stuart & hulme, 2000

Jommd of Experimental Psychology: ~ , Memory, and Cognition 2000, Vol. 26, No. 3, 796-802

OBSERVATION

Copyright 2000 by the American Psychological Association, Inc. 0278-7393/00/$5.00 DOI: I0.I0371/0278-7393.26.3.796

The Effects of Word Co-Occurrence on Short-Term Memory: Associative Links in Long-Term Memory Affect

Short-Term Memory Performance

George Smart and Charles Hulme University of York

In immediate serial recall tasks, high-frequency words are recalled better than low-frequency words. This has been attributed to high-frequency words' being better represented and providing more effective support to a redintegration process at retrieval (C. Hulme et al., 1997). In studies of free recall, there is evidence that frequency of word co-occurrence, rather than word frequency per se, may explain the recall advantage enjoyed by high-frequency words (J. Deese, 1960). The authors present evidence that preexposing pairs of low-frequency words, so as to create associative links between them, has substantial beneficial effects on immediate serial recall performance. These benefits, which are not attributable to simple familiarization with the words per se, do not occur for high-frequency words. These findings indicate that associative links between items in long-term memory have important effects on short-term memory performance and suggest that the effects of word frequency in short-term memory tasks are related to differences in interitern associations in long-term memory.

In this article, we offer empirical support for a fundamen- tal change in the way in which word fiequency effects in shcttqetm memory tasks are interpreted. In particular, we show that a~ ia f i tms b~weea lexical mpres~tafions stored in long- term memory influence short-term memory performance.

Repetition Effects

Since Ebbinghaus (1885/1964), the study of simple repetition effects has shown that repeated list items tend to be better recalled than nonrepeated list items. More recently, item repetition has been studied in the context of the spacing effect (Russo, Parkin, Taylor, & Wilks, 1998); the relevant finding was that items repeated at nonadjacent positions in a list are better recalled than items presented only once in a list (see also Crowder, 1976, for a comprehensive review of repetition effects). The repetition of whole lists can also lead to an improvement in recall performance as demonstrated in the Hebb effect (Hebb, 1961). Here, participants are pre-

George Smart and Charles Hulme, Department of Psychology, University of York, York, United Kingdom.

This research was supported by Economic and Social Research Council of the United Kingdom Grant R 000 236 216.

George Stuart is now at the Department of Psychology, Univer- sity of East London, London, United Kingdom.

Correspondence concerning this article should be addressed to Charles Hulme, Deparunent of Psychology, University of York, York, YO1 5DD, United Kingdom, or to George Stuart, Depart- ment of Psychology, University of East London, Romford Road, London, El5 4LZ United Kingdom. Electronic mail may be sent to chl @york.ac.uk or to [email protected].

sented with a series of nine-digit strings followed by immediate serial recall. One of these strings is repeated on every third trial, although participants are not informed of this. Although recall performance for the repeated digit string improves over trials, performance for the nonrepeated strings remains relatively stable. According to Hebb 0961), a momentary encounter with a set of stimuli presented in a particular order will leave a permanent record in secondary memory. More recently, the Hebb effect has been produc- tively explored within and applied to the implicit serial learning paradigm (Curran & Keele, 1993; Stadler, 1993).

Word Frequency Effects

Perhaps the most studied of all repetition phenomena is the word frequency effect. Traditional measures of word frequency offer an estimate of how often the word is encountered in either written or spoken language (Baayen, Piepenbrock, & Van Rijn, 1993; Ku~era & Francis, 1967). The common finding, across a wide range of experiments, is that high-frequency words are better recalled than low- frequency words (see Gregg, 1976, for a review). We (Hulme, Maughan, & Brown, 1991; Hulme et al., 1997) have interpreted such word frequency effects as evidence for a contribution of long-term memory mechanisms to short- term memory performance. More specifically, we have suggested that phonological information stored in long-term memory is used to "clean up" or redintegrate the degraded short-term memory trace and that the availability of a phonological trace in long-term memory is greater for high- than low-frequency words (Hulme et al., 1997). However, the finding that the typical recall advantage for high- frequency words is eliminated when high- and low-

796

OBSERVA~ON 797

frequency words are mixed within the same list (May, Cuddy, & Norton, 1979; DeLosh & McDauiel, 1996) suggests that word frequency is exerting effects that are not operating entirely at the level of individual items. Instead, these findings from studies of mixed lists suggest that the inter-relationships between fist items may be critical in determining recall performance.

