Stochastic Independence Between Two Implicit Memory Tasks

Journal of Experimental Psychology: Learning. Memory. and Cognition 1989, Vol. 15. No. I. 22-30

Copyright 1989 by the American Psychological Association, Inc. 0278-7393/89/$00.75

Stochastic Independence Between Two Implicit Memory Tasks Dawn Witherspoon

Morris Moscovitch

Brock University, St. Catharines, Ontario, Canada

Erindale College, University of Toronto, Mississauga, Ontario, Canada

Two experiments tested the statistical relation between performances on two implicit, priming tests of memory: word-fragment completion and perceptual identification. If performance on implicit priming tests is mediated by a single memory system, then stochastic dependence between them should be found. Contrary to this prediction, stochastic independence was obtained between performances on the two tests in all but one condition. For that condition (Experiment 1) the results suggest that subjects may have knowingly used information derived from the first test to complete the second. When perceived similarity was eliminated by altering contextual cues (Experiment 2), stochastic independence was found in this condition as well. The results are not consistent with current multiple memory system interpretations that assign all implicit, priming tests to one system and explicit tests to another. An alternative interpretation is that the degree of dependence between performances on memory tests is determined by the similarity of the component processes that the tests engage and of the information they use.

speed (Kolers, 1974, 1976), perceptual identification of degraded words (Jacoby & Witherspoon, 1982), lexical-decision latencies (Moscovitch, 1985; Scarborough, Cortese, & Scarborough, 1977), and word-fragment completion (Tulving, Schacter, & Stark, 1982). The effects of a single presentation of a study item is sufficient to facilitate performance for at least 24 hr on perceptual identification (Jacoby & Dallas, 1981), 48 hr on lexical decision (Scarborough et al., 1977), a week on word-fragment completion (Tulving et al., 1982), and a year on speeded reading of transformed script (Kolers, 1976). Phenomenologically, however, the subjects are often not aware that the test items had been studied earlier. Similarly, manipulating levels of processing (Jacoby, 1983; Jacoby & Dallas, 1981; Graf, Mandler, & Haden, 1982; Graf & Schacter, 1985) and frequency of occurrences (Jacoby & Witherspoon, 1982; Kates, 1986; Roediger & Blaxton, 1987) has a greater influence on explicit tests such as recognition and cued recall than on implicit tests such as perceptual identification and word-stem completion. Changes in presentation format, modality, and color have the opposite effect (Jacoby & Dallas, 1981; Morton, 1979; Moscovitch, Winocur, & McLachlan, 1986; Roediger & Blaxton, 1987). The most compelling evidence for functional independence comes from testing amnesic patients whose performance on explicit memory tests is severely impaired whereas their performance on some implicit tests is normal (Jacoby & Witherspoon, 1982; Moscovitch, 1984; Squire, 1986; Tulving, 1985). Unlike functional independence, statistical or stochastic independence is not based on comparing the average performance on two tests, but rather on determining whether performance on a particular item on one test predicts performance on that same item on another test. If no predictive relationship is found, then the tests are assumed to be independent. Performance on perceptual identification and word-fragment completion were both found to be independent of performance on recognition (Jacoby & Witherspoon, 1982; Tulving et al., 1982).

Evidence for a dissociation between performances on explicit and implicit tests of memory has been based on demonstrations of functional and statistical independence between the two types of tests. In this article we show that statistical independence can also occur between two implicit tests of memory. Explicit memory tests, such as recognition and recall, are those that require conscious recollection of an experienced event or deliberate reference to a prior episode. Implicit memory tasks assess influences of memory on behavior without the subject necessarily being aware of consulting memory, as occurs when testing the effects of previously exposed material on reading speed in which no explicit recollection of the target items is required or is even possible (e.g., Graf & Schacter, 1985; Kolers, 1974, 1976; Moscovitch, 1984; Witherspoon, 1983). Explicit and implicit tests are assumed to be functionally independent if performance on these tests is affected differently, preferably in opposite directions (as in a crossover interaction) by an independent variable. Positive evidence of functional independence can be found in studies showing that retention interval affects performance on explicit tests more than it does on implicit tests. Thus, recognition decays more rapidly with lag between study and test than does reading This research was supported by grants from the Natural Sciences and Engineering Research Council (A8347 to Morris Moscovitch and A 1295 to Dawn Witherspoon). The authors express their appreciation to F. I. M. Craik, P. Graf, L. L. Jacoby, D. Schacter, E. Tulving, and G. Winocur for valuable comments on earlier drafts of this article. We are also grateful to Doug Nelson, Walter Schneider, Henry Roediger, and some other anonymous reviewers for their suggestions and criticisms. Finally, we thank Amrit Singh of Toronto, Ontario, and Heather MacLeod of Sackville, New Brunswick, for assisting with the testing of subjects. Correspondence concerning this article should be addressed to Morris Moscovitch, Psychology Department, Erindale College, Uni[ versity of Toronto, Mississauga, Ontario, Canada L5L 1C6. 22

