~ Pergamon 0005-7967(94)E0004-3 Behar. Res. Ther. Vol. 33, No. I. pp. 1-14, 1995

Copyright :!" 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0005-7967/95 $7.00 + 0.00

IMPLICIT AND EXPLICIT MEMORY BIAS IN ANXIETY: A CONCEPTUAL REPLICATION

COLIN MACLEOD and KARl MCLAUGHLIN

Department of Psychology, The University of Western Australia, Nedlands, Perth, W.A. 6009, Australia

(Received 1 September 1993; received for publication 7January 1994)

Summary--Williams, Watts, MacLeod and Mathews' (1988) [Cognitive psychology and the emotional disorders. Chichester, Wiley] model of anxiety and cognition leads to the prediction that anxious subjects will show an implicit, but not an explicit, memory advantage for threat-related information. Mathews, Mogg, May and Eysenck 0989) [Journal of Abnormal Psychology, 98, 401-407] obtained marginally significant support for this prediction in an experiment that tested memory using word stem completion tasks following a self-referent encoding procedure. However, neither the reliability nor generality of these findings have been established. The current experiment was designed to provide a conceptual replication of Mathews et al.'s study, using different tests of implicit memory (i.e. tachistoscopic identification) and explicit memory (i.e. recognition) and an alternative type of encoding task (i.e. colour naming stimulus words). 16 generalised anxiety disorder patients, and 16 non-anxious control subjects were tested. As predicted, the anxiety patients showed a relative implicit memory advantage for threat-related stimulus words, while the two subject groups did not differ in their pattern of explicit memory performance. These results support the predictions generated by Williams et al.'s model of anxiety and cognition.

INTRODUCTION

It is now well established that clinical anxiety patients commonly display an encoding advantage for threat-related information (cf. MacLeod, 1990, 1991; MacLeod & Mathews, 1991; Mineka, 1992). Frequently, such patterns of selective encoding have been demonstrated through the use of a colour naming interference paradigm based upon the Stroop task (Stroop, 1938). Specifically, clinically anxious patients and non-anxious control subjects have been presented with threatening and non-threatening words, printed in various colours, and have been required to name the colours while ignoring the content of each word. The anxiety patients' difficulty ignoring the content of threat words has been revealed by their disproportionately long coiour naming latencies on the threatening stimuli. Such effects now have been observed in patients suffering from generalised anxiety disorder (e.g. Mathews & MacLeod, 1985; Mogg, Mathews & Weinman, 1989), specific phobia (e.g. Watts, McKenna, Sharrock & Trezise, 1986), panic disorder (e.g. Ehlers, Margraf, Davies & Roth, 1988), and post-traumatic stress disorder (e.g. McNally, Kaspi, Riemann & Zeitlin, 1990; Foa, Feske, Murdock, Kozak & McCarthy, 1991). Similar effects also have been reported for normal individuals who score high on questionnaire measures of anxiety (e.g. Richards & Millwood, 1989; MacLeod & Rutherford, 1992).

In contrast to this very reliable pattern of emotionally-congruent encoding biases, anxiety patients typically do not show facilitated recall or recognition memory for threat-related stimuli (cf. MacLeod, 1990, 1991; MacLeod & Mathews, ! 991; Mineka, 1992). Many of the colour-naming studies reported above have included such memory tests subsequently, without finding any group differences in subjects' abilities to remember each emotional class of stimulus word (e.g. Mathews & MacLeod, 1985; Mogg et al., 1989). Several other studies, that have been designed specifically to assess hypotheses concerning emotionally-congruent memory, also have failed to find any evidence of such memory biases in generalised anxiety patients (e.g. Mogg, Mathews & Weinman, 1987), specific phobics (e.g. Watts, Trezise & Sharrock, 1986), or in normal individuals with elevated anxiety scores (e.g. Foa, McNally & Murdoch, 1989).