I t em Co-Occur rence Effects

An alternative view of the mechanisms underlying the word frequency effect has been proposed by Deese (1959, 1960) in the context of studies of long-term memory. He suggested that the frequency of word co-occurrence, rather than the frequency of occurrence of the words themselves, lay behind the word frequency effect in free recall. It was this suggestion that provided the impetus for the present experiment. Deese derived an index of mean interitem association for lists of randomly selected words by obtaining free-association norms for the words within each list. He found that high-frequency words, on average, had higher interitem association ratings than low-frequency words. Furthermore, when items were selected to form lists with a zero index of interitem association, the difference between high- and low-frequency word lists in a free recall task was eliminated. Deese (1960) concluded that "This implies that there is little or no intrinsic effect of frequency of usage upon recall and that the covariation of recall scores with frequency of usage occurs because of the higher probability of association occurring between high-frequency words" (p. 342).

In the present experiment, we wished to explore the possible importance of interitem associations in short-term immediate serial recall tasks, as opposed to the long-term free recall tasks studied by Deese. To do this in as direct a way as possible, we developed a technique in which interitem associations between words are created by repeated exposure prior to the recall task. We predicted that by creating interitem associations between items in long-term memory, we would improve short-term memory performance for those items. A further, more tentative, prediction was that the creation of interitem associations in this way would have a greater beneficial effect on the recall of low-frequency words than high-frequency words. This fol- lows from the suggestion (Deese, 1960; Howes, 1957) that high-frequency words have stronger interitem associations than low-freqnency words (because high-frequency words are more likely to co-ocour in natural language than are low-frequency words). Such a pattern of findings would provide direct evidence that interitem associations in long- term memory are a determinant of immediate serial recall performance and that word frequency effects are due, at least in part, to variations in interitem associations. By including control conditions in which subjects recalled the same high- and low-frequency words without any prior familiarization, we could assess the extent to which the recall benefits produced by familiarization with item pairs are capable of accounting for the advantage in recall shown by high- over low-frequency words.

Method

Participants

A total of 88 undergraduates (79 female and 9 male) from the University of York participated in the experiment.

Materials and Design

Two sets of 12 one-syllable words were selected on the basis of Kufera and Francis's (1967) word frequency counts. One set comprised low-frequency words with an occurrence of I word per million (gild, rash, sham, cube, jest, kite, pram, perk, rift, shun, smog, weed). The other set comprised high-frequency words with an occurrence of at least 100 words per million (food, back, step, date, hall, town, land, wall, book, farm, club, w~fe ).

Procedure

Participants were randomly assigned to one of four participant groups. In two of the groups, participants recalled items from the set of 12 low-frequency words only, while in the other two groups, lists were all drawn from the set of 12 high-frequency words. One of the groups receiving high-frequency words and one of the groups receiving low-frequency words also received a familiariza- lion phase in which items from the relevant set were presented to them in pairs. Because the procedures for participants in both familiarization groups were identical, a general description will follow. For each participant, the 12 experimental words were divided randomly into two equal sets (Set A and Set B). All words were exhaustively combined within each set to form two blocks of 30 unique ordered pairs (e.g., 1-2, 2-1, 1-3, 3-1, etc.). In the familiarization phase, participants were required to read aloud a series of word pairs presented visually (in upper-case 36-point Geneva font) on a computer monitor while their utterances were recorded. Words in each pair were presented contiguously, and each word remained on the screen for 500 ms. Each word pair was followed by a 1-s interpair interval during which the screen remained blank. Presentation of the 30 word pairs formed from Set A was followed by presentation of the 30 word pairs formed from Set B. This procedure was repeated until the participants had read aloud each of the 60 ordered word pairs 10 times. For participants in the unfamifiarized conditions, the items were divided into two sets in the way described earlier, although there was no familiarization with these sets.

