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Free recall of conspicuous items
JOURNAL OF VERBALLEARNING AND VERBALBEHAVIOR, 8, 448-456
(1969)
Free Recall of Conspicuous Items 1 NANCY C. WAUGH
Harvard Medical School, Boston, Massachusetts 02115
Subjects attempted to recall lists of homogeneous unrelated words, some of which had been accompanied by a signal denoting that they were to be specially attended to. The results are consistent with the idea that "isolated" items are well retained because they are held in mind for a relatively long time. The classical explanation of the yon Restorff effect--the selective retention of an item that is obviously different from its predecessors in a series--is that a novel or perceptually "isolated" stimulus produces a more distinct initial impression, and therefore a more prominent parallel in memory, than does a run-of-the-mill event (Koffka, 1935). More recently it has been suggested that unusual events may be especially memorable because they elicit strong orienting responses (Raskin, Hattie, & Rubel, 1967), which are in turn supposed to facilitate the consolidation of their stimuli's traces in memory (Kleinsmith & Kaplan, 1963). An alternative explanation is that something novel or conspicuous simply tends to be held in mind longer than something commonPlace, and is better retained for just that reason. This hypothesis, which is by no means new, is based on the rather c o m m o n observation (Wallace, 1965) that "isolated" verbal items tend to be retained at the expense of other items in a list. It seems reasonable to assume that, when some particular item in a sequence is specially attended to (or rehearsed, cf. Posner & Rossman, 1965), then some other item must necessarily be ignored, since people 1 This work was supported by Research Grant. No. MH-08119 from the National Institute of Mental Health, United States Public Health Service, to Harvard Medical School. Mary McDonald, Frank Moses, and Patricia McPhee collected the data and helped analyze them. 448
can evidently think about only one thing at a time (Broadbent, 1958). I f one can further assume that the longer an item is thought about, the likelier it is to be remembered, then it is easy to understand how items that are selectively rehearsed, perhaps because they are unusual, will also be selectively retained. An item's perceptual "isolation" may thereby be a sufficient condition for its being closely attended to and hence readily remembered; if it is not a necessary condition, then it should be possible to simulate a " v o n Restorff effect" by requiring S to pay special attention to any arbitrarily selected item in a homogeneous list. The method of free recall provides a very convenient technique for testing the rehearsaltime hypothesis described above: it is a wellestablished fact (e.g., Murdock, 1962; Waugh, 1963) that a constant number of items are recalled from a list of fixed duration regardless of how many different items that list contains. Serial-position effects are minimal, if the last few items are excluded from consideration; so that when a list is presented at a constant rate, every item (aside from the very late ones) bears a constant probability of being recalled (Waugh, 1967). When items within the same list are presented for variable lengths of time, on the other hand, the probability that any of them will be recalled increases in direct proportion to the number of seconds for which it is presented. In point of fact, it must almost certainly be
CONSPICUOUS ITEMS the case that it is n o t really a n item's d u r a t i o n as such that determines how well it will be retained b u t rather the a m o u n t of time that S actually thinks a b o u t it. I n all likelihood, therefore, increasing a n item's d u r a t i o n increases the chances that it will be recalled b y simply p r o l o n g i n g the interval d u r i n g which it c a n be rehearsed. I f this is so, then i n s t r u c t i n g S to p a y special a t t e n t i o n to certain items in a list presented at a c o n s t a n t rate should have the same effect as increasing their d u r a t i o n : it should increase his r e t e n t i o n of these items while r e d u c i n g that of the rest by a n e q u i v a l e n t a m o u n t , so that the total n u m b e r of items recalled remains the same as when n o n e is singled out for special attention. It should further be the case that as more a n d more items w i t h i n the list are made to compete for S's limited a t t e n t i o n (so that the average a m o u n t of extra time available for rehearsing each of t h e m approaches zero), one should observe a progressive decline in the p r o b a b i l i t y that a n y one of t h e m will be recalled. The predictions j u s t described were tested in Exp. I, wherein S freely recalled lists i n which certain items, otherwise similar to the rest, were a c c o m p a n i e d by a n extraneous signal that d e n o t e d that they were to be specially a t t e n d e d to. Experiments I I a n d I I I were u n d e r t a k e n in order to determine whether the r e t e n t i o n of such " c o n s p i c u o u s " items is affected b y the priority that they are to be given in recall or b y the p o i n t in time at which they are singled out for a t t e n t i o n (before or after they occur). EXPERIMENT I
Method. The material to be recalled consisted of 80 lists of 24 common English monosyllables ordered at random. Ten experimental conditions were each represented by eight lists. An experimental condition was defined by the number of words in a list that were followed by a brief high-pitched tone. This number was either 0, 1, 2, 3, 4, 6, 9, 12, 15, or 18. None of the last six words in a list was ever followed by a tone. Under Cond. 18, therefore, each of the first 18 words was signaled. Under Cond. I, the signaled word occurred in position 1, 2, 3, 4, 9, 12, 15, or 18. The words signaled under Cond. 2 occurred in positions
449
1 and 9, 1 and 18, 2 and 12, 2 and 15, 3 and 9, 3 and 15, 4 and 12, or 4 and 18. Words 1-4, 9, 12, 15, and 18 were in fact signaled an equal number of times under each of the remaining conditions (Cond. 3 through 18), although not all of them were necessarily signaled on every list (except under Cond. 18). Signaled words also occurred in positions 5-8, 10-11, and 13-17 under Conds. 3-18; they were selected unsystematically, subject to the restrictions that obvious patterns of signals be avoided and that signals be distributed more or less uniformly throughout positions 5-18. Each condition was represented exactly once within each successive block of 10 lists. Otherwise the lists were ordered unsystematically. The Ss were given standard free recall instructions: they were to listen carefully to each list as it was presented and were then to write down as many words as they could remember from that list in any order they chose during the recall period that immediately followed. They were also told to attend "especially to the words followed by a beep" with the intent of retaining them. Each recall interval lasted approximately 2 rain., and every tenth trial within a session was followed by a 5-minute break. Two groups of 10 Ss served in the experiment. Each group listened to one of two sets of 80 lists, denoted as A and B. The two sets consisted of exactly the same lists of words arranged in exactly the same order. Any given list, however, represented a different experimental condition within either set; so that a list in which no words were signaled in Set A might, for instance, have contained nine signaled words in Set B. Both groups of Ss therefore heard the same series of words; but they underwent the experimental conditions in a different order. Lists were assigned at random to the experimental conditions, subject to the restrictions described above. Forty of the eighty lists were presented to a given group in each of two experimental sessions. Half the Ss listened to lists 1-40 in the first session and to lists 41-80 in the second, while the others listened first to lists 41-80 and then to lists 1-40. The lists were recorded onto magnetic tape by a male speaker at a rate of one word per second and were later presented to the Ss over a loudspeaker. The highpitched tone that followed the signaled words was of sufficient intensity and duration so as to be perfectly audible, although in no case did it overlap with either a signaled word or its successor. The Ss, paid volunteers, were students at Tufts University.