Recently, Williams, Watts, MacLeod & Mathews (1988) have attempted to explain this pattern of findings by proposing that anxiety is associated with the facilitated integratit~e processing of threat-related information, but with no equivalent bias in the elaboratit, e processing of such

2 COLIN MACLEOD and KAal MCLAUGHLIN

information (cf. Graf & Mandler, 1984). According to Graf and Mandler integration is an automatic process that strengthens the internal structure of a mental representation, thus making that representation more accessible in the sense that activation of any part will serve to activate the whole. In consequence, stimuli corresponding to representations that are in a high state of integration will tend to 'pop out' of a stimulus array, and hence will tend to be selectively encoded. In contrast, elaboration is a strategic process that serves to establish and strengthen associative connections between a mental representation and other existing representations in memory. A highly elaborated representation will be disproportionately easy to retrieve on intentional memory tasks, such as recall and recognition, because of these multiple retrieval paths. Williams et al.'s (1988) hypothesis, that elevated anxiety is characterised by emotionally-congruent integrative processing but not by emotionally-congruent elaborative processing, therefore accounts for the finding that clinically anxious patients show emotionally-congruent encoding biases but do not show emotionally-congruent memory biases on recall or recognition tasks.

Williams et al. (1988) point out that their account leads to a novel prediction concerning the patterns of anxiety-linked selective processing that should be observed when implicit memory tasks are considered, rather than traditional explicit memory tasks (cf. Jacoby & Witherspoon, 1982; Schacter, 1987; Roediger, 1990). Explicit memory tasks require subjects to consciously recollect previously presented stimulus items, and assess retention by examining the accuracy of this retrieval. In contrast, implicit memory tasks do not require conscious recollection of past experience, but assess retention by examining the degree to which previous exposure to stimulus items passively serves to facilitate the subsequent processing of these same stimuli. Performance on explicit memory tasks is more powerfully influenced than is performance on implicit memory tasks by the degree to which stimuli are processed elaboratively during encoding (e.g. Jacoby & Dallas, 1981; Ohta, 1984; Graf & Mandler, 1984; Dark, Johnston, Myles-Worsley & Farah, 1985; Carroll, Byrne & Kirsner, 1985). In contrast, it has often been claimed that performance on implicit memory tasks is influenced most powerfully by the degree of integrative processing taking place during stimulus encoding (e.g. Graf & Mandler, 1984; Mandler, Graf & Kraft, 1986). Conse- quently, Williams et al.'s account leads them to predict that anxiety will be associated with selectively facilitated implicit memory for threat-related information, even though this emotionally- congruent memory bias may not be observed when anxious individuals perform explicit memory tasks.

Mathews, Mogg, May and Eysenck (1989) recently have reported experimental support for this prediction. These researchers presented generalised anxiety patients and non-anxious control subjects with threat-related and non-threat-related words, in an encoding task that required subjects to imagine scenes involving both themselves and the referent of each stimulus word. Subjects were given two different memory tasks, each involving the presentation of incomplete word stems. One task tested explicit memory by means of a cued recall procedure, in which subjects were required to complete each word stem with a word that they had encountered during the initial encoding task. Explicit memory was assessed by counting the number of words correctly recalled in this task. The other task tested implicit memory, and required subjects simply to complete each stem with the first word that came to mind regardless of whether or not that word had been presented earlier. Implicit memory was assessed by counting the number of stems that were completed to make one of the previously seen (old) words, and subtracting the number that were completed to make a previously unseen (new) word. Mathews et al. found no group difference in the relative explicit memory scores observed on each class of stimulus word. However, in keeping with their experimental prediction, these researchers found a marginally significant interaction on the implicit memory task which suggested the presence of an anxiety-linked emotionally-congruent implicit memory bias. Specifically, the anxiety patients showed slightly higher implicit memory scores for the threat words than did the control subjects, but showed slightly lower implicit memory scores for the non-threat words than did these control subjects. On the basis of these results, Mathews et al. (1989) conclude that their study offers encouraging support for Williams et al.'s model of anxiety-linked processing biases.

Clearly, the findings reported by Mathews et al. (1989) may have important theoretical implications. However, there are several reasons why these findings must be confirmed by conceptual replication before they are accepted as convincing support for Williams et al.'s account.