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STOCHASTIC INDEPENDENCE Of the two types of independence, statistical independence may be a more powerful demonstration of a dissociation between explicit and implicit tests of memory (Tulving, 1983, 1985, pp. 393-395). As well as occurring between implicit and explicit tests, functional independence can also be found between two tests of explicit m e m o r y as evidenced, for example, by the differential effect of age and word frequency on recognition and recall (Craik, 1977; Deese, 1961; Hall, 1954). Functional independence cannot, therefore, serve as a strong criterion for a dissociation between explicit and implicit memory tests. Complete statistical independence, however, has only been reported between explicit and implicit tests. Comparisons between recognition and recall have always yielded at least a small, but statistically dependent, relationship (e.g., Flexser & Tulving, 1978). To account for the evidence of statistical (and functional) independence and neurological dissociations between performances on implicit and explicit tests of memory, a number of investigators have proposed that performance on the two types of tests is mediated by separate memory systems (e.g., Cohen & Squire, 1980; Mishkin, Malamut, & Bachevalier, 1984; Moscovitch, 1982a; Schacter, 1985; Squire, 1986; Tulving, 1983, 1985). The many versions of a multiple system model of memory share the assumption that performance on repetition priming tasks reflects the operations and representations of a single memory system, which Tulving (1985) dubbed the QM system, that is distinct from both episodic and semantic memory systems. It follows from this assumption that statistical independence should not occur between two implicit tests of memory that involve repetition priming and that are presumed to be mediated by the same system. Just as with two explicit tests, at least a small, but reliably dependent, relationship should always be found (Flexser & Tulving, 1978). To our knowledge, no one has ever tested this prediction. Failure to confirm this prediction would require either the rejection or substantial revision of the multiple memory systems hypothesis, or the reevaluation of the use of statistical independence as evidence for it. To test the prediction, we compared performance on two implicit tests of memory: perceptual identification and wordfragment completion. We chose these two tests because they have been identified with priming (Tulving, 1985) or with procedural m e m o r y (Cohen, 1984), and performance on them has been shown to be stochastically independent of recognition memory; In addition, the two tests struck us as sufficiently different from each other to provide a fair test of the prediction. Despite their possibly sharing c o m m o n representations of the study items and thus being susceptible to the influence of c o m m o n graphemic and lexical factors, the two tests otherwise d e m a n d quite different processing skills, strategies, and information for successful performance. Whereas perceptual identification is dependent primarily on rapid visual decoding of briefly presented, degraded visual information, word-fragment completion does not stress the processing resources of the visual system. Instead, it is a more conceptual, problem-solving task that requires lexical search, knowledge of spelling rules and of letter combination frequencies to generate the appropriate exemplars.

In our experiments, subjects studied a list of words and then were tested on word-fragment completion and perceptual identification. Word-fragment completion requires the evaluation of incomplete words, such as R _ _ _ B _ W (i.e., RAINaOW). Subjects were instructed to complete the fragment as an English word. Perceptual identification involves the identification of items that are briefly presented. Both previously studied target words and words not previously seen in the experiment served as test items. To determine whether performance on the two tests was statistically independent, a contingency analysis was performed on the results and the conditional probabilities were assessed. Because sequential ordering effects have been acknowledged as a possible source of dependency in the contingency analyses of memory retrieval (see Humphreys & Bowyer, 1980; Tulving et al., 1982), order of presentation was counterbalanced across subjects. To control for the possibility that statistical independence can occur spuriously even between performances on identical tests; two groups received the identical test, word completion or perceptual identification, twice in succession. Experiment 1

Method Subjects Sixty-four University of Toronto and Mount Allison University undergraduate students took part in the study and received course credit or $5 for their participation. Each subject was assigned randomly to one of four conditions. The conditions varied in the type and order of test presentation. The experimental group of subjects were tested either with word-fragment completion (WC) followed by perceptual identification (PI; order WC-PI) or in the other order (i.e., PI-WC). The control groups were tested twice with word-fragment completion (order WC-WC) or twice with perceptual identification (order PI-PI). Thus, all subjects were presented with a study (reading) phase followed by two test phases. Subjects were tested individually in a single session that lasted approximately 1 hr.