Results. The data are s u m m a r i z e d i n Figure 1, which shows the relative frequency with which signaled a n d u n s i g n a l e d words in positions 1-18 were recalled as a f u n c t i o n of
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the number signaled. Also shown in this figure is the total proportion of words recalled under every condition. Each of the latter points is based on 160 observations. The results are consistent with the rehearsal-time hypothesis: when only one of the first 18 words in a list is signaled (Cond. 1), it is more than twice as likely to be recalled as an unsignaled word. The proportion of signaled words recalled thereupon decreases monotonically with the number signaled, so that under Cond. 18 (words 1 through 18 each followed by a signal) the proportion recalled is the same as when no word is signaled (Cond.
o° f
•
SIGNALED
o
NOT S I G N A L E D
• TOTAL
o, •'. g
0 / 0
2
QO
0
I 4
1 6
0
I 8
0
I I "10 .12
0
[ 14
I t6
I 18
NUMBER O F WORDS SI6NALED
FIG. 1. Proportion of signaled and unsignaled words recalled, along with the total proportion recalled, as a function of the number signaled (Exp. 1). 0). The proportion of words not signaled but later recalled exhibits the expected compensatory decrease from Cond. 0 to Cond. 15, such that the total proportion of words recalled--signaled words plus unsignaled ones --remains effectively constant. Certain investigators of the yon Restorff effect (e.g., Jenkins & Postman, 1948) have suggested that "isolated" items in a list are retained at the expense of those that immediately follow them. In the present study, one would expect this negative spread of effect to be most evident under Cond. 1, where retention of a signaled word was found to be maximal. In fact, however, the relative frequency of recall for an unsignaled word immediately following a signaled one was .22 under that condition, while all other unsignaled words were recalled with a relative frequency of .21.
Under Cond. 2, the corresponding proportions were .23 and .21; and under Cond. 4, they were. 19 a n d . 19. The differences between these proportions are obviously negligible, for all practical purposes. It may therefore be concluded that signaling an item did not to any appreciable extent affect the probability that an unsignaled item immediately following it would be recalled. It is still possible, of course--and in fact even likely--that a signaled item is selectively re-rehearsed at some arbitrary point in time after it occurs, such that the item that immediately follows it is no likelier to be ignored than some later item. If signaled items were to compete with one another for rehearsal or re-rehearsal, moreover, then those that occur early in a list should enjoy an advantage over later ones, since their total chances of being rehearsed would be greater. This inference is supported by the serial position effects observed in this experiment, which indicate that items signaled early in a list are indeed likelier to be recalled than items signaled later within the same list. Thus, under Cond. 0 (no items signaled) words 1-9 and 10-18 were recalled 21 ~ and 25 ~ of the time, respectively. Under Cond. 18 (every word signaled), on the other hand, the corresponding percentages were 25 and 19. The point is perhaps more obvious in Figure 2, where the relative frequencies with which words 1, 2, 3, and 4 and words 9, 12, 15, and 18 were recalled, given that they were signaled, are plotted as a function of the total number of items signaled. (Words in those particular positions were chosen for analysis because they were signaled an equal number of times under every condition.) It is quite evident in Figure 2 .that signaled words occurring early in a list are generally recalled at the expense of signaled words occurring later, even though early words enjoy no special advantage over later ones when none of them are signaled (so that only a negligible primacy effect may be observed under Cond. 0). This result may mean, as was suggested above, that early items
451
C O N S P I C U O U S ITEMS
0.6
~ 1~ 0 . 4
o
•
0
0 0
I 2
I 4
I 6
8
I t0
I 12
I t4
•
•
0
0
I 16
being recalled, at least as long as the n u m b e r signaled did not exceed the n u m b e r recallable. U n d e r these circumstances, a signaled w o r d should be recalled with a very high probability under Cond. 1, since it should always be recalled first. The purpose o f Exp. I t was to test this hypothesis.
I 18
NUMBER OF WORDS S/GNALED
FIG. 2. Relative frequency of recall for words 1--4 (solid circles) and for words 9, 12, 15, and 18 (open circles), given that they were signaled, as a function of the total number signaled (Exp. I). The two leftmost points (solid and open triangle) denote the probability that words in those positions (1--4 or 9, 12, 15, and 18, respectively) would be recalled under Cond. 0. are selectively retained in S's limited m e m o r y store. It could, however, also m e a n that they are selectively retrieved f r o m there at the time o f recall. Inspection o f the Ss' answer sheets did in fact reveal that early signaled items tend to be recalled somewhat sooner than late ones. Thus, under Cond. 18, words 1-9 were recalled in position 7.31, on the average, while words 10-18 were recalled in position 8.07. It is conceivable that the former were recalled m o r e frequently than the latter simply because they were likelier to be selected for early recall. The existence o f a causal relation between priority a n d probability o f recall could perhaps answer one question that has been raised by the data: W h y is it that Ss can recall a signaled item no more than a b o u t half the time, even when it is the only s u c h item to have occurred in a list? The average serial position in recall for a signaled item under Cond. 1 was 5.51, so that S was writing d o w n 4 to 5 other words, on the average, before attempting to recall that item. I f deferring the retrieval o f an item increases the chances that it will be f o r g o t t e n - - a s it would, for instance, if only a fixed n u m b e r o f items could be freely recalled f r o m any given set that was stored in m e m o r y - - t h e n instructing S to write d o w n signaled items before unsignaled ones should greatly enhance the former's likelihood o f
EXPERIMENT I I Method. The Ss heard exactly the same set of lists as did one of the groups in Exp. I. The same general procedure was followed except that now the Ss were instructed not only to "pay special attention to every word followed by a beep" but also to "write these words down first" so as to recall as many of them as possible. The Ss were ten students at Tufts Summer School. They were run in two groups of five and were paid for their services. The data for one S, who apparently failed to understand the instructions, were subsequently discarded and replaced with data obtained from a new subject, who was therefore run individually. Also discarded were the data obtained from all ten Ss for the last list presented under Cond. 18, since a disturbance in the experimental room during the initial presentation of that list resulted in a sizable reduction in the number of items recalled. Results. Figure 3 shows the p r o p o r t i o n o f signaled and unsignaled words in positions 1-18 that were recalled, as well as the total p r o p o r t i o n recalled, as a function o f the n u m b e r that were signaled. As in Exp. I, an approximately constant n u m b e r o f words, signaled plus unsignaled, were recalled under every condition. A c o m p a r i s o n o f Figure 3 with Figure 1 reveals that instructions to recall • SIGNALED o NOT SIGNALED
0.6 oooO
• TOTAL
~_.R= 0 . 4
2
~000
0
I 2
0 I 4
~1 6
w 0
0 I 8
I 10
[ 12
0 0 I ~4
•
I 16
I t8
NUMBER OF WORDS SIGNALED
FIO. 3. Proportion ofsignaledandunsignaledwords recalled, along with the total proportion recalled, as a function of the number signaled (Exp. II).
