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Intelligence and short-term memory: A clandestine relationship
INTELLIGENCE 4, 319-331 (1980)
Intelligence and Short-Term Memory: A Clandestine Relationship* RONALD L. COHEN Glendon College, York University
TOR SANDBERG Regional Board of Education, Falun, Sweden
Five possible mechanisms are considered as being responsible for the systematic variation of serial STM with IQ, namely rehearsal maintenance, chunking, access, encoding of item and/or order information, and trace persistence. A STM/IQ correlational study is reported. It was concluded from analysis of a correlation matrix, and from earlier reported data, that the encoding of items-in-order into an already loaded store appears to be the critical mechanism in determining IQrelated individual differences in STM performance, at least in children. The implications of individual difference data for models of STM are also discussed.
Despite much research, the precise nature of the relationship between short-term memory (STM) and IQ has yet to be established. Studies comparing retarded subjects with controls have given rise to the current view that retarded subjects perform poorly on STM tasks because they lack proficiency in rehearsal maintenance (Ellis, 1970; Ellis, McCarver & Ashurst, 1970; Belmont & Butterfield, 19?l; Butterfield, Wambold & Belmont, 1973; Cohen & Nealon, 1979) or because they do not use executive function (Butterfield & Belmont, 1977; Cohen & Nealon, 1979). In this sense, executive function refers to the capacity to vary memorizing and/or retrieval strategy according to the demands of the task. Empirical support for the rehearsal maintenance explanation of IQ-related differences in STM capacity comes from studies in which retarded subjects show a large performance deficit (as compared with controls) on primacy items, both in the Atkinson/Ellis probed digits-in-windows test (Ellis, 1970), a variation on the Atkinson/Ellis test using cards (Ellis et al, 1970), and in the short-term recall of serial lists (Cohen & Nealon, 1979). Since performance on primacy items depends on these items being maintained in memory until recall is required, it seems logical to *The data analysis and the writing of the paper were supported by a National Sciences and Engineering Research Council Canada grant (No. A7023). Requests for reprints should be sent to Ronald L. Cohen, Glendon College, 2275 Bayview Avenue, Toronto, Canada.
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attribute the deficits shown by retarded subjects on primacy items to poor rehearsal strategies (Ellis, 1970). Short-term memory/IQ correlations within the normal range, however, are not so readily explained. In a series of studies, samples of children in the approximate IQ range of 70 to 140 were tested on the immediate probed recall of auditory 9-digit lists. Following each list was the auditory letter A, B, or C, which was the cue to recall the first three, the second three, or the final three digits respectively. In this task, reliable correlations were found between IQ and performance on the recency items (C-digits) but not between IQ and performance on the primacy items (A-digits) (Cohen & Sandberg, 1977; Cohen & Lavin, 1979). These data are, of course, precisely the opposite of what would be expected if maintenance by rehearsal were the mechanism underlying the IQ-related differences in STM performance obtained on the probed serial digits task. Given that rehearsal cannot explain these STM differences, what are the other possibilities? Four possible candidates for the critical mechanism underlying the STM/IQ correlations found with the probed serial digits task will be considered here. It should not be presumed that the following list is an exhaustive one. Rather, it represents a convenient framework for current research. (1) Chunking. In the A, B, C digit task, subjects who chunked the input into target sets of three digits could be expected to perform better than subjects who did not chunk. This explanation may be discounted on the basis of data obtained using a serial running memory task. In this task, long lists of auditory digits of varying length were presented at high rates. Immediately following list presentation, the subject was required to recall the final three digits in serial order. Performance on this task correlated with both IQ and performance on C-digits in the probed serial recall task (Cohen & Sandberg, 1977). The high rates of presentation used in the running memory task, coupled with the impossibility of identifying which three digits constituted the target set until after presentation, completely invalidates chunking as the critical mechanism responsible for the STM/IQ correlations obtained in the recall of recency items. (2) Access. An access explanation for STM/IQ correlations was invoked by Cohen and Gowen (1978) to account for the results of a study which used recognition to test performance in serial running memory. In that study it was assumed that there was no systematic variation in the availability of the target set with IQ. Instead, it was proposed that lower IQ subjects had more difficulty in locating the target set (the final 3 items) among the other items in STM, th~n did higher IQ subjects. This hypothesis was specifically tested in a later study (Cohen & Lavin, 1979), by measuring the effect of demarcating the target set on STM/IQ correlations. Increasing the ease of access to the target set in this way did not significantly reduce the difference between the lower
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and higher IQ children, as would have been expected if access were the critical operation underlying the STM/IQ correlations. Further, the large deficit shown by retarded subjects in the probed serial recall of recency items was not decreased when complete recall was substituted for probed recall (Cohen & Nealon, 1979). It is therefore extremely unlikely that access plays a major role in determining IQ-related differences in the STM for recency items. (3) Encoding ability. This explanation requires that lower IQ children do not encode incoming items as efficiently as higher IQ children. This, in turn, means that the traces laid down by lower IQ children will not be as persistent as those laid down by higher IQ children. Encoding in this case refers to item encoding, in the sense of identifying the incoming stimuli, and also to the encoding of the serial order of the items. The importance of this distinction between item and order information has already been made in the context of individual differences. Hunt, Lunneborg, and Lewis (1975), for example, report that high verbal adults are superior to lower verbals in their ability to retain order information in STM. It should be noted, however, that if encoding proficiency is critical for IQ-related individual differences in shortterm serial recall, this does not reveal itself in the recall of primacy items. (4) Trace persistence. As has already been pointed out, trace persistence should be a function of encoding efficiency. It is also possible for trace persistence to vary systematically with IQ, even if encoding efficiency does not. This viewpoint is in line with Ellis' (1963) explanation for the STM/IQ relationship, which supposed the Stimulus traces of retarded subjects to decay faster than those of nonretarded subjects. One piece of evidence against this explanation comes from a study of reading disabled children. These children showed a similar deficit in the recall of recency items to that shown by lower IQ children in both the A, B, C digits task (Cohen & Netley, 1978) and in serial running memory (Cohen & Netley, in press). This deficit is readily amenable to training, however, which result is difficult to reconcile with a hardware notion of trace decay (Cohen, 1978). Since the rehearsal maintenance, chunking, and access hypotheses have failed to receive support from previous studies, the remainder of this paper will concentrate on the two remaining hypotheses, namely encoding and trace persistence. The claims of these latter hypotheses will be examined in the light of some of the data reported by Cohen and Sandberg (1977) reanalyzed in conjunction with some further data gathered during the course of that study, but not previously published. In addition to the A, B, C digits and the serial running memory task already reported, the subjects in Experiment 2 in Cohen and Sandberg (1977), were given two tests of encoding.The validity of the above hypotheses, and more specifically the encoding hypothesis, was tested by setting up a correlation matrix based on scores from the memory tests, the encoding tests, and an IQ test and performing principal components and factor analyses.