Materials One hundred and eight words were selected from the word list provided by Tulving et al. (1982). Items were either seven or eight letters long and primarily low frequency in occurrence. Twelve items were used solely for practice prior to the test sessions (six items each). The remaining words were included as test items and were counterbalanced for test item status (target or novel). Presentation positions were randomized (two orders). Study. A list of 48 words was constructed for study-list presentation, half of which were designated as the target items (A). The remaining 24 items (B) were not presented again in the experiment. Target items were to be represented again in both test phases. Thus, these items (A) will be referred to as study-test items. All items were presented individuallyfor a 1-s duration on an Apple II Plus computer screen. Test 1 A second list of 48 words was prepared consisting of the 24 study-test items (A) and 24 new items (C) that served as a baseline control. The materials for this first test session were presented in one

24

DAWN WITHERSPOON AND MORRIS MOSCOVITCH

of two formats, depending on the condition assigned: Subjects received either the WC task (Orders WC-PI and WC-WC) or the PI task (Orders PI-WC and PI-PI). For WC, partial words (R _ _ _ B _ W) were presented individually by an Apple II Plus computer. Trials were initiated by the subjects. Each item was then in view until the subject responded with a word and terminated the display or until a maximum of 60 s had elapsed. For PI, words were exposed on a computer screen for 33.33 ms for all subjects (see Reed, 1979). At this rate novel items were perceived accurately approximately 50% of the time (Jacoby & Dallas, 1981; Jacoby & Witherspoon, 1982). Warning signals, indicating the space in which the test item would appear (- -), preceded each item for a duration of 500 ms. A mask (&&&&&) immediately followed. The brief exposure of the stimulus, in addition to the masking procedure, made identification of the item a difficult task. Test 2. The final list included the 24 original items (A) and 24 new items (D) not previously encountered in the experiment. These new items (D) yielded a measure of baseline performance. In addition, the 24 new items (C) from Test 1 were also included to determine the amount of transfer obtained between the two tasks without the benefit of a study presentation. The materials for Test 2 were presented to the subject in one of two formats, depending on group assignment: Subjects received either the WC task (Orders PI-WC and WC-WC) or the PI task (Orders WC-PI and PI-PI).

Procedure Each phase of the experiment was introduced individually, thereby minimizing task requirement expectations. The subjects were given no indication that memory was to be tested. In the study phase, each subject was seated in front of the computer at a distance of 70 to 75 cm. The subjects were told that we were interested in reading skills. They were instructed simply to attend to and read aloud each word as it was presented on the computer screen and to read it aloud. Subsequent memory tests were never mentioned. Incorrect responses were infrequent and did not alter overall levels of performance on later tests. After a 20-min delay during which subjects completed simple math problems, the 16 subjects in the first condition (WC-PI) received the WC task as Test 1 and the PI task as Test 2, whereas the 16 subjects in the second condition (PI-WC) received the tests in reverse order. The remaining subjects received either the WC task or the PI task as Tests 1 and 2. For the WC task, subjects were told that as a test of reading skills, they were to produce a legitimate English completion of a word fragment (only one accurate completion was possible, e.g., R _ _ _ B _ W to be completed as RAINBOW). For the PI tasks, subjects were told that we were interested in how they could read aloud words that were rapidly presented. Subjects were not informed of the potential memory component involved in either of these tasks. In both WC and PI, six practice trials were presented before the test officially began to familiarize the subjects with the procedure. Prior to each trial presentation, a message was printed on the computer (PRESSBUTTONWHEN READY).When the subjects were prepared, they would press a key to initiate stimulus presentation. Warning signals appeared on the screen followed immediately by the test item. Subjects were required to respond by reporting their answer out loud and pressing a response key in front of them. Guessing was encouraged. Verbal responses and response times were collected.

Results and Discussion Before p r e s e n t i n g the c o n t i n g e n c y analysis, w h i c h is o f p r i m a r y interest, we first p r e s e n t t h e a c c u r a c y a n d latency o f correct responses (see T a b l e 1).

Table 1

Experiment 1: Mean Proportion of ltems Correct and Response Timesfor PerceptualIdentification (PI) and Word Completion (WC) Study-test

Novel

Test

M

RT

M

RT

Test 1 WC-PI PI-WC WC-WC PI-PI

.51 .65 .52 .63

2,752 932 2,942 1,2t4

.29 .37 .26 .36

6,360 1,161 5,606 1,395

Test 2 WC-PI PI-WC WC-WC PI-PI

.67 .56 .51 .64

1,171 2,460 2,120 1,026

.39 .23 .23 .33

1,606 3,660 4,741 1,268

Note. RT

Test only M

RT

.47 .33 .27 .41

1,335 2,927 2,957 1,058

= response time. Response times are given in milliseconds.