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signaled items first increased the proportion of signaled items recalled under Conds. 2 through 6, but not under Cond. 9 through 18 or under Cond. 1. A progressive compensatory decrease in the recall of unsignaled words is very evident as the number of signaled words increases from 1 to 15. Again it is clear that the data support the rehearsal-time hypothesis. Figure 4 shows the relative frequency with which words 1M and words 9, 12, 15, and 18 were recalled as a function of the number signaled. Here it is evident that, as in Exp. I, early signaled words are likelier to be recalled than later ones. A comparison of Figure 4 with Figure 2 reveals that in Exp. II the early words enjoyed an even greater advantage over the late ones than in Exp. I. Words 1-9 were recalled 20 and 21 ~ of the time under Cond. 0 and 18, respectively; for words 10-18, the corresponding percentages were 17 and 10. Instructions to recall signaled words first therefore seems to depress the retention of words signaled late in a list. It may be, of course, that under Cond.
o•••
•
0.4
o o 0
Ms = . 5 ( N + 1)P(N); while for Hypothesis E, Ms = .5(ns + 1)P(ns),
0.6
~
a few words were signaled and rather more difficult when several were signaled. Nevertheless, the relation between an item's serial position in recall and its probability of being recalled at all is ambiguous at best, according to the data presented in Table 1. Here mean position in recall is shown for signaled items under Cond. 1-4, 9, and 18 in both Exps. I and II. (Only half of the data collected in Exp. I--those obtained from Ss who listened to exactly the same lists as did the Ss in Exp. II --are represented there.) The figures in italics denote an item's mean serial position in recall expected according to either of two extreme hypotheses. The first is that a signaled word is as likely to be recalled in one position as in any other (Hypothesis R, for random recall); and the second is that signaled words are always recalled before unsignaled ones (Hypothesis E, for early recall). The expected values were calculated for each condition as follows. For Hypothesis R,
0
I ] 1 1 1 2 4 6 8
1 10
NUMBER OF WORDS
o
I l 12 14 16
o I 18
S/GNALED
FIG. 4. Relative frequency of recall for words 1-4 (solid circles), and for words 9, 12, 15, a n d 18 (open
circles), given that they were signaled,as a function of the total number signaled (Exp. II). The two leftmost points (solid and open triangle) denote the probability that words in those positions (1-4 or 9, 12, 15, and 18, respectively) would be recalled under Cond. 0. 18 the Ss attempted to store the lists serially, thereby employing a rehearsal strategy that would favor the retention of early items. The Ss' instructions to recall signaled words early were evidently easy to follow when only
where Ms denotes the mean serial position in recall expected for a signaled word; P(N) denotes the probability that a total of N words (signaled plus unsignaled) were recalled, given that at least one signaled word was recalled; and P(n~)denotes the probability that ns signaled words were recalled, given that at least one was recalled. It is evident that, in Exp. I, signaled words were as likely to be recalled late as early under all conditions: all the data are much more consistent with Hypothesis R than with Hypothesis E. In Exp. II, on the other hand, the data are more consistent with Hypothesis E when only a few words are signaled and with Hypothesis R when many are signaled. A comparison of Table 1 with Figures 1 and 3 enables one to make the following statements. (1) Under Cond. 1, signaled words were equally likely to be recalled in both experiments, even though they were emitted early in
453
CONSPICUOUS ITEMS TABLE 1 MEAN SERIAL POSITION IN RECALL FOR SIGNALED WORDS AS A FUNCTION OF THE NUMBER SIGNALED Exp. I
Exp. II
Cond.
Hyp. E
Obs.
Hyp. R
1 2 4 9 18
1.00" 1.21 1.51 2.24 2.56
5.21 5.24 5.67 6.91 7.69
5.62 5.73 5.60 5.64 5.61
Hyp. E
Obs.
Hyp. R
1.00 1.23 1.79 2.00 1.85
1.58 1.74 2.78 3.80 3.71
4.38 4.44 4.24 4.43 3.98
a Note.--Entries in italics denote the values expected on the basis of Hyp. E (signaled words recalled before any of the rest) and Hyp. R (signaled words as likely to be recalled late as early).
Exp. II and late in Exp. I (no apparent relation, in other words, between priority and frequency of recall). (2) In Exp. I, signaled words were much less likely to be recalled under Cond. 4 than under Cond. 1, even though their average position in recall was virtually identical under both conditions (again no obvious relation between priority and frequency of recall); whereas in Exp. II they were recalled both earlier and more frequently under Cond. 4 than they were in Exp. I under that condition (a possible positive relation, therefore, between priority and frequency). (3) Finally, under Cond. 18, words were recalled somewhat later in Exp. I than one would expect on the basis of chance (Hypothesis R) and somewhat earlier in Exp. II; but they were nevertheless recalled somewhat more frequently in Exp. I than in Exp. II under that condition (a possible negative relation, therefore, between priority and frequency). This analysis suggests, then, that probably some factor other than the instructions to recall signaled words early in Exp. II was responsible for the disparity between the data shown in Figures 1 and 3. The nature of this factor is not immediately obvious. It is still difficult to understand, moreover, why one signaled word should be recalled with a probability of only approximately .50 when S has been instructed to pay special attention
to it and to recall it first. Two possible explanations are as follows. First, it may be that S must fully attend to a word with the intent of retaining it at the time it occurs, and not shortly thereafter, if he is to store and retain it with a high degree of certainty; perhaps some kind of arousal mechanism must be activated shortly before or simultaneously with the presentation of an item and not shortly thereafter (Berlyne et al., 1966), if the item is to be securely retained. Or perhaps a given word may sometimes be lost from primary memory (Waugh & Norman, 1965), and hence be unavailable for storage in secondary memory, by the time that S has taken cognizance of the signal that follows it. In either event, a signaled word should be better retained if it is preceded rather than followed by its signal. Alternatively, it might be the case that signaled words were recalled only half the time under Cond. 1 simply because the Ss were attempting to retain other words (both earlier and later ones) as well, even though they were not signaled, and in doing so may have failed to store the signaled item or may have lost access to it once it was stored. If this were the case, then instructing S to retain only the items that were signaled should increase their likelihood of being recalled, since only those words would be attended to. When only one item in a list is signaled under those circumstances, S
454
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should be able to hold it i n m i n d (or recirculate it i n p r i m a r y m e m o r y ) for the r e m a i n d e r of the list a n d thus be able to recall it with p r o b a b i lity 1.00. This hypothesis, of course, does n o t exclude the one j u s t described. It could be that b o t h instructing S to retain only those items that are signaled a n d signaling them before they occur are necessary to p r o d u c e perfect r e t e n t i o n of one signaled word. E x p e r i m e n t I I I was designed to test b o t h hypotheses. EXPERIMENT I I I
Method. The material to be recalled Consisted of 100 lists of 20 unrelated monosyllables, no one of which occurred in more than one list. Within each list, either 1, 2, 4, 7, or l l words were signaled. Under Cond. 1, a signal followed either word 3, 4. . . . 2, 9, 10, ... or 13. Various combinations of words were signaled under Conds. 2, 4, and 7, such that words in positions 3-7 and 9-13 were followed by signals an equal number of times over the course of the experiment. Obvious patterns of signals were avoided insofar as possible under Conds. 2-7. Under Cond. 11, every word from 3-13 was followed by a signal. A total of 10 Ss participated in two experimental sessions. All heard the same lists in the same order, 50 lists per session. Five of the Ss were instructed in Session 1 to recall only those words that had been followed by a signal, while in Session 2 they were to recall only those words preceded by a signal. The other five Ss received these instructions in the opposite order. Thus each list was recalled by five Ss. who had been instructed to obey a "retroactive" signal to attend to a given word and by five obeying a "proa~tive" signal. The serial positions of the words to be recalled ranged from 3 to 13 under the "retroactive" condition and from 4 to 14 under the "proactive" condition. The Ss, all students at Tufts Summer School, were paid for their services. They were told that one or more words would be signaled (either proactively or retroactively) within each list, that they should attend to these signaled words and only to them, and that they should recall them (and none of the others) in any order they chose after the last word had been read. They were given no explicit information as to the number of signals that would occur in any of the lists nor as to their location. The lists were recorded onto magnetic tape by a female speaker at a rate of one word per see eo'adwere later presented to the Ss over a loudspeaker. The signal, as before, consisted of a brief high-pitched tone embedded between two words but overlapping neither. Every experimental condition was represented exactly
once within each successive block of five lists. Otherwise the conditions were ordered unsystematically. The length of the recall interval varied with the number of words to be recalled, ranging from about 10 see under Cond. 1 to about 2 min. under Cond. 11. Every tenth list within a session was followed by a 5-minute rest period. The Ss wrote their responses in booklets, using a new page for each list.
