7.1, MSe = 2240, p < .001, and a three-way interaction of recall condition, constant letter order, and confusion error type, F ( 5 , 24) = 5 . 3 , MSe = 2240, p = .002. This pattern of results is evident in Table XIV , which provides mean conditional percentages of phonemic and visual confusion errors as a function of letter pair position (first or secTABLE XIV MEANCONDITIONAL PERCENTAGES OF PHONEMIC AND VISUALCONFUSION ERRORSBY LEITER PAIRPOSITION, RECALLCONDITION, AND CONSTANT LETTERORDER (EXPERIMENT 4) Confusion errors Phonemic Constant letter order BVSX

First Second BSVX First Second BPSF First Second BSPF First Second PVFX First Second PFVX First Second

Visual

Temporal

Spatial

Temporal

Spatial

56 62

40

72

19 23

15 20

65 70

24 30

12 13

47 46

54 71

35 57

14 16

20 25

45 55

28 31

20 23

39 49

35 53

30 51

24 33

18 33

38 52

11 18

24 24

35 80

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233

ond, see Section IV,B,2 for the method used to determine letter pair position), recall condition, and constant letter order. An analysis of variance yielded 3% as an estimate of the standard error of the entries in Table XIV. The finding in spatial location recall of many confusion errors involving letters in neighboring temporal positions can be understood in terms of the pattern model, which predicts a large number of interchanges of the last two letters as a result of the rapid loss of the item of pattern information representing the spatial arrangement of the last two letters. Indeed, more phonemic and visual confusion errors in spatial location recall involved the second pair of confusable letters rather than the first pair, especially when the letters in each pair were adjacent to each other in the temporal sequence. These findings are reflected in a significant main effect of letter pair position, F(1, 24) = 48.9, M S , = 660, p < .001, and significant interactions of letter pair position and recall condition, F(1, 24)= 4.7, MS, = 660, p = ,038, letter pair position and confusion error type, F(1,24) = 7.0, MS, = 270, p = .014, letter pair position, constant letter order, and confusion error type, F(5, 24) = 5.0, MS, = 270, p = .003, and letter pair position, recall condition, constant letter order and confusion error type, F ( 5 , 24) = 7.4, MS, = 270, p < .001. These effects of letter pair position may be due in part to the specific letters that enter into each pair, but the consistency of the pattern strongly suggests that the temporal positions of the letters in a pair are of major importance. 2.

Pattern Confusions

An additional analysis was conducted to test more specifically the prediction of the pattern model that in spatial location recall the correct letter pattern will be confused with the letter pattern that contains the same items of information except for the spatial arrangement of the last two letters, especially when the last two letters in the correct letter pattern do not occur in the “normal” arrangement, but from right to left (see Healy, 1978). Considering only the trials with random digit patterns, as predicted, this type of pattern confusion occurred in spatial location recall on 27 trials when the last two letters in the correct pattern were in the normal arrangement and occurred on 63 trials when the last two letters in the correct pattern were in the reverse arrangement, out of 318 trials with errors. In contrast, in temporal sequence recall, this type of pattern confusion occurred somewhat less often-on 29 trials when the last two letters in the correct pattern were in the normal arrangement and on 32 trials when they were in the reverse arrangement, out of 414 trials with errors.

Alice F. Healy

234

The frequency of these pattern confusions is especially high when compared to the chance values of 14 for spatial location recall and 18 for temporal sequence recall, which were calculated by dividing the total numbers of trials with errors by 23, the number of possible erroneous letter patterns. These results provide further support for the operation of the pattern model, especially in spatial location recall. C.

DISCUSSION

The primary goal of this experiment was to determine whether there was any evidence for visual letter coding in spatial location recall. In this experiment no more visual confusion errors were found than phonemic confusion errors in spatial location recall, and the pattern of errors observed could be explained entirely in terms of the coding of attributes of temporal-spatial patterns. It is possible that subjects in spatial location recall do not attend to features of the letters themselves, but attend exclusively to the sequence of spatial locations that include stimuli. This pattern of results is therefore consistent with the findings in Experiment 2 that pattern coding was employed even when the subjects were not shown a sequence of different letters, but were shown a single neutral stimulus letter that occurred repeatedly in different spatial locations.