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METHOD
Subjects The subjects were 80 children in the age range 12.5 to 13.5 years, taken from classes in Swedish State Schools. They had been IQ tested using the Swedish DBA group test battery (H~rnqvist, 1960) about two months prior to their experimental sessions. The IQs of these children ranged from 70 to 132, with a mean of 97.5 and an SD of 17.5. Tasks
Probed serial recall (A, B, C digits). Twelve auditory 9-digit lists were presented at a rate of four digits/second. The auditory letter A, B, or C following one second after each list served as a cue to recall thefirst three, the second three, or the final three digits, respectively. Prior to this test, six 9-digit lists were presented for complete recall, as a warm-up task. Serial running memory. Two blocks of ten auditory digit lists, whose length varied from nine to twenty digits, were presented, one block at a rate of six digits/second and the second block at a rate of nine digits/second. The task required the recall of the last three digits in each list, in serial order, as soon as presentation was complete. No post-list cues were provided to signal the end of the lists. Word encoding. Two blocks of eighteen auditory lists of common one- or two-syUable nouns were presented; one block at a rate of three words/second, and the second block at a rate of five words/second. Prior to each list, the subject was cued with a superordinate category name. All lists contained twelve words, a target word which was a member of the cueing category and eleven fdler words which were not. The task required the subject to respond with the target member. Example: Which word in the list is a FRUIT?.; Bed, cup, summer, white, apple, figure, brother, fish, tree, water, jacket, leg. This task required the subject to perform two operations, namely the recognition of the target and the recognition that the target is a member of the cueing category. By using fairly common categories and high frequency category members (according to Cohen, Bousfield & Whitmarsh, 1957), it was hoped to minimize the importance of the latter operation, thus making performance mainly dependent on target recognition. Preprobed digit encoding. Two blocks of twelve auditory 30-digit lists were presented; one block at a rate of six digits/second and the second block at a rate of nine digits/second. Prior to each list the subject was given a cueing
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digit. The task required the subject to report the digit which followed the cueing digit in the list. The cueing digit occurred once only in its list, being randomly assigned a position within that part of the list bounded by serial positions eight and sixteen. This task also involved item recognition, since both the cueing digit and the response digit must be identified before a correct response can be made. In addition, the serial nature of the relationship between cueing and response digit implicates the encoding of order information. The six and nine digits/second presentation rates were attained by recording the lists at four digits/second and playing the tapes through a Tempophone, which increased the rate of presentation without appreciably changing the other characteristics of the sound. The word lists were recorded at three words/second, and the five words/second rate attained through the use of the Tempophone. Subjects were tested individually. In all tasks, they were allowed ten seconds following presentation in which to make a verbal response, although they were not required to wait for list presentation to be complete when giving responses in the encoding tasks. The tasks were given to all subjects during the course of one testing session, in the following order: Probed serial recall, word identification (5 words/second), word identification (3 words/second), preprobed digits (9 digits/second), preprobed digits (6 digits/second), serial running memory (9 digits/second), serial running memory (6 digits/second). RESULTS Table 1 shows the performance data for the various tasks. It should be noted that the high rates of presentation produced lower performance levels than the low rates.This indicates that the high presentation rates probably had the desired effect, namely that of decreasing encoding proficiency. The correlation matrix for the 10 tasks is given in Table 2. This matrix was subjected to analysis by three different procedures, (a) a principal components analysis (see Seal, 1966), (b) a principal factor analysis with iterations (see Seal, 1966), and (c)Rao's canonical factor analysis (see Mulaik, 1972). The results of these analyses, following varimax rotation, are shown in Table 3. Using the Kaiser criterion of eigenvalues ~ 1.0 (Mulaik, 1972, p. 176), three factors were extracted in all three analyses, accounting for 68-79% of the total variance in the correlation matrix. The loadings of the various tasks follow approximately the same pattern in all three analyses on Factors I and II, but not on Factor III. Partly for this reason, and partly because the interpretation of Factor III is really unimportant in the context of this argument, only Factors I and II will be discussed.
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COHEN AND SANDBERG TABLE 1 Mean Performance Levels, Expressed as Proportions, and SDs
A-digits B-digits C-digits Fast Words Encoding Slow Words Encoding Fast Preprobcd Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory
Mean
SD
.46 .12 .46
.23 .16 .27
.50
.17
.82
.13
.40
.22
.75
.23
.49
.21
.59
.24
In addition to meeting Kaiser's eigenvalue criterion, and showing reliable loading patterns over the three analytical procedures, Factors I and II are readily interpretable. These two factors will be considered in reverse order. Factor II, which accounts for over l l % of the total variance, shows relatively high loadings from the word identification tasks and fast preprobed digits. Moderate loadings are shown by slow preprobed digits and fast running memory. The operation common to the word identification task and preprobed digits is that concerned with item identification, the target word in word identification, and tbe probeand response in preprobed digits. On this basis, Factor II can be designated an item encoding or identification factor. The importance of item identification for the performance of fast (but not. slow) running memory will be argued in the Discussion section. Factor I, which accounts for about 47% of the total variance, shows relatively high loadings from the memory tasks involving the serial recall of recency items, namely C-digits and fast and slow running memory. In addition, the fast preprobed digits and, to a lesser extent, the slow preprobed digits, load on this factor. None of the other laboratory tasks shows even a moderate loading on this factor, which is reliable across the three analyses. Since tasks involving serial relationships among items show appreeiable loadings on Factor I, whereas tasks involving the encoding of items but not order (word identification) do not, it is suggested that this factor is concerned with the serial ordering of items. It should be noted that IQ shows a consistently high loading on Factor I, but only a weak loading Factor II.