Analysis of Accuracy and of Latency of Correct Responses Separate analyses were c o n d u c t e d for Tests 1 a n d 2. (For all analyses r e p o r t e d in this article, the significance level was set at .05.) T h e results o f the analyses c a n be s u m m a r i z e d as follows. F o r Test I we c o n d u c t e d a 2 x 2 analysis o f v a r i a n c e (ANOVA). A c c u r a c y was higher for identification t h a n for c o m p l e t i o n , F(1, 62) = 10.5, MS~ = 20.4, a n d for previously p r e s e n t e d i t e m s (A) even if they were studied for 1 s, t h a n for n o v e l (C) items, F ( I , 62) = 194.4, MSe = 6.3. Analysis o f response t i m e s yielded similar results except t h a t there was a significant i n t e r a c t i o n i n d i c a t i n g t h a t p r i o r experience affected w o r d c o m p l e t i o n m o r e t h a n identification. F o r Test 2 we c o n d u c t e d a 4 x 3 ANOVA. A c c u r a c y was higher for identification t h a n for c o m p l e t i o n , F(3, 60) = 5.25, MSe = 33.24, a n d differed a m o n g i t e m s F(2, 120) = 167.9, MSe = 5.23. O r t h o g o n a l c o m p a r i s o n s i n d i c a t e d t h a t target i t e m s (A) p r e s e n t e d at b o t h study a n d Test 1 were identified m o r e accurately t h a n all o t h e r items, F(1, 120) = 317.0, MSe = 5.23, w h i c h a c c o u n t s for 94% o f t h e variance. A c c u r a c y was also higher for test-only i t e m s (C) t h a t were identified or c o m p l e t e d correctly in Test 1 t h a n for n o v e l i t e m s (D), Fz(1, 20) = 18.74, MSe = 5.23. Analysis o f response t i m e s yielded similar results.

Contingency Analysis of Study-Test Items T o assess t h e predictive relation b e t w e e n levels o f p e r f o r m ance o n the two tasks, the d a t a were evaluated by a chi-square test o f i n d e p e n d e n c e ( p r o v i d e d in T a b l e 2). A l t h o u g h routinely used for this purpose, technically the test m a y n o t b e legitimate. T h e s a m e subjects c o n t r i b u t e d different n u m b e r s o f o b s e r v a t i o n s to the various cells, thereby violating t h e a s s u m p t i o n t h a t t h e o b s e r v a t i o n s s h o u l d be i n d e p e n d e n t . M i n d f u l o f these concerns, we subjected t h e data to Flexser's a d j u s t m e n t correction, w h i c h in essence is a h o m o g e n i z i n g t e c h n i q u e for extracting subject a n d i t e m covariates, t h e r e b y e l i m i n a t i n g two possible sources o f c o n t a m i n a t i o n (see also G e n e r a l Discussion section; Flexser, 198 l; a n d H i n t z m a n , 1980, for e x t e n d e d discussion). I f t h e r e is a d e p e n d e n t relation

25

STOCHASTIC INDEPENDENCE Table 2

Experiment 1: Stochastic Independence Analysis on Correct Test 1 (T1) or Test 2 (T2) Responses and Incorrect Test Responses (T1, T2) Cells -

-

Marginals m

Condition T 1, T2 T 1, T2 T 1, T2 T 1, T2 T1 WC-PI Study-test .380 .286 .128 .206 .508 Test only .190 .281 .102 .427 .292 PI-WC Study-test .384 .179 .270 .168 .654 Test only .185 .143 .182 .490 .367 WC-WC Study-test .396 .112 .120 .372 .516 Test only .195 .073 .063 .669 .258 PI-PI Study-test .487 .151 .141 .221 .628 Test only .250 .164 .107 .479 .357 Note. CP = conditional probabilities; WC = word completion; PI = perceptual identification.

between performances on the two tasks, a significant chi square value is obtained, ×2 (1, N - 384) = 3.63, p = .05, showing that successful performance on one task would predict the outcome of the other task. Conversely, if stochastic independence is obtained, identified by a nonsignificant chisquare, the probability of successfully responding to an item on the first test is not predictive of success on the second test. For all chi-square tests reported in this article df = 1, N = 384. The significance level is set at .05. The chi-square values obtained, x 2 = 12.01, indicate that if an item is completed accurately, it will likely be identified accurately on the subsequent PI test. The conditional probabilities emphasize this result. The probability of correctly identifying a target word, given it was successfully completed, is .75, which is greater than the unconditional probability, .67, of identifying any target item. However, the same analysis provides no indication or dependence between performances on the tasks if they are presented to the subject in the opposite order, PI-WC: x 2 = 1.81. The probability of completing a target word, given it was identified, .59, is essentially equal to the overall probability of completing any target word, .56. The phi coefficients in Table 2 provide a measure of correlation for the two tasks, based on chi-square and the n u m b e r of contributing data points. When the same test is repeated, however, an item that is correct on the first attempt will likely be correct on the second attempt, regardless of whether the task was to identify a briefly presented stimulus or solve a word fragment, WC-WC: x 2 = 110.44; PI-PI: x 2 = 53.05. The difference between the W C - P I condition, in which dependence was obtained, and the P I - W C condition, in which it was not, was also apparent in the phenomenological reports offered by some of the subjects either spontaneously or in response to general questions about the experiment after it was over. In the W C - P I condition, 69% of the subjects freely commented on the fact that words were repeated between the two tests; whereas only 25% did so in the P I - W C condition. Thus, the relation was not only more readily noticed by the W C - P I subjects, but may, in fact, have influenced their performance on the second test. Rather than simply reporting the items observed in PI (which, it must be noted, is all that