Implicit and explicit memory bias in anxiety 3

First, the reliability of the implicit memory effect reported by Mathews et al. (1989) has not yet been adequately established. Even in this original study, the critical interaction between subject group and word valence, on the implicit memory task, fell slightly short of statistical significance. Furthermore, attempts to replicate this finding have been problematic. Zeitlin and McNally (1991) and Richards and French (1991) have reported some additional data, gathered on slightly different word stem completion tasks, which they argue are consistent with the existence of an anxiety-linked implicit memory bias. However, in a recent review, Nugent and Mineka (1994) have identified serious methodological problems within Zeitlin and McNally's experimental design, and have argued that interpretation of Richards and French's findings is severely compromised by the highly unusual pattern of results obtained by these researchers, within which several well-established effects failed to materialise. Nugent and Mineka themselves attempted to replicate Mathews et al.'s earlier findings using the exact word stem completion tasks employed by these original researchers, but could find no evidence that high trait anxious students showed a different pattern of implicit memory, across the two classes of emotionally valenced words, than was shown by low trait anxious students. It could perhaps be argued that Nugent and Mineka's negative results might reflect the fact that these researchers were studying a non-clinical population. However, Mathews himself (1994) also has reported a failure to replicate his original finding in a more recent study examining generalised anxiety disorder patients. In view of such replication failures, the validity of the original finding must remain in question.

Even if Mathews et al.'s (1989) findings already had been shown to replicate reliably using the same experimental methodology as was employed in that original study, the generality of the observed implicit memory bias would remain uncertain. According to Williams et al.'s account, this effect should be observed even when the initial encoding task involves little or none of the elaborate self-referent processing that was required in Mathews et al.'s encoding task. Williams et al.'s proposal that anxiety is associated with the facilitated integrative processing of threat-related information was introduced to account for the patterns of biases shown by clinically anxious patients on tasks requiring little or no elaborative processing, such as the simple colour naming paradigm described earlier. It follows, therefore, that clinically anxious patients should show implicit memory advantages for threat-related information even following this kind of superficial encoding procedure.

Furthermore, according to Williams et al.'s account, Mathews et al.'s (1989) observed dis- sociation between the patterns of memory shown by clinically anxious patients on their explicit and implicit memory tasks should generalise to quite different types of memory measures. Specifically, an anxiety-linked advantage for threat-related information should be observed on any task that assesses implicit memory, but should not be observed on any task assessing explicit memory alone. The two word completion tasks employed by Mathews et al. (1989) certainly are not the only implicit and explicit memory tasks that could be considered.

Indeed, Perruchet and Baveux (1989) recently have argued that word stem completion may provide a particularly impure measure of implicit memory. These researchers have demonstrated that performance on certain tasks which supposedly measure implicit memory is highly correlated with performance on traditional explicit memory tasks. The implicit memory task that Perruchet and Baveux found to show the highest correlations with explicit memory task performance was the word stem completion task employed by Mathews et al. to assess implicit memory for emotional information. Perruchet and Baveux specifically conclude that "the effects exerted by prior word exposure in a . . . word completion task are associated with explicit memory performance" (p. 82). However, Perruchet and Baveux found that not all implicit memory tasks yielded correlated performance with every explicit memory task. The lowest correlation, which fell slightly below zero, was found between explicit recognition memory and a measure of implicit memory assessed by a tachistoscopic word identification procedure introduced by Jacoby and Dallas (1981). In this tachistoscopic identification task, implicit memory is revealed by the increased accuracy with which previously seen (old) words, relative to previously unseen (new) words, can be identified at brief exposure durations. On the basis of their observed patterns of correlations between explicit and implicit memory tests, Perruchet and Baveux conclude that "tachistoscopic identification may serve as a relatively uncontaminated and ubiquitous indicator of implicit memory" (p. 77). Consequently, Williams et al.'s account leads us to predict that clinically anxious patients should show a reliable

4 COLIN MACLEOD and KARl McLAUGHLIN

selective advantage for threat-related information when implicit memory is assessed by means of this tachistoscopic identification task, but not when explicit memory is assessed by means of a recognition task.

We designed the current study to provide a conceptual replication of Mathews et al.'s (1989) original experiment, while also testing the generality of these researchers' findings. A rather different encoding task was employed in the present study. This was more similar to the colour naming procedure which originally yielded the pattern of anxiety-linked interference effects that motivated the development of Williams et al.'s theoretical account. Also, different memory measures were employed in the current experiment, selected on the basis of Perruchet and Baveux's (1989) research, to provide relatively uncontaminated indices of explicit and implicit retention. Nevertheless, the study addressed the same predictions as were derived from Williams et al.'s theoretical account by Mathews et al. (1989). Specifically, the current experiment was designed to test the hypothesis that generalised anxiety patients, but not non-anxious control subjects, would show an advantage for threatening information on the implicit memory task, but not on the explicit memory task.