t, J hi,
1.00
,18 1.00
.10 .24 1.00
C-digits
1.00
.01 .16 .23
The critical r value corresponding to the ,05 significance level = 0,22
Fast Words Encoding Slow Words Encoding Fast Preprobed Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory 1Q
C-digits
A-digits B-digits
A-digits B-digits
Fast Words Encoding
.56 .59 1.00
.44 .75 1.00
.42 1.00
1.00
,56
.45
.58
.45 .73
.35
.60
.55
1.00
.32
-.03 .30 .46
IQ
.50
.19
,07 .34 .62
Slow Running Memory
.58
.38
,17 .34 .53
Fast Running Memory
1.00
.24
.13 .31 .44
Slow Preprobed Digits
.50
.15 .43 ,56
Digits
Fast Preprohed
.56
.03 .28 .38
Slow Words Encoding
TABLE 2 Correlation Matrix
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TABLE 3 Varimax Rotated Factor Loadings from (a) Principal Components Analysis; (b) Principal Factor Analysis and (c) Rao's Cononical Factor Analysis Factor I
A-digits B-digits C-digits Fast Word Identification Slow Word Identification Fast Preprobed Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory IQ Eigenvalues:
Factor II
(a)
(b)
(c)
(a)
-.30 .36 .77 .08 .36 .65 .55 .75 .90 .73 4.67
.01 .05 .04 .26 .18 -.18 .53 .61 -.11 .12 .12 -.89 ,27~ .28 -.78 .46 .49 -.54 .36 ~ .37 -.36 .60 .-~]3--~.3~_ .96 .93 -.04 .83 .55 -.30 4.77 4 . 6 7 1 . 1 3
(b)
Factor III (e)
.01 .06 .20 .10 .26 .28 .67 .65 .72 .68 .61 .68 .40 .44 .44 .45 .15 .08 .28 .36 1.16 1.13
(a)
(b)
(c)
-.92 .36 .19 -.50 .36 .87 -.09 .31 .13 .00 .00 .07 -.07 .14 .17 -.24 .44 .32 -.23 .38 .25 -.20 .36 .19 -.08 .13 .19 .08 .01 .18 1 . 0 2 1 . 1 2 1.02
DISCUSSION As an aid to establishing more precisely the possible mechanisms underlying Factor I, which is obviously the critical factor concerned in the STM/IQ relationships observed in these children, improbable candidates will first be eliminated. In the introduction, maintenance rehearsal and access were dismissed as possible mechanisms underlying the correlations obtained between IQ and the serial recall of recency items. The results of the factor analyses extends this argument to deal with maintenance under conditions of fast presentation, where rehearsal strategies would be of doubtful importance. The ability to encode serial lists of items, in the sense of identifying the incoming items would also appear to be eliminated as a likely explanation for Factor I, since the word encoding task shows high loadings on Factor II, but not on Factor I. It could, of course, be argued that the word encoding tasks involved semantic encoding whereas the digit tasks did not. Consequently, Factor II should be regarded as a semantic encoding factor, rather than a more general item identification factor. This objection is not really valid, however, since Factor II also showed a high loading from fast preprobed digits and a moderate loading from slow preprobed digits. The above process of elimination leaves trace persistence and the encoding of order information as possible mechanisms underlying Factor I. At first glance, trace persistence would appear to be ruled out by the fairly high loadings on this factor from the preprobed digits tasks, since these tasks should have involved the minimal amount of retention. However, a post hoc analysis based partly on information obtained from the subjects, revealed
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that retention was indeed involved in preprobed digits. Because of the high rates of presentation, subjects did not react to the cueing digit until several more items had been presented. Thus, the response digit was not selected from the presented list, but had to be retrieved from memory. A strong trace would enable this to be done successfully, whereas a weak trace would make this retrieval difficult. There are other data which are inconsistent with a trace persistence hypothesis, however. First, even at the relatively high presentation rate of 4 digits/see, the persistence of primacy items (A-digits) is apparently unrelated to IQ. Second, as already mentioned in the introduction, the ready trainability of A, B, C digits in reading disabled children does not support a hardware notion of trace persistence. And third, Ferretti (1979) has reported similar forgetting curves for retarded and nonretarded subjects, when the initial load is adjusted to allow for differences in immediate memory capacity between the two types of subject. Retention in this task was passive, active maintenance being hindered through the use of a detection task in the retention interval. Under such conditions, it could be argued that the retention curves represent trace decay curves. While retarded/nonretarded comparisons may involve different mechanisms from those involved in comparisons within the normal population, the lack of a trace persistence deficit in retarded subjects seems inconsistent with the notion that such a deficit exists in lower IQ subjects within the normal range. At this point, then, the encoding of order information appears to be the most likely mechanism underlying Factor I. An attempt has been made to test this hypothesis directly, using recognition to measure performance in the serial running memory task (Cohen & Gowen, 1978). Through the use of suitable instructions and lures, item and order memory were measured separately. A significant correlation was found between IQ and recognition memory for the items in the target set, but not between IQ and the memory for the order of these items. However, a post hoc analysis of the stages involved in this task suggested that recognition is probably an inappropriate method for measuring the availability of order information. If the proficiency with which the subject encodes the order of the incoming items is indeed the critical operation underlying Factor I in this study, then certain limitations can be imposed on the importance of this mechanism for determining STM performance on serial tasks. First, although it is possible to separate item information from order information (Factor II) it is really impossible to divorce order information from item information. A more useful distinction would be that between the enco~ing of items and the encoding of items-in-order. Second, the encoding of items-in-order does not present a general problem for lower IQ subjects, as is evidenced by the complete lack of a correlation between performance on A-digits and IQ. Third, rate of presentation does not appear to have much effect on individual
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differences in the encoding of items-in-order. In Cohen and Sandberg's Experiment 3b (1977), the correlation found between IQ and performance on C-digits presented at 1 digit/2 seconds was only fractionally lower than that obtained with the same subjects using a 4 digits/second presentation rate. A similar result has been reported by Lyon (1977), who demonstrated that the difference between good and poor adult performers on slow serial lists did not decrease when presentation rate was increased. This trend is also apparent in Waugh and Norman's (1965) data. Very high rates of presentation may influence individual differences in serial recall, however, by affecting how well defined the actual items are in the items-in-order. This could account for the tendency of fast running memory, but not slow running memory, to load on Factor II. More critical for determining IQ-related individual differences appears to be the state of the STM system when the items are encoded. Items entering an empty system, for example A-digits, do not appear to constitute an encoding problem for the lower IQ children. Items entering an already loaded system do present an encoding problem. In Cohen and Sandberg's (1977) Experiment 3a no systematic differences were found in STM/IQ correlations with length of list. The task used was serial running memory requiring the recall of the final three items. Since the shortest list was only six digits in length, it is concluded that three items may be sufficient to load the system to the extent that a deficit in the encoding of further items-in-order will appear in lower IQ children. This conclusion gains support from the STM/IQ correlations which have been found with B-digits in some of the groups tested, in spite of the low performance levels generally obtained with these items. A further property of the system is that a certain depth of encoding must be involved in processing the items in order to produce loading. Children, representing a wide range of IQ, were given the serial running memory task using a four digits/second rate. The target sets, the final three items in each list, were separated from earlier list items by a one second pause. When the instruction was to recall the final three items in each list, in serial order, all subjects performed perfectly, regardless of IQ. When the instructions were to recall either the three digits preceding the pause or the three digits following the pause, in response to a post-cue, lower IQ children perfomed worse than higher IQ children in recalling the post-pause digits (Cohen & Lavin, 1979). The difference between the two conditions is that in the first condition the children knew that they did not have to retain the items preceding the pause. In the second condition all items had to be encoded for retention in case the recall of items preceding the pause was signalled. These data indicate that simply listening to the first part of the list is not sufficient to load the system. Some encoding in depth on the part of the subject is necessary to put the system on load.