T2

Phi

CP

.666 .471

.18 .23

P(PIIWC) = .75 P(PIIWC) = .40

.563 .328

.07 .29

P(WCIPI) = .59 P(WCIPI) = .23

.508 .268

.54 .65

P(WCIWC) = .78 P(WCIWC) = .10

.638 .414

.37 .43

P(PIIWC) = .78 P(PIIWC) = .26

is required to meet successfully the demands of the implicit task), perhaps subjects searched their memory for items encountered in WC and verified their responses by noting whether the item had been presented previously. That is, the contextual aspects of a task may have altered the manner in which the subject approached the task and converted an ostensibly implicit test of memory to an explicit one. This change in strategy is reflected in a change of the dependency relation between the two measures on the first five study-test items, where independence is found, and the last five items, where dependency is strong (Table 3). A similar effect is not obtained in the P I - W C condition. The distribution of common responses between the tests in the two conditions may account for the differences between them. By examining the responses of the subjects, it became evident that a much higher percentage of omission errors (i.e., no response) was produced in the WC task than in the PI task. In WC there is only one possible English completion for the given distribution of letters. If the subjects are unable to solve the word fragment, they frequently respond with "don't know" rather than suggest a word that cannot possibly be correct. For example, if the word presented was

Table 3

Experiment 1: Stochastic Independence Analysis on Correct Test 1 (TI) or Test 2 (7"2) Responses and Incorrect Test Responses (T1, T2) for First Five and Last Five Study- Test ltems Cells

Marginals

Condition TI, T2 TI, T2 T1, T2 T1, T2 T1

T2 Phi CP

WC-PI First .300 .275 .163 .262 .463 .575 .14 .65a La~ .450 .238 .063 .249 .513 .688 .42 .88a PI-WC Fi~t .456 .232 .156 .156 .612 .688 .16 .66b Last .375 .200 .225 .200 .600 .575 .12 .65b Note. CP = conditional probabilities; WC = word completion; PI = perceptual identification. Results of P(PIIWC). b Results of P(WCIPI).

26


R W, subjects cannot fill in the allotted blanks with an incorrect response that suits the fragment. Candidates, such as RICEBOWL,simply do not fit. For PI, in contrast, there are a n u m b e r of responses that may be deemed appropriate. Depending on what letters of the array are identified, the number of possible solutions varies. Thus, if the item presented was RAINBOW,but the subject only detected the letters R, I, B, O, they are more likely to produce a response that is consistent with the information, for example, RIBBON, rather than no response. In this case, then, subjects will be more likely to produce some response, even if it is incorrect. Table 4 provides the response distributions for the two tests. On examining these distributions, it is clear that .59 of the responses given in word completion for the W C - P I condition were omissions. This means that the proportion of responses in c o m m o n with the second test phase was .98 (i.e., of the .4l committed responses given, .40 responses were correct and therefore c o m m o n to the second test). However, o n l y . 10 of all responses were omissions in PI for the P I - W C condition. This translates into only .57 responses in c o m m o n with the second test phase (i.e., of the .90 committed responses given, .51 were correct and therefore c o m m o n to the second test). Thus, the proportion of responses in c o m m o n with the second test phase is extremely different for these two conditions. The high overlap between responses in the W C - P I condition is itself a contextual cue that alerts the subject to the similarity between the two tasks and leads the subject to use the information available from the WC phase when performing the PI task. Because this relation is considerably obscured in the PIWC condition, memory is not explicitly consulted for information obtained during the earlier test. Before testing some predictions that follow from this analysis in the next experiment, it is instructive to examine the subjects' performances on the items they first saw in Test 1, the test-only items (C). _ _

_ _

_ _

B

_ _

Contingency Analysis of Test-Only Items (C) For these items, only some were correctly reported during Test 1 and this disproportionate exposure produces a dependent relation between task performances (WC-PI: ×2 = 20.66;