Results. The data are s u m m a r i z e d i n Figure 5, where the p r o p o r t i o n of (signaled) items recalled is plotted as a f u n c t i o n of the total n u m b e r signaled. It is evident that this p r o p o r t i o n decreases progressively f r o m a value that is very close to 1.00 w h e n only one or two words are signaled to a value of approximately .35 w h e n 11 words (out of 20) are signaled. This result is consistent with those of Exps. I a n d I I : the greater the n u m b e r of signaled words c o m p e t i n g for S's a t t e n t i o n the smaller "the chances that a n y one of them will be recalled. It is also quite obvious i n Figure 5 that the location of the signal makes n o practical difference whatsoever: r e t e n t i o n is equally good when the signal t o retain a w o r d follows it as w h e n it precedes it. A n analysis of serial-position effects reveals a n even greater disparity between the recall of early a n d late signaled words t h a n was seen in Exps. I a n d II. Figure 6 shows the p r o p o r tion of items that were signaled (both "proactively" a n d "retroactively") a n d later • PROACTIVE SIGNAL
1,0
S I G N A L 0.8 0.6 0.4 Q.
0.2 0
0
I
I
2
I
I
4
I
I
6
I
I
8
I
I
I
I
t0 ,~,12
NUMBER OF WORDS SIGNALED
FIG. 5. Proportion of pro- and retroactively signaled words recalled as a function of the total number signaled (Exp. III).
CONSPICUOUS ITEMS
455
recalled as a function of their serial position in Exp. II even when they intend to recall within the signaled set under Cond. 11; the signaled words before any of the rest. horizontal line indicates the total proportion recalled. The strong primacy effect evident in DISCUSSION Figure 6 suggests that the Ss may have been The main results of the present study may attempting to store the items in serial order, and inspection of the Ss' answer sheets be summarized as follows. 1. Instructing a subject to "pay special indicated that an item's order in recall was in fact positively correlated with its order in attention" to certain arbitrarily designated items in a list makes him much likelier to recall presentation. Interestingly enough, the probability that them than he would otherwise, as long as only an unsignaled word would be recalled, a few items are so designated. When many estimated across all conditions, was approxi- items are signaled, however, the probability mately .01, while the probability that a word that any one of them will be retained apthat was recalled was an unsignaled one was proaches the proportion of items recalled approximately .04. The proportion of "false when none is signaled. Signaled items are positives," moreover, did not vary systemati- retained at the expense of unsignaled ones, cally with the number of words to be recalled. such that the total number recalled remains Evidently, then, the Ss were able to discrimi- constant regardless of how many are signaled. nate very accurately between the words which These facts are entirely consistent with the had been signaled and those which had not. hypothesis, previously suggested elsewhere Unfortunately there is no way of estimating (Waugh, 1967), that only a fixed number of the analogous proportion of "false positive" words can be freely recalled from a list of responses occurring in Exps. I and II, since fixed duration, with the likelihood that any the appropriate data were not collected (i.e., given word will be recalled increasing in direct proportion to the time allotted to it. "Special attention" may accordingly be taken to mean protracted attention. 2. The role of order in recallis not entirely . clear: a specially-attended-to item, when it is 0.4 .. the only such item to occur in a list, is not 0.2 °Oo remembered any more frequently when S is I I I L I~ instructed to recall it first (and does in fact 0 2 4 6 8 t 0 12 recall it first, if he recalls it at all) than when SERIAL POSI T/ON FIG. 6. Relative frequency of recall for a signaled he has been given no special instruction (and word as a function of its serial position (Exp. III, is therefore as likely to recall the item very late as very early). When 2, 3, 4, or 6 signaled Cond. 11). items (in a list of 24) are to be recalled before the Ss were not asked to indicate whether or any of the rest, on the other hand, they are not they thought each word that they recalled recalled more frequently than they would be had been signaled). Inspection of Table 1 otherwise. It is possible that under these suggests, however, that when Ss have the circumstances signaled items are stored as one option of storing a set of unsignalcd words cluster, and are hence more easily retrievable along with a set of signaled ones, they may lose than if they were stored independently of one their ability to discriminate between the two another (Tulving & Patterson, 1968). The at the time of recall, and may therefore maximum size which such a cluster can attain erroneously defer the recall of a signaled word is apparently limited to about four items.
0.f6[
456
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3. It makes no difference whether the signal to attend to a given word is presented just before or just after the word itself. Apparently a retroactive set to retain an item is just as effective as a Pr0active o n e - - a t least, one would have to suppose, as long as the item was still available in primary memory (Waugh & Norman, 1965). 4. One particular word can be freely recalled from a supraspan list with a probability approaching 1.00 when S has been instructed to retain only that word, but not when he has the option of retaining others as well. This fact suggests that diverting one's attention from an item, perhaps even very briefly, can result in its becoming irretrievable no matter how carefully it has been stored. (For discussions of the distinction between storage and retrieval in free recall, see Tulving & Pearlstone, 1966; Waugh, 1967, Slamecka, 1968.) Obviously these results are not incompatible with an "arousal" hypothesis of selective retention: it could be that stimuli which produce a strong orienting reflex, perhaps because they are especially novel or especially significant, will also tend to be selected for attention at the expense of trivial or inconspicuous stimuli. It should be equally clear that the present results cannot be taken as direct evidence for the hypothesis that the von Restorff effect reflects the selective rehearsal of such items rather than their inherent perceptual discriminability. These results do mean, however, that an item's novelty or significance as such are not necessary for its selective retention. They m a y be sufficient, although perhaps only insofar as they serve to fix S's attention on the item. In order to show that conspicuous items tend to be well retained in conventional tests of the von Restorff effect simply because they tend to be well rehearsed, one would have to perform an experiment analogous to Exp. I, but with items that were isolated from their neighbors
in a list by virtue of some intrinsic property of meaning or appearance rather than through their association with an arbitrary signal. REFERENCES
BERLYNE,D. E., BORSA,D. M., HAMACHER,J. H., & KOENIG,I. D. V. Paired-associate learning and the timing of arousal. Journal of Experimental Psychology, 1966, 72, 1-6. BROADBENT,D. E. Perception and communication. New York: Pergamon, 1958. JENKINS, W. O., & POSTMAN,L. Isolation and the spread of effectin serial learning. AmericanJournal of Psychology, 1948, 61,214-221. KLEINSMITH,L. J., & KAVLAN,.S. Paired-associate learning as a function of arousal and interpolated interval. Journal of Experimental Psychology, 1963, 65, 190-193. KOFFKA, K. Principles of gestalt psychology. New York: Harcourt, Brace, 1935. MURDOCK,B. B., JR. The serial position effect of free recall. Journal of Experimental Psychology, 1962, 64, 482-488. POSNER,M. I., & ROSSMAN,E. Effect of size and location of informational transforms upon short-term retention. Journal of Experimental Psychology, 1965, 70, 496-505. RASKIN,D. C., HATTLE,M., & RUEEL,E. W. The effects of electric shock isolation in serial learning. Psychonomic Science, 1967, 8, 413-414. SLAMECKA,N. An examination of trace storage in free recall. Journal of Experimental Psychology, 1968, 76, 504-513. TULVING,E., & PATTERSON,R. D. Functional units and
retrieval processes in free recall. Journal of Experimental Psychology, 1968, 77, 239-248. TULVING, E., & PEARLSTONE,Z. Availability versus
accessibility of information in memory for words. Journal of Verbal Learning and Verbal Behavior, 1966, 5, 381"-391. WALLACE,W. P. Review of the historical, empirical, and theoretical status of the yon Restorff phenomenon. Psychological Bulletin, 1965, 63, 410-424. WAUG~, N. C. Immediate memory as a function of repetition. Journal of Verbal Learning and Verbal Behavior, 1963, 2, 107-112. WAUGH, N. C. Presentation time and free recall. Journal of Experimental Psychology, 1967, 73, 39-44. WAUGH, N. C., • NORMAN,D. A. Primary memory.