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The above conclusions lead to a picture of a STM system which deals with actively encoded serial items. This system can readily process serial items when empty, regardless of the IQ of its owner, within the limits of the normal range. When loaded with approximately three items, its encoding proficiency decreases, the size of this decrease varying according to the child's IQ (or perhaps reading ability). Encoding proficiency for serial items on overload is trainable, at least in the case of reading disabled children, which suggests that the difference between good and poor encoders should not be regarded as being due to fixed structural properties of the system. At this point, it is perhaps appropriate to make some comment on the items-in-order i n f o r m a t i o n assumed to be critical in the S T M / I Q correlations under study. The suggestion is not that lower IQ children can encode the items into the system, even on overload, hut that the serial order of these items somehow becomes jumbled. If this were the case, then the STM / IQ correlation obtained in the running memory task should disappear if a free recall instruction and scoring were used instead of serial recall. In fact, free order running memory results in a correlation of about the same magnitude as serial running memory (Cohen & Gowen, 1978). Analysis of the output order of the items in the free order condition showed that there was a strong tendency to recall these in serial order in spite of instructions. What appears to differentiate between lower and higher IQ children is not the general capacity to encode items or items-in=order, but the capacity to encode items into an already loaded STM system, an important property of which is that it retains not individual items, but items-in-serial=order. The state of these items-in=order is conceived as constituting a phonological pattern, or what Dornic (1975) calls a phonologically encoded 'chain trace', rather than an ordered series of individual items. In Dornic's model the items-in=order represent a more primitive state of information than do individually encoded items. This conclusion is based on the finding that the recall of items-in-order information does not vary as a function of the amount of attention available for its encoding, whereas the recall of item information does show such a variation. It should be noted that the individual differences data indicate the need for revision of earlier views of the STM system, at least for serial lists. For example, maintenance rehearsal was an important mechanism in determining performance level in the early STM stores (Broadbent, 1958; Atkinson & Shiffrin, 1969); this process does not appear to be involved in determining the performance level of individuals, except perhaps in the case of retarded subjects. Further, in earlier models, overloading the store meant the retention of later items at the expense of earlier items, if these were not rehearsed (Waugh & Norman, 1965). The individual differences data stress the failure of the STM system to encode later items because of the load constituted by
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earlier items. A n d finally, t h e i m p o r t a n c e o f t h e state o f the s y s t e m ( e m p t y or l o a d e d ) i n d e t e r m i n i n g S T M / I Q c o r r e l a t i o n s is o b l i q u e l y at o d d s w i t h D o r n i c ' s (1975) i t e m s - i n - o r d e r e n c o d i n g w h i c h is u n a f f e c t e d by the p r e s e n c e o f a c o m p e t i n g task, at least i n adults. H u n t 0 9 7 8 ) d i s t i n g u i s h e s b e t w e e n "the' d i f f e r e n t i a l efficiency o f m e m o r y a c t i v a t i o n across i n d i v i d u a l s " a n d " t h e details o f the process itself." It m a y be, h o w e v e r , t h a t the s t u d y of differential efficiency in m e m o r y will n o t o n l y a d v a n c e o u r k n o w l e d g e in t h e a r e a o f i n d i v i d u a l differences, b u t will also have i m p o r t a n t i m p l i c a t i o n s for f u t u r e m o d e l s of S T M .
REFERENCES Atkinson, R. C., & Shiffrin, R. M. Human memory: A proposed system and its control processes. In K. W. Spence & J. T. Spence (Eds.), Thepsychology of learning andmotivation: Advances in research and theory. Vol. 2. New York: Academic Press, 1968. Beimont, J. M.,&Butterfield, E.C. Learningstrategiesasdeterminantsofmemorydeficiencies. Cognitive Psychology, 1971, 2, 411~$20. Broadbent, D. E. Perception and communication. New York: Pergamon Press, 1958. Butterfield, E. C., & Belmont, J. M. Assessing and improving the executive cognitive functions of mentally retarded people. In I. Bailer & M. Sternlicht (Eds.), Psychological issues in mental retardation. New York: Psychological Dimensions, Inc., 1977. Butterfield, E. C., Wamboid, C., & Belmont, J. M. On the theory and practice of improving short-term memory. American Journal of Mental Deficiency, 1973, 77, 654-669. Cohen, B. J., Bousfield, W. A., & Whitmarch, G. A. Cultural norms for verbal items in 43 categories. Tech. Rep. No. 22, University of Connecticut, Dept. of Psychology, 1957. Cohen, R. L. A comparison of short-term memory patterns as related to intelligence and reading intelligence. Paper given at the Gatlinburg Conference on Research in Mental Retardation, March, 1978. Cohen, R. L., & Gowen, A. Recall and recognition of order and item information in probed running memory, as a function of IQ. Intelligence, 1978, 2, 353-362. Cohen, R. L., & Lavin, K. The effect of demarcating the target set on IQ-related individual differences in the probed serial recall of very recent items. In M. M. Gruneberg, P. E. Morris, & R. N. Sykes (Eds.), Practical aspects of memory. New York: Academic Press, 1979. Cohen, R. L., & Nealon, J. An analysis of short-term memory differences between retardates and nonretardates. Intelligence, 1979, 3, 65-72. Cohen, R. L., & Netley, C. Cognitive deficits, learning disabilities, and WISC verbalperformance consistency. Developmental Psychology, 1978, 4, 624-634. Cohen, R. L., & Netley, C. Short-term memory deficits in reading disabled children, in the absence of opportunity for rehearsal strategies. Intelligence, in press. Cohen, R. L., & Sandberg, T. The relationship between intelligence and short-term memory. Cognitive Psychology, 1977, 9, 534-554. Domic, S. Some studies on the retention of order information. In P. Rabbitt & S. Dornic (Eds.), Attention and performance. Vol. 5. London: Academic Press, 1975. Ellis, N. R. The stimulus trace and behavioral inadequacy. In N. R. Ellis (Ed.), Handbook of mental deficiency: psychological theory and research. New York: McGraw-Hill, 1963. Ellis, N. R. Memory processes in retardates and normals: theoretical and empirical considerations. In N. R. Ellis (Ed.), International review of research in mental retardation. Vol. 4. New York: Academic Press, 1970.
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Ellis, N. R., McCarver, R. B., & Ashurst, H. M. Short-term memory in the retarded: ability level and stimulus meaningfulness. American Journal of Mental Deficiency, 1970, 75, 72-80. Ferretti, R. P. An analysis of passive memory in normal and mentally retarded persons. Paper given at the Gatlinburg Conference on Research in Mental Retardation and Developmental Disabilities, April, 1979. H/irnqvist, K. Individuella differencer och skoldifferentiering. Statens offentliga utredningar. Stockholm, 1960, 1960, nr. 13. Hunt, E. Mechanics of verbal ability. Psychological Review, 1978, 85, 109-130. Hunt, E., Lunneborg, C., & Lewis, J. What does it mean to be high verbal? Cognitive Psychology, 1975, 7, 194-227. Lyon, D. R. Individual differences in immediate serial recall: a matter of mnemonics7 Cognitive Psychology, 1977, 9, 403--41 I. Mulaik, S. A. The foundations of factor analysis. New York: McGraw-Hill, 1972. Seal, H. Multivariate statistical analysis for biologists. London: Methuen, 1966. Waugh, N. C., & Norman, D. A. Primary memory. Psychological Review, 1965, 72, 89-104.