All participants undertook the serial recall task, which for unfamiliarized groups was the start of the experiment and for familiarized groups was stage two of the experiment. Participants were presented with 40 lists to recall, each containing six words. Lists were presented visually in the same format as in the familiarization phase, but with a presentation rate of one word per second and a 500-ms interstimulus interval (ISI). Each trial was initiated by the participant's pressing the computer mouse button. The 40 experimental lists comprised 10 lists of four types; words presented in pure fists were entirely from Set A or Set B ~ A and Pure B), while altemaling lists began either with a word from Set A or from Set B (Alternating AB or Alternating BA). The order of trials was randomized for each participant. Participants recalled each list by writing the words in order on one page of a response booklet. Each page of the booklet had the numbers 1 to 6 in a column on the left-hand side indicating the order of presentation, and participants were required to draw a line to indicate when they could not recall a word in a particular position. A strict serial recall protocol was enforced by ensuring that participants wrote their

798 OBSERVATION

answers in the order of presentation and did not return to any unanswered items.

Results

Responses were scored as correct when the correct item was recalled in the correct serial position. For each participant group, data from the two alternating list conditions were combined, as were data from the two pure list conditions. Figure I shows the proportion of items correctly recalled in each serial position for the pure and alternating lists, collapsed across lists and participants, in the four participant groups: Performance in the high-frequency conditions is plotted in Panel 1 and for the low-frequency conditions in Panel 2. An alpha level of .05 was applied to all statistical tests unless otherwise stated.

The data in Figure 1 were first entered into a 2 × 2 × 2 × 6 analysis of variance (A_NOVA) with word frequency (high vs. low) and familiarization (familiarized vs. unfamiliarized) as between-subjects variables, and list type (pure vs. alternating) and serial position (1 to 6) as within-subject variables. This analysis revealed significant main effects of word frequency, F(1, 84) -- 7.41, M S E = .326, indicating that high-frequency woods (68.9% correct) were better recalled than low-frequency words (59.4% correct); list type, F(1, 84) -- 12.82, M S E -- .014, reflecting the fact that pure lists (65.4% correct) were better recalled than alternating lists (62.8% correct), and serial position, F(5, 420) = 113.75, M S E = .040. The main effect of familiarization was not significant, F(I , 84) = .93, M S E = .326, ns, nor was the interaction between word frequency and familiarization, F(1, 84) -- 2.20, M S E = .326, ns. Thus familiarization, per

se, had no significant effect on recall accuracy. However, there was a significant interaction between list type and familiarization, F(1, 84) = 12.09, M S E = .014, showing that the co-occurrence effect occurred with familiarized fists but not with unfamiliarized lists, and most critically there was also a significant three-way interaction between list type, word frequency and familiarization, F(1, 84) = 5.70, M S E = .014. From Figure 1 it can be seen that this interaction reflects the fact that an increase in recall for the pure lists after familiarization occurs for low-frequency words, but not for the high-frequency words. There was also a significant interaction between word frequency and serial position, F(5, 420) = 2.35, M S E = .040, which reflects the fact that recall declines across serial positions somewhat more steeply for low-frequency than for high-frequency words (cf. Hulme et al., 1997). No other effects in this analysis were significant.