Psychological Review, 1965, 72, 89-104. (Received December 6, 1968)
(1969)
Free Recall of Conspicuous Items 1 NANCY C. WAUGH
Harvard Medical School, Boston, Massachusetts 02115
Subjects attempted to recall lists of homogeneous unrelated words, some of which had been accompanied by a signal denoting that they were to be specially attended to. The results are consistent with the idea that "isolated" items are well retained because they are held in mind for a relatively long time. The classical explanation of the yon Restorff effect--the selective retention of an item that is obviously different from its predecessors in a series--is that a novel or perceptually "isolated" stimulus produces a more distinct initial impression, and therefore a more prominent parallel in memory, than does a run-of-the-mill event (Koffka, 1935). More recently it has been suggested that unusual events may be especially memorable because they elicit strong orienting responses (Raskin, Hattie, & Rubel, 1967), which are in turn supposed to facilitate the consolidation of their stimuli's traces in memory (Kleinsmith & Kaplan, 1963). An alternative explanation is that something novel or conspicuous simply tends to be held in mind longer than something commonPlace, and is better retained for just that reason. This hypothesis, which is by no means new, is based on the rather c o m m o n observation (Wallace, 1965) that "isolated" verbal items tend to be retained at the expense of other items in a list. It seems reasonable to assume that, when some particular item in a sequence is specially attended to (or rehearsed, cf. Posner & Rossman, 1965), then some other item must necessarily be ignored, since people 1 This work was supported by Research Grant. No. MH-08119 from the National Institute of Mental Health, United States Public Health Service, to Harvard Medical School. Mary McDonald, Frank Moses, and Patricia McPhee collected the data and helped analyze them. 448
can evidently think about only one thing at a time (Broadbent, 1958). I f one can further assume that the longer an item is thought about, the likelier it is to be remembered, then it is easy to understand how items that are selectively rehearsed, perhaps because they are unusual, will also be selectively retained. An item's perceptual "isolation" may thereby be a sufficient condition for its being closely attended to and hence readily remembered; if it is not a necessary condition, then it should be possible to simulate a " v o n Restorff effect" by requiring S to pay special attention to any arbitrarily selected item in a homogeneous list. The method of free recall provides a very convenient technique for testing the rehearsaltime hypothesis described above: it is a wellestablished fact (e.g., Murdock, 1962; Waugh, 1963) that a constant number of items are recalled from a list of fixed duration regardless of how many different items that list contains. Serial-position effects are minimal, if the last few items are excluded from consideration; so that when a list is presented at a constant rate, every item (aside from the very late ones) bears a constant probability of being recalled (Waugh, 1967). When items within the same list are presented for variable lengths of time, on the other hand, the probability that any of them will be recalled increases in direct proportion to the number of seconds for which it is presented. In point of fact, it must almost certainly be
CONSPICUOUS ITEMS the case that it is n o t really a n item's d u r a t i o n as such that determines how well it will be retained b u t rather the a m o u n t of time that S actually thinks a b o u t it. I n all likelihood, therefore, increasing a n item's d u r a t i o n increases the chances that it will be recalled b y simply p r o l o n g i n g the interval d u r i n g which it c a n be rehearsed. I f this is so, then i n s t r u c t i n g S to p a y special a t t e n t i o n to certain items in a list presented at a c o n s t a n t rate should have the same effect as increasing their d u r a t i o n : it should increase his r e t e n t i o n of these items while r e d u c i n g that of the rest by a n e q u i v a l e n t a m o u n t , so that the total n u m b e r of items recalled remains the same as when n o n e is singled out for special attention. It should further be the case that as more a n d more items w i t h i n the list are made to compete for S's limited a t t e n t i o n (so that the average a m o u n t of extra time available for rehearsing each of t h e m approaches zero), one should observe a progressive decline in the p r o b a b i l i t y that a n y one of t h e m will be recalled. The predictions j u s t described were tested in Exp. I, wherein S freely recalled lists i n which certain items, otherwise similar to the rest, were a c c o m p a n i e d by a n extraneous signal that d e n o t e d that they were to be specially a t t e n d e d to. Experiments I I a n d I I I were u n d e r t a k e n in order to determine whether the r e t e n t i o n of such " c o n s p i c u o u s " items is affected b y the priority that they are to be given in recall or b y the p o i n t in time at which they are singled out for a t t e n t i o n (before or after they occur). EXPERIMENT I
Method. The material to be recalled consisted of 80 lists of 24 common English monosyllables ordered at random. Ten experimental conditions were each represented by eight lists. An experimental condition was defined by the number of words in a list that were followed by a brief high-pitched tone. This number was either 0, 1, 2, 3, 4, 6, 9, 12, 15, or 18. None of the last six words in a list was ever followed by a tone. Under Cond. 18, therefore, each of the first 18 words was signaled. Under Cond. I, the signaled word occurred in position 1, 2, 3, 4, 9, 12, 15, or 18. The words signaled under Cond. 2 occurred in positions
449
1 and 9, 1 and 18, 2 and 12, 2 and 15, 3 and 9, 3 and 15, 4 and 12, or 4 and 18. Words 1-4, 9, 12, 15, and 18 were in fact signaled an equal number of times under each of the remaining conditions (Cond. 3 through 18), although not all of them were necessarily signaled on every list (except under Cond. 18). Signaled words also occurred in positions 5-8, 10-11, and 13-17 under Conds. 3-18; they were selected unsystematically, subject to the restrictions that obvious patterns of signals be avoided and that signals be distributed more or less uniformly throughout positions 5-18. Each condition was represented exactly once within each successive block of 10 lists. Otherwise the lists were ordered unsystematically. The Ss were given standard free recall instructions: they were to listen carefully to each list as it was presented and were then to write down as many words as they could remember from that list in any order they chose during the recall period that immediately followed. They were also told to attend "especially to the words followed by a beep" with the intent of retaining them. Each recall interval lasted approximately 2 rain., and every tenth trial within a session was followed by a 5-minute break. Two groups of 10 Ss served in the experiment. Each group listened to one of two sets of 80 lists, denoted as A and B. The two sets consisted of exactly the same lists of words arranged in exactly the same order. Any given list, however, represented a different experimental condition within either set; so that a list in which no words were signaled in Set A might, for instance, have contained nine signaled words in Set B. Both groups of Ss therefore heard the same series of words; but they underwent the experimental conditions in a different order. Lists were assigned at random to the experimental conditions, subject to the restrictions described above. Forty of the eighty lists were presented to a given group in each of two experimental sessions. Half the Ss listened to lists 1-40 in the first session and to lists 41-80 in the second, while the others listened first to lists 41-80 and then to lists 1-40. The lists were recorded onto magnetic tape by a male speaker at a rate of one word per second and were later presented to the Ss over a loudspeaker. The highpitched tone that followed the signaled words was of sufficient intensity and duration so as to be perfectly audible, although in no case did it overlap with either a signaled word or its successor. The Ss, paid volunteers, were students at Tufts University.