Intelligence and Short-Term Memory: A Clandestine Relationship* RONALD L. COHEN Glendon College, York University
TOR SANDBERG Regional Board of Education, Falun, Sweden
Five possible mechanisms are considered as being responsible for the systematic variation of serial STM with IQ, namely rehearsal maintenance, chunking, access, encoding of item and/or order information, and trace persistence. A STM/IQ correlational study is reported. It was concluded from analysis of a correlation matrix, and from earlier reported data, that the encoding of items-in-order into an already loaded store appears to be the critical mechanism in determining IQrelated individual differences in STM performance, at least in children. The implications of individual difference data for models of STM are also discussed.
Despite much research, the precise nature of the relationship between short-term memory (STM) and IQ has yet to be established. Studies comparing retarded subjects with controls have given rise to the current view that retarded subjects perform poorly on STM tasks because they lack proficiency in rehearsal maintenance (Ellis, 1970; Ellis, McCarver & Ashurst, 1970; Belmont & Butterfield, 19?l; Butterfield, Wambold & Belmont, 1973; Cohen & Nealon, 1979) or because they do not use executive function (Butterfield & Belmont, 1977; Cohen & Nealon, 1979). In this sense, executive function refers to the capacity to vary memorizing and/or retrieval strategy according to the demands of the task. Empirical support for the rehearsal maintenance explanation of IQ-related differences in STM capacity comes from studies in which retarded subjects show a large performance deficit (as compared with controls) on primacy items, both in the Atkinson/Ellis probed digits-in-windows test (Ellis, 1970), a variation on the Atkinson/Ellis test using cards (Ellis et al, 1970), and in the short-term recall of serial lists (Cohen & Nealon, 1979). Since performance on primacy items depends on these items being maintained in memory until recall is required, it seems logical to *The data analysis and the writing of the paper were supported by a National Sciences and Engineering Research Council Canada grant (No. A7023). Requests for reprints should be sent to Ronald L. Cohen, Glendon College, 2275 Bayview Avenue, Toronto, Canada.
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attribute the deficits shown by retarded subjects on primacy items to poor rehearsal strategies (Ellis, 1970). Short-term memory/IQ correlations within the normal range, however, are not so readily explained. In a series of studies, samples of children in the approximate IQ range of 70 to 140 were tested on the immediate probed recall of auditory 9-digit lists. Following each list was the auditory letter A, B, or C, which was the cue to recall the first three, the second three, or the final three digits respectively. In this task, reliable correlations were found between IQ and performance on the recency items (C-digits) but not between IQ and performance on the primacy items (A-digits) (Cohen & Sandberg, 1977; Cohen & Lavin, 1979). These data are, of course, precisely the opposite of what would be expected if maintenance by rehearsal were the mechanism underlying the IQ-related differences in STM performance obtained on the probed serial digits task. Given that rehearsal cannot explain these STM differences, what are the other possibilities? Four possible candidates for the critical mechanism underlying the STM/IQ correlations found with the probed serial digits task will be considered here. It should not be presumed that the following list is an exhaustive one. Rather, it represents a convenient framework for current research. (1) Chunking. In the A, B, C digit task, subjects who chunked the input into target sets of three digits could be expected to perform better than subjects who did not chunk. This explanation may be discounted on the basis of data obtained using a serial running memory task. In this task, long lists of auditory digits of varying length were presented at high rates. Immediately following list presentation, the subject was required to recall the final three digits in serial order. Performance on this task correlated with both IQ and performance on C-digits in the probed serial recall task (Cohen & Sandberg, 1977). The high rates of presentation used in the running memory task, coupled with the impossibility of identifying which three digits constituted the target set until after presentation, completely invalidates chunking as the critical mechanism responsible for the STM/IQ correlations obtained in the recall of recency items. (2) Access. An access explanation for STM/IQ correlations was invoked by Cohen and Gowen (1978) to account for the results of a study which used recognition to test performance in serial running memory. In that study it was assumed that there was no systematic variation in the availability of the target set with IQ. Instead, it was proposed that lower IQ subjects had more difficulty in locating the target set (the final 3 items) among the other items in STM, th~n did higher IQ subjects. This hypothesis was specifically tested in a later study (Cohen & Lavin, 1979), by measuring the effect of demarcating the target set on STM/IQ correlations. Increasing the ease of access to the target set in this way did not significantly reduce the difference between the lower
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and higher IQ children, as would have been expected if access were the critical operation underlying the STM/IQ correlations. Further, the large deficit shown by retarded subjects in the probed serial recall of recency items was not decreased when complete recall was substituted for probed recall (Cohen & Nealon, 1979). It is therefore extremely unlikely that access plays a major role in determining IQ-related differences in the STM for recency items. (3) Encoding ability. This explanation requires that lower IQ children do not encode incoming items as efficiently as higher IQ children. This, in turn, means that the traces laid down by lower IQ children will not be as persistent as those laid down by higher IQ children. Encoding in this case refers to item encoding, in the sense of identifying the incoming stimuli, and also to the encoding of the serial order of the items. The importance of this distinction between item and order information has already been made in the context of individual differences. Hunt, Lunneborg, and Lewis (1975), for example, report that high verbal adults are superior to lower verbals in their ability to retain order information in STM. It should be noted, however, that if encoding proficiency is critical for IQ-related individual differences in shortterm serial recall, this does not reveal itself in the recall of primacy items. (4) Trace persistence. As has already been pointed out, trace persistence should be a function of encoding efficiency. It is also possible for trace persistence to vary systematically with IQ, even if encoding efficiency does not. This viewpoint is in line with Ellis' (1963) explanation for the STM/IQ relationship, which supposed the Stimulus traces of retarded subjects to decay faster than those of nonretarded subjects. One piece of evidence against this explanation comes from a study of reading disabled children. These children showed a similar deficit in the recall of recency items to that shown by lower IQ children in both the A, B, C digits task (Cohen & Netley, 1978) and in serial running memory (Cohen & Netley, in press). This deficit is readily amenable to training, however, which result is difficult to reconcile with a hardware notion of trace decay (Cohen, 1978). Since the rehearsal maintenance, chunking, and access hypotheses have failed to receive support from previous studies, the remainder of this paper will concentrate on the two remaining hypotheses, namely encoding and trace persistence. The claims of these latter hypotheses will be examined in the light of some of the data reported by Cohen and Sandberg (1977) reanalyzed in conjunction with some further data gathered during the course of that study, but not previously published. In addition to the A, B, C digits and the serial running memory task already reported, the subjects in Experiment 2 in Cohen and Sandberg (1977), were given two tests of encoding.The validity of the above hypotheses, and more specifically the encoding hypothesis, was tested by setting up a correlation matrix based on scores from the memory tests, the encoding tests, and an IQ test and performing principal components and factor analyses.