METHOD

In this experiment, clinically anxious patients and non-anxious control subjects were presented with threatening and non-threatening words in a colour naming task somewhat similar to that employed in previous research on anxiety-linked attentional biases (e.g. Mathews & MacLeod, 1985; Watts et al., 1986; Ehlers et al., 1988; Mogg et al., 1989; McNally et al., 1990; Foa et al., 1991). In the current study, however, in order to ensure a degree of memory for these stimulus words, subjects were instructed both to name the ink colour and to say the word itself. Following this encoding task, explicit and implicit memory for the two classes of stimulus words was tested. In the explicit memory test, subjects were presented with previously seen words, and with valence-matched distractor words, and instructed to circle those words that they recognised as having been exposed in the colour naming task. Explicit memory was revealed by a subject's elevated tendency to endorse previously seen words, relative to previously unseen words, on this recognition task. In contrast, the implicit memory test employed a tachistoscopic identification task, in which words were presented briefly, at a pre-calibrated exposure duration, on a VDU screen (cf. Jacoby & Dallas, 1981; Perruchet & Baveux, 1989). Once again, some of these words had been exposed in the initial colour naming task, whereas other valence-matched words had not been presented previously. However, the instructions on this task were to simply identify each briefly exposed word by naming it aloud. Implicit memory was revealed by a subject's elevated tendency to accurately identify the previously seen words, relative to the previously unseen words. The experimental prediction, that the anxiety patients will show disproportionate retention of the threat-related words on the implicit memory test alone, can be tested by contrasting the patterns of performance shown by the two group of subjects on each of the two memory tasks.

Subjects

A total of 32 Ss were tested. 16 Ss (11 women and 5 men) comprised the Anxiety Patient Group. All these individuals were receiving treatment for generalised anxiety disorder (GAD), and were referred by Clinical Psychologists working in the State Health Service. Inclusion in this group required that a S met DSM IIIR criteria for the diagnosis of GAD, without also meeting criteria for the diagnosis of major depression, obsessive-compulsive disorder, specific phobia, or post traumatic stress disorder. The Control Group consisted of 16 Ss (9 women and 7 men) with no history of emotional disorder. These control Ss were employees of the State Health Service, and were selected from occupational groups that matched, as closely as possible, those from which the anxiety patients had been drawn. Subjects in the Anxiety Patient Group ranged in age from 22 to 54 years, with a mean of 36.87 years, while those in the Control Group ranged from 22 to 56 years, with a mean of 35.44 years. A t-test revealed no significant age difference between the groups (t = 0.38, df = 30, NS).

Implicit and explicit memory bias in anxiety 5

Materials Stimulus words. 96 words that we considered to be threat-related (e.g. violence, disaster,

humiliated) comprised the set of Threat Words employed in this study, while 96 words that we considered to be unrelated to threat (e.g. symphony, superior, celebrates) made up the set of Non-Threat Words. These two sets of words, which are presented in Table 1, were matched exactly on average letter length, and on average frequency of usage according to Carroll, Davies & Richman's (1971) word frequency norms.

Our initial intuitions concerning the emotional valence of the stimulus words were confirmed by having 5 Clinical Psychologists independently rate each word on a scale ranging from i (non-threatening) to 5 (very threatening). Every one of the Threat Words received a higher mean rating than any of the Non-Threat Words. The mean rating given for the Threat Words was 4.11, while the mean rating given for the Non-Threat Words was 1.32. The difference between these ratings was highly significant (t = 8.96, df = 4, P < 0.001).

This total pool of words was divided into two Presentation Sets (A and B), each consisting of 48 Threat Words and 48 Non-Threat Words. Only one of these sets was to be presented in the encoding task for any individual S, while the other set would provide the distractor items in the memory tests given to that S. Two Memory Test Sets (1 and 2), each consisting of 96 words, also were created by drawing 24 Threat Words and 24 Non-Threat Words from each of the two Presentation Sets. For any S, one of these Memory Test Sets was to be employed in the explicit memory test, while the other would be employed in the implicit memory test.