Results. The data are s u m m a r i z e d i n Figure 1, which shows the relative frequency with which signaled a n d u n s i g n a l e d words in positions 1-18 were recalled as a f u n c t i o n of
450
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the number signaled. Also shown in this figure is the total proportion of words recalled under every condition. Each of the latter points is based on 160 observations. The results are consistent with the rehearsal-time hypothesis: when only one of the first 18 words in a list is signaled (Cond. 1), it is more than twice as likely to be recalled as an unsignaled word. The proportion of signaled words recalled thereupon decreases monotonically with the number signaled, so that under Cond. 18 (words 1 through 18 each followed by a signal) the proportion recalled is the same as when no word is signaled (Cond.
o° f
•
SIGNALED
o
NOT S I G N A L E D
• TOTAL
o, •'. g
0 / 0
2
QO
0
I 4
1 6
0
I 8
0
I I "10 .12
0
[ 14
I t6
I 18
NUMBER O F WORDS SI6NALED
FIG. 1. Proportion of signaled and unsignaled words recalled, along with the total proportion recalled, as a function of the number signaled (Exp. 1). 0). The proportion of words not signaled but later recalled exhibits the expected compensatory decrease from Cond. 0 to Cond. 15, such that the total proportion of words recalled--signaled words plus unsignaled ones --remains effectively constant. Certain investigators of the yon Restorff effect (e.g., Jenkins & Postman, 1948) have suggested that "isolated" items in a list are retained at the expense of those that immediately follow them. In the present study, one would expect this negative spread of effect to be most evident under Cond. 1, where retention of a signaled word was found to be maximal. In fact, however, the relative frequency of recall for an unsignaled word immediately following a signaled one was .22 under that condition, while all other unsignaled words were recalled with a relative frequency of .21.
Under Cond. 2, the corresponding proportions were .23 and .21; and under Cond. 4, they were. 19 a n d . 19. The differences between these proportions are obviously negligible, for all practical purposes. It may therefore be concluded that signaling an item did not to any appreciable extent affect the probability that an unsignaled item immediately following it would be recalled. It is still possible, of course--and in fact even likely--that a signaled item is selectively re-rehearsed at some arbitrary point in time after it occurs, such that the item that immediately follows it is no likelier to be ignored than some later item. If signaled items were to compete with one another for rehearsal or re-rehearsal, moreover, then those that occur early in a list should enjoy an advantage over later ones, since their total chances of being rehearsed would be greater. This inference is supported by the serial position effects observed in this experiment, which indicate that items signaled early in a list are indeed likelier to be recalled than items signaled later within the same list. Thus, under Cond. 0 (no items signaled) words 1-9 and 10-18 were recalled 21 ~ and 25 ~ of the time, respectively. Under Cond. 18 (every word signaled), on the other hand, the corresponding percentages were 25 and 19. The point is perhaps more obvious in Figure 2, where the relative frequencies with which words 1, 2, 3, and 4 and words 9, 12, 15, and 18 were recalled, given that they were signaled, are plotted as a function of the total number of items signaled. (Words in those particular positions were chosen for analysis because they were signaled an equal number of times under every condition.) It is quite evident in Figure 2 .that signaled words occurring early in a list are generally recalled at the expense of signaled words occurring later, even though early words enjoy no special advantage over later ones when none of them are signaled (so that only a negligible primacy effect may be observed under Cond. 0). This result may mean, as was suggested above, that early items
451
C O N S P I C U O U S ITEMS
0.6
~ 1~ 0 . 4
o
•
0
0 0
I 2
I 4
I 6
8
I t0
I 12
I t4
•
•
0
0
I 16
being recalled, at least as long as the n u m b e r signaled did not exceed the n u m b e r recallable. U n d e r these circumstances, a signaled w o r d should be recalled with a very high probability under Cond. 1, since it should always be recalled first. The purpose o f Exp. I t was to test this hypothesis.
I 18
NUMBER OF WORDS S/GNALED
FIG. 2. Relative frequency of recall for words 1--4 (solid circles) and for words 9, 12, 15, and 18 (open circles), given that they were signaled, as a function of the total number signaled (Exp. I). The two leftmost points (solid and open triangle) denote the probability that words in those positions (1--4 or 9, 12, 15, and 18, respectively) would be recalled under Cond. 0. are selectively retained in S's limited m e m o r y store. It could, however, also m e a n that they are selectively retrieved f r o m there at the time o f recall. Inspection o f the Ss' answer sheets did in fact reveal that early signaled items tend to be recalled somewhat sooner than late ones. Thus, under Cond. 18, words 1-9 were recalled in position 7.31, on the average, while words 10-18 were recalled in position 8.07. It is conceivable that the former were recalled m o r e frequently than the latter simply because they were likelier to be selected for early recall. The existence o f a causal relation between priority a n d probability o f recall could perhaps answer one question that has been raised by the data: W h y is it that Ss can recall a signaled item no more than a b o u t half the time, even when it is the only s u c h item to have occurred in a list? The average serial position in recall for a signaled item under Cond. 1 was 5.51, so that S was writing d o w n 4 to 5 other words, on the average, before attempting to recall that item. I f deferring the retrieval o f an item increases the chances that it will be f o r g o t t e n - - a s it would, for instance, if only a fixed n u m b e r o f items could be freely recalled f r o m any given set that was stored in m e m o r y - - t h e n instructing S to write d o w n signaled items before unsignaled ones should greatly enhance the former's likelihood o f
EXPERIMENT I I Method. The Ss heard exactly the same set of lists as did one of the groups in Exp. I. The same general procedure was followed except that now the Ss were instructed not only to "pay special attention to every word followed by a beep" but also to "write these words down first" so as to recall as many of them as possible. The Ss were ten students at Tufts Summer School. They were run in two groups of five and were paid for their services. The data for one S, who apparently failed to understand the instructions, were subsequently discarded and replaced with data obtained from a new subject, who was therefore run individually. Also discarded were the data obtained from all ten Ss for the last list presented under Cond. 18, since a disturbance in the experimental room during the initial presentation of that list resulted in a sizable reduction in the number of items recalled. Results. Figure 3 shows the p r o p o r t i o n o f signaled and unsignaled words in positions 1-18 that were recalled, as well as the total p r o p o r t i o n recalled, as a function o f the n u m b e r that were signaled. As in Exp. I, an approximately constant n u m b e r o f words, signaled plus unsignaled, were recalled under every condition. A c o m p a r i s o n o f Figure 3 with Figure 1 reveals that instructions to recall • SIGNALED o NOT SIGNALED
0.6 oooO
• TOTAL
~_.R= 0 . 4
2
~000
0
I 2
0 I 4
~1 6
w 0
0 I 8
I 10
[ 12
0 0 I ~4
•
I 16
I t8
NUMBER OF WORDS SIGNALED
FIO. 3. Proportion ofsignaledandunsignaledwords recalled, along with the total proportion recalled, as a function of the number signaled (Exp. II).