The critical three-way interaction between list type, word frequency and familiarization was further explored by performing three-way (2 × 2 × 6) ANOVAs on the high- and low-frequency word data separately. For high-frequency words, the only significant effect was that of serial position F(5, 210) = 73.50, M S E = .031. Neither the effect of list type nor familiarization was significant, F(1, 42) = 1.91, M S E = .014 and F(1, 42) = .17, M S E = .254, respectively, nor did these two factors interact, F(1, 42) = .58, M S E -- .014, ns. By contrast, for low-frequency words there were significant main effects of serial position, F(5, 210) = 49.53,

• M S E = .049, and list type, F(1, 42) = 14.67, M S E = .016, showing that pure lists were recalled significantly better than alternating lists. This main effect of list type was modified

1.0

0.8

0.6

0.4

0.2

0.0

High.frequency words Low-frequency words

1.0

~ ~ ~ , ~ 0.8

" , . . . . ; ¢ , ,Q , 0.6

0.4

: ~ ~ rna~ng - - ' . a - - UnfamCWtzed Pure 0.2 - - - 0 - - ~ , ~ ' ~ g

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¢:1~. "° A, I=amlliarlzed Pure ........ I

I - Familiarized Altes'naUng - - - ~ - - Unfamllladzed Pure - - .-o- - Unfamlliartzed N t ~ n g

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1 2 3 4 5 6 1 2 3 4 5 6

Sedal Position Sedal Pot ion

Figure 1. Spoken recall of high- and low-frequency words as a function of familiarization and list type.

OBSERVATION 799

by a significant interaction between list type and familiarization, F(1, 42) = 1411/7, MSE = .016, reflecting the fact that list type had a significant effect on familiarized words, but not on unfamiliarized words. Tests of simple effects confirmed that recall of the pure lists of familiarized words was superior to recall of the alternating lists of familiarized words F(1, 42) = 25.41, MSE = .01, but that there was no reliable difference between recall of the pure and alternating lists in the unfamiliarized condition F(1, 42) = 0.01, MSE = .01. R is also important to note that the main effect of familiarization, F(1, 42) = 1.16, MSE = .41, was not significant in this analysis; thus, even in the case of low-frequency words, simple familiarization with the words used in the experiment had no significant effect on recall

One other critical effect apparent from Figure 1 is that recall of the pure familiarized lists of low-frequency words appears to be at an equivalent level to that for familiarized high-frequency words (mean correct recall of 67.7% and 69.1%, respectively). A t-test confirmed that recall of the familiarized pure list low-frequency words and the familiarized pure list high-frequency words did not differ significantly, t(21) = .242, ns. Thus, the associative link~ between low-frequency words produced as a result of our paired familiarization procedure have completely eliminated the effects of word frequency on serial recall that are present in the unfamiliarized conditions of this experiment.

The pattern of recall errors made in the experiment was analyzed to see what light it shed on the processes affected by inducing interitem associations. Errors in recall were classified as order errors, where an item that had been presented in the list was recalled in the wrong position, and item errors, where participants failed to recall the correct item. Item errors were in turn broken down into three subtypes: (a) omissions, where no response was given; (b) extraset intrusions, where a nonexperimental word was recalled; and (c) intraset intrusions, where an item recalled was from one of the two pools of experimental items that had not been presented on that particular trial. These error

rates are shown in Table 1, which, for completeness, also shows the proportion of items correct in each condition.

We will first consider the data for order and item errors, collapsing across the different item error types. For the purposes of these analyses, order errors were conditional- ized on the number of items correctly recalled in each condition. It is clear that the rate of order errors is fairly constant across all eight conditions of the experiment (ranging from 11% to 14%), and this was confirmed by a 2 (high vs. low-frequency) X 2 (familiarized vs. unfamiliarized) × 2 (pure vs. alternating) ANOVA that showed no main effects of either word-frequency, F(1, 84) = 1.57, MSE = .034; list-type, F(1, 84) = .001, MSE = .002; or familiarization, F(1, 84) = .52, MSE = .034. None of the interactions were significant.