Finally, an additional set of 20 emotionally neutral words was constructed for use in an initial series of exposure calibration trials. This set of words shared the same average length and frequency as the total pool of 192 stimulus words described above.

Experimental hardware. The experiment was run using an Archimedes 310 microcomputer with a high resolution colour video display unit. A two key 'response scoring box' was attached to this computer by a 2 m cable connected to the mouse port. The left and right buttons were labelled 'Incorrect' and 'Correct' respectively. An Epsom LX-800 printer also was attached to the Archimedes via a 2 m cable.

Table I. Total set of Threat and Non-Threat Stimulus Words

Threat Words blood brutal failure wrecked disabled strangled nightmare inquest agony savage corpse disdain despised fracture disgraced hurt abuse embarrassed cursed useless scorned abortion worthless inadequate harm spite mocked despair anguish defeated ridiculed punishment sick infect hearse ashamed coronary casualty incurable malignancy grief attack plague assault insecure harrowed collapsed incompetent angry molest ignored seizure insanity deathbed mutilated opposed fatal injury poison snubbed accident deranged injection kill inept catastrophe stupid shunned loathed violence holocaust paralysed gore wound cancer disease torture pathetic ambulance criticised gash hated lonely idiotic hopeless lacerate emergency indecisive rape hazard coffin foolish disaster inferior mortality humiliated

Non-Threat Words daily elbows clothes sundown enduring portrayed motivated balcony oddly refund longed wayward omission acrobats modernize navy folks backgrounds smiles pockets cadence molecule lecturing collective star guess spires prairie pitched turnpike campsites dedication ages equine burger dragged pensions inflated foreheads effortless bacon health upheld mileage profiles clicking outboard camaraderie prime clears lighted graphic endeared bounding lamplight display brass passes earthy neptune symphony imitates recalling grow caked compartment ruling oatmeal storing superior ancestral cartilage clad laugh parade largest thrifty westward duplicate structured cask shade viewed ditties uniform midpoint furniture celebrates peck tribes threads bounced identify proposes additions juxtaposed

6 COLIN MACLEOD and KARl MCLAUOHLIN

Experimental tasks

Each subject was required to perform four rather different types of tasks during the test session. First, the subject was given a set of Exposure Calibration Trials, to determine the exposure duration that would be employed subsequently for that S in the Implicit Memory Task. Next, the S was exposed to one of the Presentation Word Sets in the Colour Naming Encoding Task. Following this, the S was given two memory tasks. One was an Explicit Memory Task (Recognition), while the other was an Implicit Memory Task (Tachistoscopic Identification). Each of these four tasks is described in more detail below.

Exposure calibration trials. These calibration trials employed a procedure previously described by Perruchet and Baveux (1989) to identify an exposure duration for each S that would permit approximately 50% accuracy on a simple tachistoscopic identification task. Each trial began with the presentation of two white horizontal lines in the centre of the VDU, that served to define the area within which a stimulus word was about to be presented. 1 sec later, a single word was presented in 7 mm high white capital letters for a variable brief exposure duration, following which it was replaced by a white pattern mask consisting of rotated and inverted letters. Ss were required simply to name the briefly exposed word aloud. The experimenter, who simultaneously was presented with a printed output of the exposed word on the LX-800 printer, recorded the accuracy of the S's vocal response using the two button response scoring box. The next trial began 1 sec later.

The exposure duration was initially set at 160 msec. This duration was decreased by 20 msec each time a word was identified correctly. If the S was unable to identify a word, then the next word also was presented at that same exposure duration. If this second word was identified correctly then the exposure duration again was decreased by 20 msec, and trials continued. Trials ceased when the S failed to correctly identify two consecutively presented words. The critical exposure duration (CED) for that S was then set to the shortest duration at which the S had made a correct word identification.

These exposure calibration trials used only the set of 20 stimulus words created specifically for this purpose. The order in which those words were presented was fully randomised for every S.

Colour naming encoding task. In this encoding task each S was exposed to all 96 words from either Presentation Set A or Presentation Set B. On each trial a single word was presented to a central location of the VDU in upper case letter 7 mm high. This was displayed in one of the following four colours; red, green, yellow, or blue. The display was maintained for 3 sec, during which time the Ss was required first to name the ink colour aloud, then to name the word aloud. A blank screen was presented for 1 sec between individual trials. Every one of the 96 words in the presentation set employed for this encoding task was exposed once in each of the four possible ink colours, resulting in a total of 384 trials. The presentation order was fully randomised for every S.