452
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signaled items first increased the proportion of signaled items recalled under Conds. 2 through 6, but not under Cond. 9 through 18 or under Cond. 1. A progressive compensatory decrease in the recall of unsignaled words is very evident as the number of signaled words increases from 1 to 15. Again it is clear that the data support the rehearsal-time hypothesis. Figure 4 shows the relative frequency with which words 1M and words 9, 12, 15, and 18 were recalled as a function of the number signaled. Here it is evident that, as in Exp. I, early signaled words are likelier to be recalled than later ones. A comparison of Figure 4 with Figure 2 reveals that in Exp. II the early words enjoyed an even greater advantage over the late ones than in Exp. I. Words 1-9 were recalled 20 and 21 ~ of the time under Cond. 0 and 18, respectively; for words 10-18, the corresponding percentages were 17 and 10. Instructions to recall signaled words first therefore seems to depress the retention of words signaled late in a list. It may be, of course, that under Cond.
o•••
•
0.4
o o 0
Ms = . 5 ( N + 1)P(N); while for Hypothesis E, Ms = .5(ns + 1)P(ns),
0.6
~
a few words were signaled and rather more difficult when several were signaled. Nevertheless, the relation between an item's serial position in recall and its probability of being recalled at all is ambiguous at best, according to the data presented in Table 1. Here mean position in recall is shown for signaled items under Cond. 1-4, 9, and 18 in both Exps. I and II. (Only half of the data collected in Exp. I--those obtained from Ss who listened to exactly the same lists as did the Ss in Exp. II --are represented there.) The figures in italics denote an item's mean serial position in recall expected according to either of two extreme hypotheses. The first is that a signaled word is as likely to be recalled in one position as in any other (Hypothesis R, for random recall); and the second is that signaled words are always recalled before unsignaled ones (Hypothesis E, for early recall). The expected values were calculated for each condition as follows. For Hypothesis R,
0
I ] 1 1 1 2 4 6 8
1 10
NUMBER OF WORDS
o
I l 12 14 16
o I 18
S/GNALED
FIG. 4. Relative frequency of recall for words 1-4 (solid circles), and for words 9, 12, 15, a n d 18 (open
circles), given that they were signaled,as a function of the total number signaled (Exp. II). The two leftmost points (solid and open triangle) denote the probability that words in those positions (1-4 or 9, 12, 15, and 18, respectively) would be recalled under Cond. 0. 18 the Ss attempted to store the lists serially, thereby employing a rehearsal strategy that would favor the retention of early items. The Ss' instructions to recall signaled words early were evidently easy to follow when only
where Ms denotes the mean serial position in recall expected for a signaled word; P(N) denotes the probability that a total of N words (signaled plus unsignaled) were recalled, given that at least one signaled word was recalled; and P(n~)denotes the probability that ns signaled words were recalled, given that at least one was recalled. It is evident that, in Exp. I, signaled words were as likely to be recalled late as early under all conditions: all the data are much more consistent with Hypothesis R than with Hypothesis E. In Exp. II, on the other hand, the data are more consistent with Hypothesis E when only a few words are signaled and with Hypothesis R when many are signaled. A comparison of Table 1 with Figures 1 and 3 enables one to make the following statements. (1) Under Cond. 1, signaled words were equally likely to be recalled in both experiments, even though they were emitted early in
453
CONSPICUOUS ITEMS TABLE 1 MEAN SERIAL POSITION IN RECALL FOR SIGNALED WORDS AS A FUNCTION OF THE NUMBER SIGNALED Exp. I
Exp. II
Cond.
Hyp. E
Obs.
Hyp. R
1 2 4 9 18
1.00" 1.21 1.51 2.24 2.56
5.21 5.24 5.67 6.91 7.69
5.62 5.73 5.60 5.64 5.61
Hyp. E
Obs.
Hyp. R
1.00 1.23 1.79 2.00 1.85
1.58 1.74 2.78 3.80 3.71
4.38 4.44 4.24 4.43 3.98
a Note.--Entries in italics denote the values expected on the basis of Hyp. E (signaled words recalled before any of the rest) and Hyp. R (signaled words as likely to be recalled late as early).
Exp. II and late in Exp. I (no apparent relation, in other words, between priority and frequency of recall). (2) In Exp. I, signaled words were much less likely to be recalled under Cond. 4 than under Cond. 1, even though their average position in recall was virtually identical under both conditions (again no obvious relation between priority and frequency of recall); whereas in Exp. II they were recalled both earlier and more frequently under Cond. 4 than they were in Exp. I under that condition (a possible positive relation, therefore, between priority and frequency). (3) Finally, under Cond. 18, words were recalled somewhat later in Exp. I than one would expect on the basis of chance (Hypothesis R) and somewhat earlier in Exp. II; but they were nevertheless recalled somewhat more frequently in Exp. I than in Exp. II under that condition (a possible negative relation, therefore, between priority and frequency). This analysis suggests, then, that probably some factor other than the instructions to recall signaled words early in Exp. II was responsible for the disparity between the data shown in Figures 1 and 3. The nature of this factor is not immediately obvious. It is still difficult to understand, moreover, why one signaled word should be recalled with a probability of only approximately .50 when S has been instructed to pay special attention
to it and to recall it first. Two possible explanations are as follows. First, it may be that S must fully attend to a word with the intent of retaining it at the time it occurs, and not shortly thereafter, if he is to store and retain it with a high degree of certainty; perhaps some kind of arousal mechanism must be activated shortly before or simultaneously with the presentation of an item and not shortly thereafter (Berlyne et al., 1966), if the item is to be securely retained. Or perhaps a given word may sometimes be lost from primary memory (Waugh & Norman, 1965), and hence be unavailable for storage in secondary memory, by the time that S has taken cognizance of the signal that follows it. In either event, a signaled word should be better retained if it is preceded rather than followed by its signal. Alternatively, it might be the case that signaled words were recalled only half the time under Cond. 1 simply because the Ss were attempting to retain other words (both earlier and later ones) as well, even though they were not signaled, and in doing so may have failed to store the signaled item or may have lost access to it once it was stored. If this were the case, then instructing S to retain only the items that were signaled should increase their likelihood of being recalled, since only those words would be attended to. When only one item in a list is signaled under those circumstances, S
454
WAUGH
should be able to hold it i n m i n d (or recirculate it i n p r i m a r y m e m o r y ) for the r e m a i n d e r of the list a n d thus be able to recall it with p r o b a b i lity 1.00. This hypothesis, of course, does n o t exclude the one j u s t described. It could be that b o t h instructing S to retain only those items that are signaled a n d signaling them before they occur are necessary to p r o d u c e perfect r e t e n t i o n of one signaled word. E x p e r i m e n t I I I was designed to test b o t h hypotheses. EXPERIMENT I I I
Method. The material to be recalled Consisted of 100 lists of 20 unrelated monosyllables, no one of which occurred in more than one list. Within each list, either 1, 2, 4, 7, or l l words were signaled. Under Cond. 1, a signal followed either word 3, 4. . . . 2, 9, 10, ... or 13. Various combinations of words were signaled under Conds. 2, 4, and 7, such that words in positions 3-7 and 9-13 were followed by signals an equal number of times over the course of the experiment. Obvious patterns of signals were avoided insofar as possible under Conds. 2-7. Under Cond. 11, every word from 3-13 was followed by a signal. A total of 10 Ss participated in two experimental sessions. All heard the same lists in the same order, 50 lists per session. Five of the Ss were instructed in Session 1 to recall only those words that had been followed by a signal, while in Session 2 they were to recall only those words preceded by a signal. The other five Ss received these instructions in the opposite order. Thus each list was recalled by five Ss. who had been instructed to obey a "retroactive" signal to attend to a given word and by five obeying a "proa~tive" signal. The serial positions of the words to be recalled ranged from 3 to 13 under the "retroactive" condition and from 4 to 14 under the "proactive" condition. The Ss, all students at Tufts Summer School, were paid for their services. They were told that one or more words would be signaled (either proactively or retroactively) within each list, that they should attend to these signaled words and only to them, and that they should recall them (and none of the others) in any order they chose after the last word had been read. They were given no explicit information as to the number of signals that would occur in any of the lists nor as to their location. The lists were recorded onto magnetic tape by a female speaker at a rate of one word per see eo'adwere later presented to the Ss over a loudspeaker. The signal, as before, consisted of a brief high-pitched tone embedded between two words but overlapping neither. Every experimental condition was represented exactly
once within each successive block of five lists. Otherwise the conditions were ordered unsystematically. The length of the recall interval varied with the number of words to be recalled, ranging from about 10 see under Cond. 1 to about 2 min. under Cond. 11. Every tenth list within a session was followed by a 5-minute rest period. The Ss wrote their responses in booklets, using a new page for each list.