In contrast, there were large differences between conditions in the proportion of item errors made. An equivalent 2 × 2 × 2 ANOVA for the item error data revealed a significant main effect of list type, F(1, 84) = 33.02, MSE = .002, confirming that there were significantly more item errors in the alternating lists than in the pure lists. The main effect of word frequency was also significant, showing that more item errors occurred for low-frequency words than for high-frequency words, F(1, 84) = 20.47, MSE = .026. These main effects were modified by a significant two-way interaction between list type and familiarization, F(1, 84) = 6.27, MSE = .002, indicating that familiarization produced a selective reduction in item errors in the pure lists. Finally, a significant three-way interaction between familiarization, list type, and word frequency, F(1, 84) = 4.26, MSE = .002, reflects the fact that after familiarization, the difference between pure and alternating lists was particularly marked for low-frequency words. This pattern precisely parallels the pattern found for the overall recall scores reported earlier. In summary, these analyses of item and order errors support the conclusion that the effects of intefitem association obtained in this experiment reflect changes in memory for item

Table 1 Proportion of Items Correctly Recalled and Proportion of Different Error Categories in Each Condition in the Experiment

EITors

Item

Extraset Intraset Condition Correct Order Total Omissions intrusions intrusions

Familiarized Pure I-IF .691 .137 .159 .122 .000 .038 Alternating HF .669 .127 .193 .132 ,000 .061 Pure LF .677 .106 .202 .161 ,001 .040 Alternating LF .593 .117 .276 .213 .000 .063

Unfaraifiarized Pure I-IF .702 .139 .152 .099 .003 .050 Alternating I-IF .695 .119 .178 .121 .002 .055 Pure LF .548 .137 .304 .224 .028 .052 Alternating LF .553 .126 .319 .225 .024 .066

Note. HF ffi high-frequency words; LF = low-frequency words.

800 OBSERVATION

information without any selective changes in memory for order information.

When item errors, in turn, are broken down into the subcategories shown in Table 1, an interesting pattern emerges. First, it is noteworthy that participants in the familiarized groups virtually never made extraset intrusions; that is, they did not recall items that had not been presented in the experiment. In the unfamiliarized groups, these errors did occur but only with any regularity for the low-frequency word lists. A 2 x 2 X 2 ANOVA on the proportion of extraset intrusions for each condition revealed significant main effects for both word frequency, F(1, 84) = 12.96, M S E -- .001, and familiarization, F(1, 84) = 17.37, M S E = .001. There was no significant effect of list type, F(1, 84) = 1.70, M S E = .000. The only significant interaction was between word frequency and familiarization, F(1, 84) = 11.63, M S E = .001, reflecting the fact that the extralist errors occurred almost exclusively for the recall of low- frequency unfamiliarized lists.

By comparison, there were a moderate number of intraset intrusions in all conditions (rangitig from roughly 4% to 7% across conditions). An ANOVA on the proportions of intraset intiusions revealed only a main effect of fist type, F(1, 84) = 21.69, M S E = .001, confirming that there were more of these errors in the alternating list than the pure list conditions. This effect shows that in the pure list conditions participants were able to benefit from having the response set restricted to just six items that were presented in different orders on any given trial, whereas in the alternating lists the response set consisted of 12 possible items only half of which were present on any given trial. This effect, however, is small in magnitude.

By far the largest proportion of item errors were omissions (ranging from roughly 10% to 23% across conditions). An ANOVA on the proportion of omission errors revealed significant main effects of word frequency, F(1, 84) = 14.64, M S E = .024, and list type, F(1, 84) = 18.78, M S E = .001, and a significant three-way interaction between word frequency, list type, and familiarization, F(1, 84) = 9.81, M S E = .001. This interaction reflects the fact that after familiarization the difference between pure and alternating lists was particularly marked for low-frequency words. This pattern parallels that found for the overall recall scores and for overall item errors reported earlier. These analyses support the idea that the selective beneficial effect of item co-occurrence on the recall of low-frequency words found here reflects a selective reduction in the number of omission errors made.

Discussion

We have investigated the effect of familiarization with word pairs on the subsequent recall of those same words in an immediate serial recall task. The familiarization with pairs of words was designed to increase the strength of interitem associative links between the representations of these words in long-term memory. As predicted, we found a substantial improvement in the subsequent serial recall of low-frequency words following familiarization with item pairs, but no equivalent change in the recall of high-

frequency words. Perhaps one of the most striking aspects of our findings is that after familiarizatila with item pairs, low-frequency words were recalled as accurately as high- frequency words. Finally, comparison of the two alternating preexposure conditions with the no-preexposure conditions (in which participants were required to recall lists of low- or high-frequency words without any familiarization) showed that preexposure alone had no significant effect on recall performance. This, coupled with the finding that inducing interitem associative links between low-frequency words boosted their recall to the same level as high-frequency words, suggests that the recall advantage for high-frequency words in immediate serial recall tasks may be explained entirely in terms of greater interitem associative links between these items.