PI-WC: x 2 = 31.10; WC-WC: ×2 = 162.37; PI-PI: x 2 = 72.02, all ps < .01). Thus, if a test-only item is successfully identified or completed in Test 1, then there is some positive transfer for those items to the next test. Also, for changed test conditions (WC-PI and PI-WC), if the response to the novel item had not been correct in Test 1, then there is no representation of the completed item in memory and the item is treated like any other novel item (for WC-PI = .40 vs..39 and for P I - W C = .23 vs..23; compare Table 2 with Table 1). For the repeated test conditions, however, the dependency pattern was different in this regard. For correct Test 1-only items, a benefit is observed; but if a Test 1-only item was not correct on Test 1, then the probability of responding correctly to these items on Test 2 was lower than to novel items, D (for W C - W C = . 10 vs..23 and for PI-PI = .26 vs..33). Other investigators have obtained similar dependency results for test-only items (e.g., Tulving et al., 1982). The contrasting findings for test-only transfer items emphasize an important difference between repeated test conditions and changed test conditions. For specific tests, dependency occurs because it is likely that the same information is dealt with in the same way in both conditions. As a result, performance on failed items was even poorer than on novel items in Test 2. In the changed test conditions, however, the benefit gained by correctly identifying or completing test-only items on Test 1 results simply from the fact that these items are now represented in memory and not from their having been processed or retrieved similarly on the two occasions. Once the test changes, items that were incorrect on the first test were not penalized in comparison with novel items. In summary, the major point of Experiment 1 was that performance on PI was independent of that on WC unless the overlap in responses made the subject aware of a relation between the two tests. In addition, it was found that those items that received correct responses on Test 1 were reported more correctly on Test 2. Repeating the identical tests had beneficial effects for correct items but deleterious effects for incorrectly reported items, whereas no such relation was observed for incorrect items if Tests 1 and 2 were different. Experiment 2

Table 4

Distribution of All Responses Given During the First Test Session (N = 48 items presented) in Experiment 1 Response category Correct target Correct novel Incorrect responses Total responses Omissions (no response) Summary:

WC-PI .25 .15 .01

PI-WC .33 .18 .39

.41 .59 .98a

.90 .10 .57h

Proportion of items common to the two tests (i.e., of the .41 committed responses given, .40 responses were correct and therefore common to the second test). hProportion of items common to the two tests (i.e., of the .90 committed responses given, .51 responses were correct and therefore common to the second test).

The results of the first experiment suggest that stochastic dependence between two implicit priming tests of memory depends on the information-processing demands of the two tasks and the cognitive strategies used to perform them, and not on their presumed mediation by a common memory system. The overlap between the items in the WC-PI condition was noticed by most subjects and may have led them to search their memory explicitly during PI for words encountered during WC. As a result, stochastic dependence was found. When the similarity between the two conditions was not noticed, the natural tendency to treat PI as a test of visual perception, and WC as a problem-solving test, prevailed. The difference in the cognitive processes used to execute tests was sufficient to produce stochastic independence. According to this interpretation, contextual similarity induced by response overlap between the two tests was respon-

STOCHASTIC INDEPENDENCE sible for influencing the strategies subjects adopted in the W C - P I condition. Reducing the response overlap between the tests should lower the dependency between them. To test this hypothesis, the test-only items presented in Test 1, and again in Test-2, were omitted from the experiment. By thus reducing the n u m b e r of common items between the two tests, the contextual similarity produced by the repeated items would also be lessened.

Method

27

Table 5

Mean Proportion of Items Correct and Response Times in Experiment 2 for Perceptual Identification (PI) and Word Completion (WC) Study-test Test Test 1 WC-PI PI-WC Test 2 WC-PI PI-WC

Novel

M

RT

M

RT

.51 .58

2,558 1,406

.25 .34

3,741 1,668

.69 .59

1,302 2,453

.39 .32

1,944 3,741

Study only M

RT

.60 .49

1,268 2,644

Note. RT = response time. Response times are given in millise-

Subjects

conds.

Another 32 University of Toronto undergraduate students took part in the study and received $5 for participation.

Overlap Contingency Analysis Procedure Experiment 2 differed from Experiment 1 in only one respect. The new items in Test 1 session were not retested in Test 2. Instead, the remaining 24 items from the original study phase were re-presented in Test 2. These items will be referred to as study-only items to distinguish them from items that were also presented in a prior test (study-test items) and items that were only presented in test phases (test-only). This alteration provided for reduced similarity between the two tests, because none of the new items from the first test were repeated in the second test. Thus, in the study phase, 24 study-test items (A) and 24 studyonly items (B) were presented for reading. In Test l, the 24 studytest items (A) and 24 novel items (C) were presented. Finally, in Test 2, the 24 study-test items (A), 24 study-only items (B), and an additional 24 novel items (D) were presented. In all other respects the procedure was identical to Experiment 1.