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METHOD
Subjects The subjects were 80 children in the age range 12.5 to 13.5 years, taken from classes in Swedish State Schools. They had been IQ tested using the Swedish DBA group test battery (H~rnqvist, 1960) about two months prior to their experimental sessions. The IQs of these children ranged from 70 to 132, with a mean of 97.5 and an SD of 17.5. Tasks
Probed serial recall (A, B, C digits). Twelve auditory 9-digit lists were presented at a rate of four digits/second. The auditory letter A, B, or C following one second after each list served as a cue to recall thefirst three, the second three, or the final three digits, respectively. Prior to this test, six 9-digit lists were presented for complete recall, as a warm-up task. Serial running memory. Two blocks of ten auditory digit lists, whose length varied from nine to twenty digits, were presented, one block at a rate of six digits/second and the second block at a rate of nine digits/second. The task required the recall of the last three digits in each list, in serial order, as soon as presentation was complete. No post-list cues were provided to signal the end of the lists. Word encoding. Two blocks of eighteen auditory lists of common one- or two-syUable nouns were presented; one block at a rate of three words/second, and the second block at a rate of five words/second. Prior to each list, the subject was cued with a superordinate category name. All lists contained twelve words, a target word which was a member of the cueing category and eleven fdler words which were not. The task required the subject to respond with the target member. Example: Which word in the list is a FRUIT?.; Bed, cup, summer, white, apple, figure, brother, fish, tree, water, jacket, leg. This task required the subject to perform two operations, namely the recognition of the target and the recognition that the target is a member of the cueing category. By using fairly common categories and high frequency category members (according to Cohen, Bousfield & Whitmarsh, 1957), it was hoped to minimize the importance of the latter operation, thus making performance mainly dependent on target recognition. Preprobed digit encoding. Two blocks of twelve auditory 30-digit lists were presented; one block at a rate of six digits/second and the second block at a rate of nine digits/second. Prior to each list the subject was given a cueing
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digit. The task required the subject to report the digit which followed the cueing digit in the list. The cueing digit occurred once only in its list, being randomly assigned a position within that part of the list bounded by serial positions eight and sixteen. This task also involved item recognition, since both the cueing digit and the response digit must be identified before a correct response can be made. In addition, the serial nature of the relationship between cueing and response digit implicates the encoding of order information. The six and nine digits/second presentation rates were attained by recording the lists at four digits/second and playing the tapes through a Tempophone, which increased the rate of presentation without appreciably changing the other characteristics of the sound. The word lists were recorded at three words/second, and the five words/second rate attained through the use of the Tempophone. Subjects were tested individually. In all tasks, they were allowed ten seconds following presentation in which to make a verbal response, although they were not required to wait for list presentation to be complete when giving responses in the encoding tasks. The tasks were given to all subjects during the course of one testing session, in the following order: Probed serial recall, word identification (5 words/second), word identification (3 words/second), preprobed digits (9 digits/second), preprobed digits (6 digits/second), serial running memory (9 digits/second), serial running memory (6 digits/second). RESULTS Table 1 shows the performance data for the various tasks. It should be noted that the high rates of presentation produced lower performance levels than the low rates.This indicates that the high presentation rates probably had the desired effect, namely that of decreasing encoding proficiency. The correlation matrix for the 10 tasks is given in Table 2. This matrix was subjected to analysis by three different procedures, (a) a principal components analysis (see Seal, 1966), (b) a principal factor analysis with iterations (see Seal, 1966), and (c)Rao's canonical factor analysis (see Mulaik, 1972). The results of these analyses, following varimax rotation, are shown in Table 3. Using the Kaiser criterion of eigenvalues ~ 1.0 (Mulaik, 1972, p. 176), three factors were extracted in all three analyses, accounting for 68-79% of the total variance in the correlation matrix. The loadings of the various tasks follow approximately the same pattern in all three analyses on Factors I and II, but not on Factor III. Partly for this reason, and partly because the interpretation of Factor III is really unimportant in the context of this argument, only Factors I and II will be discussed.
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COHEN AND SANDBERG TABLE 1 Mean Performance Levels, Expressed as Proportions, and SDs
A-digits B-digits C-digits Fast Words Encoding Slow Words Encoding Fast Preprobcd Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory
Mean
SD
.46 .12 .46
.23 .16 .27
.50
.17
.82
.13
.40
.22
.75
.23
.49
.21
.59
.24
In addition to meeting Kaiser's eigenvalue criterion, and showing reliable loading patterns over the three analytical procedures, Factors I and II are readily interpretable. These two factors will be considered in reverse order. Factor II, which accounts for over l l % of the total variance, shows relatively high loadings from the word identification tasks and fast preprobed digits. Moderate loadings are shown by slow preprobed digits and fast running memory. The operation common to the word identification task and preprobed digits is that concerned with item identification, the target word in word identification, and tbe probeand response in preprobed digits. On this basis, Factor II can be designated an item encoding or identification factor. The importance of item identification for the performance of fast (but not. slow) running memory will be argued in the Discussion section. Factor I, which accounts for about 47% of the total variance, shows relatively high loadings from the memory tasks involving the serial recall of recency items, namely C-digits and fast and slow running memory. In addition, the fast preprobed digits and, to a lesser extent, the slow preprobed digits, load on this factor. None of the other laboratory tasks shows even a moderate loading on this factor, which is reliable across the three analyses. Since tasks involving serial relationships among items show appreeiable loadings on Factor I, whereas tasks involving the encoding of items but not order (word identification) do not, it is suggested that this factor is concerned with the serial ordering of items. It should be noted that IQ shows a consistently high loading on Factor I, but only a weak loading Factor II.