Results. The data are s u m m a r i z e d i n Figure 5, where the p r o p o r t i o n of (signaled) items recalled is plotted as a f u n c t i o n of the total n u m b e r signaled. It is evident that this p r o p o r t i o n decreases progressively f r o m a value that is very close to 1.00 w h e n only one or two words are signaled to a value of approximately .35 w h e n 11 words (out of 20) are signaled. This result is consistent with those of Exps. I a n d I I : the greater the n u m b e r of signaled words c o m p e t i n g for S's a t t e n t i o n the smaller "the chances that a n y one of them will be recalled. It is also quite obvious i n Figure 5 that the location of the signal makes n o practical difference whatsoever: r e t e n t i o n is equally good when the signal t o retain a w o r d follows it as w h e n it precedes it. A n analysis of serial-position effects reveals a n even greater disparity between the recall of early a n d late signaled words t h a n was seen in Exps. I a n d II. Figure 6 shows the p r o p o r tion of items that were signaled (both "proactively" a n d "retroactively") a n d later • PROACTIVE SIGNAL
1,0
S I G N A L 0.8 0.6 0.4 Q.
0.2 0
0
I
I
2
I
I
4
I
I
6
I
I
8
I
I
I
I
t0 ,~,12
NUMBER OF WORDS SIGNALED
FIG. 5. Proportion of pro- and retroactively signaled words recalled as a function of the total number signaled (Exp. III).
CONSPICUOUS ITEMS
455
recalled as a function of their serial position in Exp. II even when they intend to recall within the signaled set under Cond. 11; the signaled words before any of the rest. horizontal line indicates the total proportion recalled. The strong primacy effect evident in DISCUSSION Figure 6 suggests that the Ss may have been The main results of the present study may attempting to store the items in serial order, and inspection of the Ss' answer sheets be summarized as follows. 1. Instructing a subject to "pay special indicated that an item's order in recall was in fact positively correlated with its order in attention" to certain arbitrarily designated items in a list makes him much likelier to recall presentation. Interestingly enough, the probability that them than he would otherwise, as long as only an unsignaled word would be recalled, a few items are so designated. When many estimated across all conditions, was approxi- items are signaled, however, the probability mately .01, while the probability that a word that any one of them will be retained apthat was recalled was an unsignaled one was proaches the proportion of items recalled approximately .04. The proportion of "false when none is signaled. Signaled items are positives," moreover, did not vary systemati- retained at the expense of unsignaled ones, cally with the number of words to be recalled. such that the total number recalled remains Evidently, then, the Ss were able to discrimi- constant regardless of how many are signaled. nate very accurately between the words which These facts are entirely consistent with the had been signaled and those which had not. hypothesis, previously suggested elsewhere Unfortunately there is no way of estimating (Waugh, 1967), that only a fixed number of the analogous proportion of "false positive" words can be freely recalled from a list of responses occurring in Exps. I and II, since fixed duration, with the likelihood that any the appropriate data were not collected (i.e., given word will be recalled increasing in direct proportion to the time allotted to it. "Special attention" may accordingly be taken to mean protracted attention. 2. The role of order in recallis not entirely . clear: a specially-attended-to item, when it is 0.4 .. the only such item to occur in a list, is not 0.2 °Oo remembered any more frequently when S is I I I L I~ instructed to recall it first (and does in fact 0 2 4 6 8 t 0 12 recall it first, if he recalls it at all) than when SERIAL POSI T/ON FIG. 6. Relative frequency of recall for a signaled he has been given no special instruction (and word as a function of its serial position (Exp. III, is therefore as likely to recall the item very late as very early). When 2, 3, 4, or 6 signaled Cond. 11). items (in a list of 24) are to be recalled before the Ss were not asked to indicate whether or any of the rest, on the other hand, they are not they thought each word that they recalled recalled more frequently than they would be had been signaled). Inspection of Table 1 otherwise. It is possible that under these suggests, however, that when Ss have the circumstances signaled items are stored as one option of storing a set of unsignalcd words cluster, and are hence more easily retrievable along with a set of signaled ones, they may lose than if they were stored independently of one their ability to discriminate between the two another (Tulving & Patterson, 1968). The at the time of recall, and may therefore maximum size which such a cluster can attain erroneously defer the recall of a signaled word is apparently limited to about four items.
0.f6[
456
WAUGH
3. It makes no difference whether the signal to attend to a given word is presented just before or just after the word itself. Apparently a retroactive set to retain an item is just as effective as a Pr0active o n e - - a t least, one would have to suppose, as long as the item was still available in primary memory (Waugh & Norman, 1965). 4. One particular word can be freely recalled from a supraspan list with a probability approaching 1.00 when S has been instructed to retain only that word, but not when he has the option of retaining others as well. This fact suggests that diverting one's attention from an item, perhaps even very briefly, can result in its becoming irretrievable no matter how carefully it has been stored. (For discussions of the distinction between storage and retrieval in free recall, see Tulving & Pearlstone, 1966; Waugh, 1967, Slamecka, 1968.) Obviously these results are not incompatible with an "arousal" hypothesis of selective retention: it could be that stimuli which produce a strong orienting reflex, perhaps because they are especially novel or especially significant, will also tend to be selected for attention at the expense of trivial or inconspicuous stimuli. It should be equally clear that the present results cannot be taken as direct evidence for the hypothesis that the von Restorff effect reflects the selective rehearsal of such items rather than their inherent perceptual discriminability. These results do mean, however, that an item's novelty or significance as such are not necessary for its selective retention. They m a y be sufficient, although perhaps only insofar as they serve to fix S's attention on the item. In order to show that conspicuous items tend to be well retained in conventional tests of the von Restorff effect simply because they tend to be well rehearsed, one would have to perform an experiment analogous to Exp. I, but with items that were isolated from their neighbors
in a list by virtue of some intrinsic property of meaning or appearance rather than through their association with an arbitrary signal. REFERENCES
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Psychological Review, 1965, 72, 89-104. (Received December 6, 1968)