Results and Discussion Analysis of Accuracy, and of Latency of Correct Responses (Table 5) For Test 1, study-test items were reported significantly more accurately (2 x 2 ANOVA),F(1, 30) = 85.97, MSe = 6.70, and more quickly, F(1, 30) = 5.74, than novel items. No other significant effects were found for accuracy. Latency scores were shorter for PI than WC, F(1, 30) = 7.80; and the effects of task interacted with novelty, F(1, 30) = 29.82, presumably because the range for improvement of WC items was wider than for PI items (stimuli remained on the screen for 60 s for WC and only for 33 ms for PI). A similar pattern of results was obtained for Test 2. Previously seen items, A and B, were reported more accurately (2 x 3 ANOVA), F(2, 60) = 20.29, MS~ = 19.72, and more quickly, F(2, 60) = 5.31, than novel items. This result was confirmed by orthogonal comparisons: For accuracy, F(I, 60) = 36.69, MSe = 19.12; for latency, F(1, 60) --- 18.81. Responses were also more accurate, F(1, 30) = 6.89, MSe = 17.63, and faster, F(1, 30) = 6.36, for PI than for WC. No other main effects or interactions were significant.

The main purpose of this experiment was to reduce the perceived similarity between the two tests as to prevent subjects from using conscious recollection to help in perceptual identification. The design succeeded in this effort by reducing the proportion of common responses between the tests. As can be seen in Table 6, the proportion of responses that were omissions for W C - P I was .61, making the proportion of items that the two tests had in common only .64 (i.e., of the .39 committed responses given, only .25 were correct target items that would be repeated in the second test). For the P I - W C condition, the proportion of responses that were omissions was. 14, making the proportion of common items .34 (i.e., of the .86 committed responses given, only .29 were correct target items). The overlap between the two tests had been considerably diminished in comparison with Experiment i, and, presumably, so had the perceived similarity between them. No subjects reported noticing any overlap between the items on the two tests. The stochastic independence analyses are provided in Table 7. It is clear from the chi-square values that no significant relationship between the measures can be inferred, WC-PI: x 2 = 1.32; PI-WC: x 2 = 1.96, ps > .05. For the WC-PI condition, perceptual identification of successfully completed items is no greater than identification for unconditionalized Table 6

Distribution of All Responses Given During the First Test Session (N = 48 Items Presented) in Experiment 2 Response category Correct target Correct novel Incorrect responses Total responses Omissions (no response) Summary

WC-PI .25 .13 .01

PI-WC .29 .17 .40

.39 .61 .64a

.86 .14 .340

a Proportion of items common to the two tests (i.e., of the .39 committed responses given, only .25 were correct target items that would be repeated in the second test). b Proportion of items common to the two tests (i.e., of the .86 committed responses given, only .29 were covert target items).

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Table 7

Stochastic Independence Analysis on Correct Test 1 (T1) or Test 2 (T2) Responses and Incorrect Test Responses (T1, T2) for Target (Study-Only) Items in Experiment 2 Cells

Marginals

Condition TI, T2 TI, T2 T1, T2 TI, T2 T1 T2 Phi CP WC-PI .362 .328 .143 .167 .505 .690 .06 .72a PI-WC .359 .224 .227 .190 .586 .583 .07 .62b Note. CP = conditional probabilities; WC = word completion; PI = perceptual identification. a Result of P(PIIWC). b Result of P(WCIPI).

items [i.e., P(PIIWC) essentially equals P(WC)]. A similar relationship is observed for the P I - W C condition. In contrast to Experiment 1, both conditions demonstrate statistical independence. Successful retrieval of an item from memory on one test does not predict the successful retrieval of that same item on a subsequent test. The performance of subjects in W C - P I condition indicates that when test context similarity is reduced, the amount of dependence in performances between the tests is also reduced. By reducing the perceived test similarity, the subjects' responses are no longer influenced by strategies designed to gain access to information that is available from the previous test. Thus, without extraneous variables, the processes usually required for word completion or perceptual identification are sufficiently different from one another to allow for the independent sampling of items from a c o m m o n memory pool. The results call into question either the hypothesis that implicit priming tests are mediated by a c o m m o n memory system or that tests of stochastic independence are useful in distinguishing one memory system from another or both. General Discussion There is a tendency for some cognitive psychologists to infer the existence of two or more memory systems if performance on one test of memory is functionally and stochastically independent of performance on another test (Graf & Schacter, 1985; Graf, Squire, & Mandler, 1984; Johnson, 1983; Tulving, 1983; Tulving et al., 1982). These bold assumptions derive their strength, in part, from neuropsychological studies on amnesia in which damage to a number of limbic system structures is known to produce profound memory impairment on some tasks but not on others. Neuropsychologists have considered such findings as constituting evidence for dissociation of at least two memory functions (Cohen & Squire, 1980; Gaffan, 1976; Hirsch, 1974; Milner, 1966; Mishkin et al., 1984; Moscovitch, 1982a, 1982b; O'Keefe & Nadel, 1978; Weiskrantz, 1977), which, not surprisingly, bear a strong correspondence to the independent memory systems postulated by cognitive psychologists. The general consensus has been that one system mediates performance on implicit tests of memory and the other on explicit tests of memory and that the modes in which information is represented and