Explicit memory task (Recognition). In this recognition memory task Ss were presented with an A4 sheet of paper containing all the words from either Memory Set 1 or from Memory Set 2, arranged in a fixed random order. These 96 words, therefore, consisted of 48 words (24 Threat Words and 24 Non-Threat Words) that had been exposed during the encoding task, and 48 words (24 Threat Words and 24 Non-Threat Words) that had not been exposed previously in the test session. The following instructions were written on the top of the sheet: "Some of the words listed below were shown to you in the colour naming task, while some words are new. Please place a circle around all the words that you recognise from the colour naming task". All Ss completed this task within five minutes.

Implicit memory task (Tachistoscopic identification). The trials in this task were very similar to those in the Exposure Calibration Task. Each implicit memory trial began with the presentation of two horizontal white lines in the centre of the VDU for I sec. A single stimulus word in 7 mm high white capital letters then was exposed briefly between these lines, before being replaced by a pattern mask consisting of rotated and inverted letters. The 96 stimulus words employed in this task were those from either Memory Set I or Memory Set 2, and hence consisted of 48 words that had been exposed during the encoding task (24 Threat Words and 24 Non-Threat Words), and

Implicit and explicit memory bias in anxiety 7

48 words that had not been exposed previously in the test session (24 Threat Words and 24 Non-Threat Words). A fixed word exposure duration was employed which, for each S, was the critical exposure duration (CED) established for that S in the initial Exposure Calibration Trials. The order of these 96 presentations was fully randomised for every S.

Subjects were required simply to name each of the briefly exposed words aloud when it appeared on the screen. The experimenter, who was presented simultaneously with these words on the LX-800 printer, recorded the accuracy of the response on each trial using the two button response scoring box. The computer stored this accuracy data, and presented the next trial 1 sec later.

Procedure Each S was tested individually in a quiet room, seated approximately 60 cm in front of the

VDU screen. The printer and response scoring box were positioned behind the S's line of vision, and were screened from the S's view. The experimenter sat before the printer and response scoring box throughout the experimental session.

Each session began with the Exposure Calibration Trials, which were always followed by the Colour Naming Encoding Task. Half of the Ss in each group then received the Implicit Memory Task (Tachistoscopic Identification) followed by the Explicit Memory Task (Recognition). The remaining Ss received the implicit and explicit memory tasks in the reverse order. The orders in which the memory tasks were given, together with the allocation of Presentation Sets to the encoding task and the allocation of Memory Sets to each of the two different memory tasks, were fully counterbalanced across Ss within both groups.

Finally, all Ss completed both the state and trait sections of the Spielberger State Trait Anxiety Inventory (STAI: Spielberger, Gorsuch, Lushene, Vagg & Jacobs, 1983), the Beck Depression Inventory (BDI: Beck, Ward, Mendelsohn, Mock & Erbaugh, 1961), and the Mill Hill Synonyms Test, before being debriefed and thanked for their participation.

RESULTS

Subject characteristics The scores obtained by each group of Ss on the questionnaires measures are summarised in

Table 2, which also reports the significance of any observed group differences. As can be seen, the Anxiety Patients reported a significantly higher mean level of trait anxiety

than did the Control Subjects (t = 6.70, df = 30, P < 0.001). The Anxiety Patients also reported a higher mean level of state anxiety (t = 4.76, df = 30, P < 0.001), and a higher mean level of depression (t = 4.32, df= 30, P < 0.001) than was reported by the Control Subjects. However, there was no significant group difference in the scores obtained on the Mill Hill Synonyms Test (t = 1.04, df = 30, NS). Thus, while the Anxiety Patients showed inflated levels of trait anxiety, state anxiety and depression relative to the Control Subjects, both groups of Ss showed an equivalent level of verbal ability.