VI. A.

INDEPENDENCE

OF

Conclusions

ITEM AND ORDERINFORMATION

The question of central interest in this study was whether the retention of item and order information draws on the same processing capacity. A clear negative answer to this question was provided for both temporal sequence and spatial location information. In Experiment 1, the percentages of correct responses were just as high in the item order - item condition as in the order only condition, despite the fact that the memory load for item information was considerably greater in the item + order item condition. This result indicates that the amount of to-be-remembered item information does not influence the retention of temporal sequence information. In Experiment 2, the pattern of results for the count condition was strictly analogous to that for the previous spatial location recall conditions; although essentially no item information was to be remembered in the count condition, since only a single neutral stimulus occurred in different spatial locations. This result suggests that the coding strategy used to retain spatial location information is not affected by the amount of

+

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associated item information. Experiments 3 and 4 gave further indications that spatial location retention is not affected by the quality of the to-beremembered items, because there were not consistent effects in these experiments of either the phonemic or the visual characteristics of the items. What implications do these results have for the models of short-term memory that have been proposed? These findings provide devastating evidence against Conrad’s (1965) slot theory. According to that model, order errors are a by-product of item errors and completely dependent on them. Thus, the slot model cannot accommodate the finding that order errors were just as frequent in the item order - item condition as in the item order condition of Experiment 1, even though no item errors were made in the former condition. It is also hard to reconcile this model with the finding that order errors were just as common in the count condition of Experiment 2 as in the spatial location recall condition of the earlier experiment by Healy (1978), although all items were the same in the count condition. The independence between the retention of item and order information demonstrated in these experiments is also difficult to reconcile with the models proposed by Murdock (1976), Shiffrin and Cook (1978), and Drewnowski (1980), because each of these models postulates a certain degree of interdependence between the loss of item and order information. In contrast, the augmented version of the perturbation model of Lee and Estes (1981) can accommodate these findings quite easily, since order and item errors are due to perturbations at two different levels of this system. Likewise, the more specific model proposed by Healy (1978) is consistent with the present findings because it postulates that subjects code information about the temporal-spatial pattern of letter presentations, rather than information about the letters themselves, when recalling the spatial arrangement of a sequence of letters.

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B.

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CODINGSTRATEGIES

A second question of the present study was why subjects use different coding strategies for the retention of temporal sequence and spatial location information. This question was answered by referring to the striking differences found between the count and position conditions in Experiment 2. These two conditions differed only in what the subjects said during the presentation of the to-be-remembered stimuli; in the count condition, like the previous spatial location recall conditions, the subjects’ utterances contained no useful information, whereas in the position condition, like the previous temporal sequence recall conditions, the sub-

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jects’ utterances did contain useful information. Since the count condition showed a pattern of results analogous to that in the previous spatial location recall conditions, and the position condition showed results comparable to those in the previous temporal sequence recall conditions, the conclusion was reached that what the subjects say during stimulus presentation influences the recall strategy that they use. More specifically, subjects make use of a phonemic code that reflects what they said during stimulus presentation whenever that code contains some useful information. C . MENTALREPRESENTATIONS

The present findings also have implications for a much broader issue in cognitive psychology concerning the representation of information in memory (e.g., Anderson, 1978; Kosslyn & Pomerantz, 1977). The present results suggest that the pattern code used to retain spatial location information is analogical rather than verbal. Subjects in the position condition of Experiment 2, who were required to vocalize the sequence of spatial location position numbers of the stimuli, showed less evidence of pattern coding than those in the count condition, whose vocalizations contained no useful information. This result indicates that pattern coding is not equivalent to a verbal coding of spatial position numbers, which may not be the only possible verbal code for spatial location information, but is surely the most reasonable one available in this situation. Although the present results ruled out verbal coding as a likely basis for spatial location retention, a code based on a static visual image also seems unlikely for two reasons: First, visual confusion errors were not consistently found in Experiments 3 and 4, and those that were obtained could be explained in terms of factors that were irrelevant to visual similarity per se. Second, a static visual image could not be used to retain the spatial location information in Experiment 2, since only a single stimulus item (the letter X) occurred in different spatial locations. Although a static visual image revealing the spatial arrangement of the tobe-recalled items might have provided the basis for retention in the spatial location recall conditions with letters, such an image would be a redundant string of Xs in the conditions of Experiment 2. What is the nature of the analogical pattern code used for spatial location retention? The modality in which pattern information is stored cannot be determined from the present findings. The pattern information could be stored in terms of a kinesthetic code, a visual code, or a more abstract code. Nevertheless, it is clear from the present results that this code, like that postulated by Healy (1978), makes reference to both temporal and spatial properties of the stimulus configuration.

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ACKNOWLEDGMENTS This research was supported in part by NSF Grants BNS77-00077 and BNS80-00263 to Yale University and BNS80-25020 to the Institute of Cognitive Science at the University of Colorado. The author was supported by a Senior Faculty Fellowship from Yale University during the preparation of this article. The author is indebted to William K. Estes for generously providing many helpful suggestions at numerous phases of this research, to Lorretta T. Polka for her careful help with the construction of the experimental materials and the conduct and analyses of all four experiments, to Maureen McNamara for her aid in constructing the materials used in Experiment 1, and to Mary LaRue for her aid in the data analyses of Experiment 4.

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