It should be emphasized that, in the familiarized conditions, all items presented in the serial recall task had been presented to participants equally often during the course of the experiment and that only the frequency of co-occurrence differed between the pure and alternating lists. Whereas all item pairs presented in the pure list conditions had co- occurred during familiarization, adjacent test items in the alternating lists had not. Because both ordered combinations of each pair (e.g., 1-2 vs. 2-1) had been presented equally often in the familiarized pure lists, it cannot be said that participants had simply learned a particular order of items. Similarly, because all items had been presented equally often, simple familiarity with the items themselves cannot account for this effect. This finding has important implica- tions both for theories of short-term memory and for the way in which word frequency effects are interpreted. We will consider each of these issues in the remainder of this discussion.

Our previous studies of the effects of word frequency on short-term memory (Huhne et al., 1997) were interpreted as evidence that long-term memory processes influence short- term memory performance. We argued, essentially, that word frequency was an item-level variable and that high- frequency words were intrinsically easier to recall. More specifically, we argued that the phonological representations of high-frequency words were better specified or easier to access than the representations of low-frequency words, and that these differences between the representations of words affected a process of redintegration (trace reconstruction) at retrieval. The present experiment also demonstrates an influence of long-term knowledge on short-term memory performance, but one that is independent of the frequency with which items have been presented in the course of an experimental session. Crucially, however, this effect of long-term knowledge on short-term memory performance cannot be explained at the level of individual items. Instead it is the relationship between items, or more specifically the artificially induced associative links between items, that must underfie the item co-occurrence effect.

The impetus for the present experiment came from the suggestion by Deese (1960) that in free recall, the recall advantage for high-frequency words was attributable to interitem associations. Deese's argument was that high- frequency words co-occur statistically more often in natural language than do low-frequency words (cf. Howes, 1957).

OBSERVATION 801

Strikingly, he demonstrated that there was no difference in the free recall of high- and low-frequency word lists when the words were selected to have equivalent (zero) interitem associations (Deese, 1960). This finding is consistent with the idea that in free recall interitem associations are entirely responsible for the recall advantage of high-frequency words. The present experiment demonstrates beyond doubt that interitem associations, produced as a result of preexposure to word pairs, exert a powerful effect on immediate serial recall. This in turn suggests that the recall advantage of high-frequency words in immediate serial recall tasks may be explained in terms of interitem association differences.

An important additional question to consider is the extent to which word frequency effects in serial recall can be explained purely in terms of interitem association values. The data from the present experiment are certainly consistent with the idea that they might be. Statistically, the recall of preassociated low-frequency words was indistinguishable from the recall of high-frequency words, and the recall of high-frequency words was unaffected by such preassocia- tion training. Furthermore, prccxposure alone did not significantly affect recall performance. This is exactly the pattern that would be predicted by the view that the frequency effect in serial recall is entirely mediated by interitem associations, but this is a radical suggestion and would require further experiments to support it. For example, it is possible that there are weaker effects of the frequency of item occurrence that are independent of item association frequency, but that were masked in the present experiment, It is important to note that in the present experiment, small sets of items were presented repeatedly for participants to recall. It is conceiv- able that, if items were ever presented only once in an experiment, differences in recall between high- and low- frequency words might be greater. In the present study there was a small, though statistically nonsignificant, recall advantage for familiarized over unfamiliarized lists of low- frequency words, and the pattern of recall errors showed that extraset intrusions occasionally occurred but only to an appreciable extent in the unfamiliarized low-frequency conditions. This suggests that for low-frequency items familiarity with the item set used in an experiment may have a small effect in helping participants to retrieve the correct items. However, it is certainly clear that in the present experiment any effect of item familiarity per se was very small in comparison to the effect of item co-occurrence.