t, J hi,
1.00
,18 1.00
.10 .24 1.00
C-digits
1.00
.01 .16 .23
The critical r value corresponding to the ,05 significance level = 0,22
Fast Words Encoding Slow Words Encoding Fast Preprobed Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory 1Q
C-digits
A-digits B-digits
A-digits B-digits
Fast Words Encoding
.56 .59 1.00
.44 .75 1.00
.42 1.00
1.00
,56
.45
.58
.45 .73
.35
.60
.55
1.00
.32
-.03 .30 .46
IQ
.50
.19
,07 .34 .62
Slow Running Memory
.58
.38
,17 .34 .53
Fast Running Memory
1.00
.24
.13 .31 .44
Slow Preprobed Digits
.50
.15 .43 ,56
Digits
Fast Preprohed
.56
.03 .28 .38
Slow Words Encoding
TABLE 2 Correlation Matrix
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TABLE 3 Varimax Rotated Factor Loadings from (a) Principal Components Analysis; (b) Principal Factor Analysis and (c) Rao's Cononical Factor Analysis Factor I
A-digits B-digits C-digits Fast Word Identification Slow Word Identification Fast Preprobed Digits Slow Preprobed Digits Fast Running Memory Slow Running Memory IQ Eigenvalues:
Factor II
(a)
(b)
(c)
(a)
-.30 .36 .77 .08 .36 .65 .55 .75 .90 .73 4.67
.01 .05 .04 .26 .18 -.18 .53 .61 -.11 .12 .12 -.89 ,27~ .28 -.78 .46 .49 -.54 .36 ~ .37 -.36 .60 .-~]3--~.3~_ .96 .93 -.04 .83 .55 -.30 4.77 4 . 6 7 1 . 1 3
(b)
Factor III (e)
.01 .06 .20 .10 .26 .28 .67 .65 .72 .68 .61 .68 .40 .44 .44 .45 .15 .08 .28 .36 1.16 1.13
(a)
(b)
(c)
-.92 .36 .19 -.50 .36 .87 -.09 .31 .13 .00 .00 .07 -.07 .14 .17 -.24 .44 .32 -.23 .38 .25 -.20 .36 .19 -.08 .13 .19 .08 .01 .18 1 . 0 2 1 . 1 2 1.02
DISCUSSION As an aid to establishing more precisely the possible mechanisms underlying Factor I, which is obviously the critical factor concerned in the STM/IQ relationships observed in these children, improbable candidates will first be eliminated. In the introduction, maintenance rehearsal and access were dismissed as possible mechanisms underlying the correlations obtained between IQ and the serial recall of recency items. The results of the factor analyses extends this argument to deal with maintenance under conditions of fast presentation, where rehearsal strategies would be of doubtful importance. The ability to encode serial lists of items, in the sense of identifying the incoming items would also appear to be eliminated as a likely explanation for Factor I, since the word encoding task shows high loadings on Factor II, but not on Factor I. It could, of course, be argued that the word encoding tasks involved semantic encoding whereas the digit tasks did not. Consequently, Factor II should be regarded as a semantic encoding factor, rather than a more general item identification factor. This objection is not really valid, however, since Factor II also showed a high loading from fast preprobed digits and a moderate loading from slow preprobed digits. The above process of elimination leaves trace persistence and the encoding of order information as possible mechanisms underlying Factor I. At first glance, trace persistence would appear to be ruled out by the fairly high loadings on this factor from the preprobed digits tasks, since these tasks should have involved the minimal amount of retention. However, a post hoc analysis based partly on information obtained from the subjects, revealed
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that retention was indeed involved in preprobed digits. Because of the high rates of presentation, subjects did not react to the cueing digit until several more items had been presented. Thus, the response digit was not selected from the presented list, but had to be retrieved from memory. A strong trace would enable this to be done successfully, whereas a weak trace would make this retrieval difficult. There are other data which are inconsistent with a trace persistence hypothesis, however. First, even at the relatively high presentation rate of 4 digits/see, the persistence of primacy items (A-digits) is apparently unrelated to IQ. Second, as already mentioned in the introduction, the ready trainability of A, B, C digits in reading disabled children does not support a hardware notion of trace persistence. And third, Ferretti (1979) has reported similar forgetting curves for retarded and nonretarded subjects, when the initial load is adjusted to allow for differences in immediate memory capacity between the two types of subject. Retention in this task was passive, active maintenance being hindered through the use of a detection task in the retention interval. Under such conditions, it could be argued that the retention curves represent trace decay curves. While retarded/nonretarded comparisons may involve different mechanisms from those involved in comparisons within the normal population, the lack of a trace persistence deficit in retarded subjects seems inconsistent with the notion that such a deficit exists in lower IQ subjects within the normal range. At this point, then, the encoding of order information appears to be the most likely mechanism underlying Factor I. An attempt has been made to test this hypothesis directly, using recognition to measure performance in the serial running memory task (Cohen & Gowen, 1978). Through the use of suitable instructions and lures, item and order memory were measured separately. A significant correlation was found between IQ and recognition memory for the items in the target set, but not between IQ and the memory for the order of these items. However, a post hoc analysis of the stages involved in this task suggested that recognition is probably an inappropriate method for measuring the availability of order information. If the proficiency with which the subject encodes the order of the incoming items is indeed the critical operation underlying Factor I in this study, then certain limitations can be imposed on the importance of this mechanism for determining STM performance on serial tasks. First, although it is possible to separate item information from order information (Factor II) it is really impossible to divorce order information from item information. A more useful distinction would be that between the enco~ing of items and the encoding of items-in-order. Second, the encoding of items-in-order does not present a general problem for lower IQ subjects, as is evidenced by the complete lack of a correlation between performance on A-digits and IQ. Third, rate of presentation does not appear to have much effect on individual
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differences in the encoding of items-in-order. In Cohen and Sandberg's Experiment 3b (1977), the correlation found between IQ and performance on C-digits presented at 1 digit/2 seconds was only fractionally lower than that obtained with the same subjects using a 4 digits/second presentation rate. A similar result has been reported by Lyon (1977), who demonstrated that the difference between good and poor adult performers on slow serial lists did not decrease when presentation rate was increased. This trend is also apparent in Waugh and Norman's (1965) data. Very high rates of presentation may influence individual differences in serial recall, however, by affecting how well defined the actual items are in the items-in-order. This could account for the tendency of fast running memory, but not slow running memory, to load on Factor II. More critical for determining IQ-related individual differences appears to be the state of the STM system when the items are encoded. Items entering an empty system, for example A-digits, do not appear to constitute an encoding problem for the lower IQ children. Items entering an already loaded system do present an encoding problem. In Cohen and Sandberg's (1977) Experiment 3a no systematic differences were found in STM/IQ correlations with length of list. The task used was serial running memory requiring the recall of the final three items. Since the shortest list was only six digits in length, it is concluded that three items may be sufficient to load the system to the extent that a deficit in the encoding of further items-in-order will appear in lower IQ children. This conclusion gains support from the STM/IQ correlations which have been found with B-digits in some of the groups tested, in spite of the low performance levels generally obtained with these items. A further property of the system is that a certain depth of encoding must be involved in processing the items in order to produce loading. Children, representing a wide range of IQ, were given the serial running memory task using a four digits/second rate. The target sets, the final three items in each list, were separated from earlier list items by a one second pause. When the instruction was to recall the final three items in each list, in serial order, all subjects performed perfectly, regardless of IQ. When the instructions were to recall either the three digits preceding the pause or the three digits following the pause, in response to a post-cue, lower IQ children perfomed worse than higher IQ children in recalling the post-pause digits (Cohen & Lavin, 1979). The difference between the two conditions is that in the first condition the children knew that they did not have to retain the items preceding the pause. In the second condition all items had to be encoded for retention in case the recall of items preceding the pause was signalled. These data indicate that simply listening to the first part of the list is not sufficient to load the system. Some encoding in depth on the part of the subject is necessary to put the system on load.