processed in the two systems are fundamentally different from each other. It is this belief, in fact, that underlies the assumption that stochastic independence between two tasks is evidence that they are served by different memory systems. Insofar as perceptual identification and word-fragment completion are both implicit tests of memory, it was expected that performances in the two tests would not be found to be statistically independent of each other because both tests are presumed to be mediated by a common memory system. On the contrary, independence was found in all conditions but W C - P I of Experiment 1. That was the only condition in which the subjects were aware of the overlap between the two tests. As a result, there was an increased likelihood of their adopting retrieval strategies and information sampling procedures on the PI task that were either similar to those used in the WC task or were derived from it. In all other conditions, contrary to the multiple memory system hypothesis, stochastic independence was obtained between performances on the two implicit tests. It is conceivable that rather than being mediated by a single implicit system, PI and WC are mediated by separate systems, such as sensory and perceptual, that are dissociable one from the other (Johnson, 1983). At first glance such an interpretation may appear to be consistent with a multiple memory system account that considers tests of stochastic independence as strong evidence for dissociable memory systems. On reflection, however, it is unclear how this interpretation could accommodate both the results of Experiment 1 and Experiment 2 in which dependent and independent relationships, respectively, were found between performances on the identical two tests. (See also Witherspoon, 1983, for similar findings in studies comparing explicit with implicit tests of memory.) Hintzman (1980) argued this very point. He advised that independence analysis in memory research be interpreted cautiously because the covariance terms of subject, item, and interactive effects could potentially alter the level of dependency observed between memory tests. We appreciate Hintzman's (1980) concerns but believe they become less relevant when interest is focused less on absolute levels of dependency and more on predictable alterations in the pattern of dependency between tasks. Such predictable changes, first reported by Witherspoon (1983), were also found in the present study. Rather than calling into question the use to which tests of stochastic independence are put in memory research, the results of our study suggest that the multiple memory system hypothesis as it is currently formulated is probably incorrect. Although evidence of functional independence does not weigh as heavily as that of statistical independence, it is instructive that perceptual identification and word completion have also been shown to be functionally independent of each other. Schwartz (1988) reasoned, like us, that in comparison with perceptual identification, word completion is more of a problem-solving task in which conceptual, orthographic knowledge is used to generate the appropriate exemplars. Consequently, it is likely to benefit more than perceptual identification from orienting tasks that recruit similar components in processing the target at study. The results of

STOCHASTIC INDEPENDENCE her study confirmed her hypothesis. Generating targets as solutions to anagrams led to better performance in subsequent tests of word completion than of perceptual identification, whereas simply reading the target had equivalent effects on the two tests. Recent evidence from studies of neurologically impaired patients is also consistent with the observation that performance on different implicit tests of memory can be statistically and functionally independent of each other. Patients in the early stages of Huntington's disease are impaired in the acquisition of a mirror-reading and pursuit motor skill, yet are normal on word completion (Heindel, Salmon, Butters, & Shults, 1988; Martone, Butters, Payne, Becker, & Sax, 1984), whereas the opposite may be true of patients in the early stage of Alzheimer's disease (Shimamura, Salmon, Squire, & Butters, 1987; but see Huberman, Freedman, & Moscovitch, 1988). As we noted earlier, the multiple memory system view grew from the radical assumption that dissociations in performances between two tasks constitute evidence that each task is mediated by independent memory systems. This assumption is a departure from the more modest claim that dissociations merely imply that one or more of the components of the tasks are different (e.g., Tulving & Thomson, 1973). This alternate, and older, view is based on the assumption that performance of each task requires the operation of many components, some of which are c o m m o n to other tasks and some of which are not. Performances on two tasks may be independent from each other to the extent that their components differ (or the information they use is different), leaving open the possibility that some components (or type of information) may be more critical in this regard than others. From a neuropsychological perspective, damage to different critical component structures will produce correspondingly different memory disorders. The proper description and classification of these disorders will require the development and administration of a variety of memory tests that tap different critical component processes and that use different types of information as encoding and retrieval cues. This enterprise is just beginning (cf. Tulving, 1985). With respect to the larger issue of whether there are one, two, or many memory systems, the results of the present study are neutral. They speak only against the current versions of the multiple memory system hypothesis and caution against testing the hypothesis primarily with demonstrations of dissociations. What is first needed to resolve the issue is a clearer formulation of what constitutes a memory system and what evidence is needed to distinguish it from a collection of components. Until the issue is resolved, there is no harm in remaining agnostic while attempting to specify the components and types of information that mediate performance on different memory tests (Roediger & Blaxton, 1987). References Cohen, N. (1984). Amnesia and the distinction between procedural and declarative knowledge. In N. Butters & L. R. Squire (Eds.), The neuropsychology of memory (pp. 83-103). New York: Guilford Press.

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