The present findings may usefully be related to our earlier suggestion that word frequency operates to facilitate a redintegrative process in serial recall (Hulme et al., 1997). That account was based on the idea that redintegration was more efficient for high-frequency items because they had more accessible or better specified representations in long- term memory. As a modification to this view, the present experiment suggests that the availability of item representations in long-term memory necessary to support redintegration is also dependent upon interitem associative links. We suggest that interitem associations may create a mutually supportive network of item nodes that makes a long-term memory representation of each candidate item more accessible at the point of item retrieval. This view, in turn, goes some way toward accounting for the finding that the typical

recall advantage for high-frequency words is eliminated when high- and low-frequency words are mixed within the same list (DcLosh & McDaniel, 1996; May et al., 1979) because the relationships between list items are critical in determining recall performance. However, our finding that interitem associative links in long-term memory are critical for explaining the typical recall advantage for high= frequency words cannot explain the finding that low- frequency words are sometimes recalled better than high- frequency words within a mixed frequency list (DcLosh & McDaniel, 1996; Nalrne, Rieglcr, & Serra, 1991). Further- more, without further assumptions, the idea that interitem associations underlie the superiority in recall of high- frequency words would lead us to expect that in mixed lists of high- and low-frequency words recall should be better in lists in which the proportion of high frequency words is greater. However, data presented in DeLosh and McDaniel (1996) do not support this prediction. Such effects were interpreted by DeLosh and McDaniel in terms of variations in the strength of item and order encoding processes. Specifically, they argued that low-frequency words embed- ded in lists of high-frequency words might attract dispropor- tionate attentional resources to their encoding, but at the expense of allocating fewer attentional resources to the encoding of order information across the rest of the list. We have not compared the effects of mixing words of different frequency in the same list in this study, but it is clear that such effects cannot be explained by attributing the effects of word frequency on immediate serial recall entirely to differences in interitem associative links in long-term memory.

It is of some interest to contrast the results of the present study with findings from studies of the Hebb effect. The parallel between the Hebb effect and the item co-occurrence effect demonstrated here for low-frequency words is clear in that a relatively small number of exposures to the stimuli was sufficient to improve short-term recall performance. However, the order learning assumed to be operating in the Hebb effect is plainly absent in the present experiment. On the basis of the co-occurrence hypothesis, we argue that the association strength between items presented in any repeated sequence will strengthen with each repetition, and so our study differs from the Hebb effect insofar as we have demonstrated that it is not order, per se, that is remembered. Rather, there is some support between the representations of the items themselves, based on association strength. Indeed, it is a striking aspect of the present data that interitem associations produce improvements in item memory for low-frequency words, with no effect on memory for order information.

Finally, it is interesting to note certain similarities between the present findings (and our interpretation of them) and recent accounts of semantic priming. Moss, Hare, Day, and Tyler (1994) have argued that associative priming may also be explained in terms of lexical co-occurrence. They noted that associates such as pil/ar and society will prime one another in the absence of any semantic association between the words involved. In line with Fischler (1977), they suggested that these associations arise through lexical

802 OBSI~VATION

co-occurrence. Using a simple recurrent network, Moss et ai. were able to demonstrate how frequent item co- occurrence during a training phase could lead to increased associative priming over and above that from semantic priming.

In summary, inducing associative links between pairs of low-frequency words has powerful effects on how well such words are recalled in a short-term memory task, but there is no equivalent effect for high-frequency words. Perhaps even more strikingly, in the present experiment a brief period of such associative training completely eliminated the usual recall advantage for high- over low-frequency words. These findings show that interitem associative llnk~ play a major role in accounting for differences in the recall of high- and low-frequency words in immediate serial recall tasks, just as they do in free recall tasks (Deese, 1960).

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Received March 5, 1999 Revision received July 20, 1999

Accepted September 23, 1999 •

stuart & hulme, 2000

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