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The above conclusions lead to a picture of a STM system which deals with actively encoded serial items. This system can readily process serial items when empty, regardless of the IQ of its owner, within the limits of the normal range. When loaded with approximately three items, its encoding proficiency decreases, the size of this decrease varying according to the child's IQ (or perhaps reading ability). Encoding proficiency for serial items on overload is trainable, at least in the case of reading disabled children, which suggests that the difference between good and poor encoders should not be regarded as being due to fixed structural properties of the system. At this point, it is perhaps appropriate to make some comment on the items-in-order i n f o r m a t i o n assumed to be critical in the S T M / I Q correlations under study. The suggestion is not that lower IQ children can encode the items into the system, even on overload, hut that the serial order of these items somehow becomes jumbled. If this were the case, then the STM / IQ correlation obtained in the running memory task should disappear if a free recall instruction and scoring were used instead of serial recall. In fact, free order running memory results in a correlation of about the same magnitude as serial running memory (Cohen & Gowen, 1978). Analysis of the output order of the items in the free order condition showed that there was a strong tendency to recall these in serial order in spite of instructions. What appears to differentiate between lower and higher IQ children is not the general capacity to encode items or items-in=order, but the capacity to encode items into an already loaded STM system, an important property of which is that it retains not individual items, but items-in-serial=order. The state of these items-in=order is conceived as constituting a phonological pattern, or what Dornic (1975) calls a phonologically encoded 'chain trace', rather than an ordered series of individual items. In Dornic's model the items-in=order represent a more primitive state of information than do individually encoded items. This conclusion is based on the finding that the recall of items-in-order information does not vary as a function of the amount of attention available for its encoding, whereas the recall of item information does show such a variation. It should be noted that the individual differences data indicate the need for revision of earlier views of the STM system, at least for serial lists. For example, maintenance rehearsal was an important mechanism in determining performance level in the early STM stores (Broadbent, 1958; Atkinson & Shiffrin, 1969); this process does not appear to be involved in determining the performance level of individuals, except perhaps in the case of retarded subjects. Further, in earlier models, overloading the store meant the retention of later items at the expense of earlier items, if these were not rehearsed (Waugh & Norman, 1965). The individual differences data stress the failure of the STM system to encode later items because of the load constituted by
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earlier items. A n d finally, t h e i m p o r t a n c e o f t h e state o f the s y s t e m ( e m p t y or l o a d e d ) i n d e t e r m i n i n g S T M / I Q c o r r e l a t i o n s is o b l i q u e l y at o d d s w i t h D o r n i c ' s (1975) i t e m s - i n - o r d e r e n c o d i n g w h i c h is u n a f f e c t e d by the p r e s e n c e o f a c o m p e t i n g task, at least i n adults. H u n t 0 9 7 8 ) d i s t i n g u i s h e s b e t w e e n "the' d i f f e r e n t i a l efficiency o f m e m o r y a c t i v a t i o n across i n d i v i d u a l s " a n d " t h e details o f the process itself." It m a y be, h o w e v e r , t h a t the s t u d y of differential efficiency in m e m o r y will n o t o n l y a d v a n c e o u r k n o w l e d g e in t h e a r e a o f i n d i v i d u a l differences, b u t will also have i m p o r t a n t i m p l i c a t i o n s for f u t u r e m o d e l s of S T M .
REFERENCES Atkinson, R. C., & Shiffrin, R. M. Human memory: A proposed system and its control processes. In K. W. Spence & J. T. Spence (Eds.), Thepsychology of learning andmotivation: Advances in research and theory. Vol. 2. New York: Academic Press, 1968. Beimont, J. M.,&Butterfield, E.C. Learningstrategiesasdeterminantsofmemorydeficiencies. Cognitive Psychology, 1971, 2, 411~$20. Broadbent, D. E. Perception and communication. New York: Pergamon Press, 1958. Butterfield, E. C., & Belmont, J. M. Assessing and improving the executive cognitive functions of mentally retarded people. In I. Bailer & M. Sternlicht (Eds.), Psychological issues in mental retardation. New York: Psychological Dimensions, Inc., 1977. Butterfield, E. C., Wamboid, C., & Belmont, J. M. On the theory and practice of improving short-term memory. American Journal of Mental Deficiency, 1973, 77, 654-669. Cohen, B. J., Bousfield, W. A., & Whitmarch, G. A. Cultural norms for verbal items in 43 categories. Tech. Rep. No. 22, University of Connecticut, Dept. of Psychology, 1957. Cohen, R. L. A comparison of short-term memory patterns as related to intelligence and reading intelligence. Paper given at the Gatlinburg Conference on Research in Mental Retardation, March, 1978. Cohen, R. L., & Gowen, A. Recall and recognition of order and item information in probed running memory, as a function of IQ. Intelligence, 1978, 2, 353-362. Cohen, R. L., & Lavin, K. The effect of demarcating the target set on IQ-related individual differences in the probed serial recall of very recent items. In M. M. Gruneberg, P. E. Morris, & R. N. Sykes (Eds.), Practical aspects of memory. New York: Academic Press, 1979. Cohen, R. L., & Nealon, J. An analysis of short-term memory differences between retardates and nonretardates. Intelligence, 1979, 3, 65-72. Cohen, R. L., & Netley, C. Cognitive deficits, learning disabilities, and WISC verbalperformance consistency. Developmental Psychology, 1978, 4, 624-634. Cohen, R. L., & Netley, C. Short-term memory deficits in reading disabled children, in the absence of opportunity for rehearsal strategies. Intelligence, in press. Cohen, R. L., & Sandberg, T. The relationship between intelligence and short-term memory. Cognitive Psychology, 1977, 9, 534-554. Domic, S. Some studies on the retention of order information. In P. Rabbitt & S. Dornic (Eds.), Attention and performance. Vol. 5. London: Academic Press, 1975. Ellis, N. R. The stimulus trace and behavioral inadequacy. In N. R. Ellis (Ed.), Handbook of mental deficiency: psychological theory and research. New York: McGraw-Hill, 1963. Ellis, N. R. Memory processes in retardates and normals: theoretical and empirical considerations. In N. R. Ellis (Ed.), International review of research in mental retardation. Vol. 4. New York: Academic Press, 1970.
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Ellis, N. R., McCarver, R. B., & Ashurst, H. M. Short-term memory in the retarded: ability level and stimulus meaningfulness. American Journal of Mental Deficiency, 1970, 75, 72-80. Ferretti, R. P. An analysis of passive memory in normal and mentally retarded persons. Paper given at the Gatlinburg Conference on Research in Mental Retardation and Developmental Disabilities, April, 1979. H/irnqvist, K. Individuella differencer och skoldifferentiering. Statens offentliga utredningar. Stockholm, 1960, 1960, nr. 13. Hunt, E. Mechanics of verbal ability. Psychological Review, 1978, 85, 109-130. Hunt, E., Lunneborg, C., & Lewis, J. What does it mean to be high verbal? Cognitive Psychology, 1975, 7, 194-227. Lyon, D. R. Individual differences in immediate serial recall: a matter of mnemonics7 Cognitive Psychology, 1977, 9, 403--41 I. Mulaik, S. A. The foundations of factor analysis. New York: McGraw-Hill, 1972. Seal, H. Multivariate statistical analysis for biologists. London: Methuen, 1966. Waugh, N. C., & Norman, D. A. Primary memory. Psychological Review, 1965, 72, 89-104.