Explicit memory task performance In order to evaluate performance on the explicit memory test, we first calculated the number of

Threat Words and the number of Non-Threat Words, from both the previously exposed Presentation Set (i.e. 'Old' Words) and the word set that had not been presented in the colour

Table 2. Mean questionnaire scores for each group of Ss (standard deviations in parentheses)

Control Anxiety Subjects Patients P

STAI trait 32.94 51.94

8 COLIN MACLEOD and KARl MCLAUGHLIN

naming encoding task (i.e. 'New' Words), that each subject claimed to recognise on this memory task. The average number of words, from each experimental condition, that Ss claimed to recognise (all out of a possible maximum of 24) are presented in Table 3. These data were subjected to a mixed design analysis of variance (ANOVA) that considered two within-group factors and one between-group factor. The within-group factors were Presentation Status (Old Words vs New Words), and Word Valence (Threat Words vs Non-Threat Words). The between-group factor was Subject Group (i.e. Anxiety Patients vs Control Subjects).

Several significant effects emerged from this analysis. First, not surprisingly, there was a very highly significant main effect of Presentation Status (F[1,30] = 121.96, P < 0.001), reflecting the fact that subjects claimed to recognise more Old Words (16.24) than New Words (1.81). This ability to discriminate Old from New Words on this recognition memory task indicates that Ss did indeed show significant explicit memory for the words encountered in the colour naming encoding task.

The ANOVA also revealed a significant main effect of Subject Group (F[1,30] = 4.39, P < 0.05). The Control Subjects claimed to recognise more words on average than did the Anxiety Patients (9.97 vs 8.07 respectively). Additionally, there was a marginally significant main effect of Word Valence (F[1,30] = 3.29, P = 0.07), reflecting a general trend for Ss to endorse more Threat Words than Non-Threat Words on this recognition task (9.51 vs 8.53 respectively).

No other effects approached significance in this analysis. Of most importance, there was no tendency whatsoever for Presentation Status to interact with Word Valence (F[1,30] = 0.16, NS), and no tendency for these two factors to be involved in a three-way interaction with Subject Group (F [1,30] = 0.15). Consequently, it must be concluded that both groups of Ss showed an equivalent ability to discriminate Old from New Threat Words, and to discriminate Old from New Non-Threat Words on this explicit memory task.

For both groups of Ss, we calculated an Explicit Memory Index for each valence of word by subtracting the number of New Words that Ss claimed to recognise from the number of Old Words that they claimed to recognise. A high score on this index would reflect a high level of explicit memory. Figure 1 plots this Explicit Memory Index as a function of both Subject Group and Word Valence.

As can be seen from this figure, both groups of Ss showed a very similar ability to accurately recognise the Threat Words and the Non-Threat Words. Indeed, scores on the Explicit Memory Index were virtually identical for the Threat Words and the Non-Threat Words, both for the Anxiety Patients (13.31 vs 13.13 respectively), and for the Control Subjects (15.63 vs 15.63 respectively).

Implicit memory task performance The numbers of words from each experimental condition that were identified accurately in the

Tachistoscopic Identification task are presented in Table 4. Once again, the maximum score possible in each cell was 24. Given that the overall mean accuracy score was 11.32 (representing a 47.17% level of accuracy), we can conclude that the calibration procedure was successful in identifying exposure durations that permitted approximately 50% accuracy on this task.

These data were subjected to an analysis of variance (ANOVA) employing the same design as was used for the explicit memory task data. As before, this ANOVA considered the two within-group factors of Presentation Status (Old Words vs New Words) and Word Valence (Threat Words vs Non-Threat Words), and the between-group factor of Subject Group (Anxiety Patients vs Control Subjects). Once again, a number of significant effects emerged from the analysis. First, a highly significant main effect of Presentation Status (F[1,30] = 131.12, P < 0.001) was obtained,

Table 3. Mean number of words endorsed in the explicit memory task (standard deviations in parentheses)

Threat Words Non-Threat Words Old Words New Words Old Words New Words

Control Subjects 18.06 2.44 17.50 1.88 (3.48) (2.10) (4.41) (2.61)

Anxiety Patients 15.44 2.13 13.94 0.8 l (4.70) (2.25) (5.00) (1.22)

Implicit and explicit memory bias in anxiety 9

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Fig. 1. Explicit memory scores for Threat and Non-Threat Words.

due to the fact that Ss correctly identified more Old Words (14.47) than New Words (8.16). This finding confirms that Ss did indeed demonstrate significant implicit memory for those words exposed during the initial colour naming encoding task.

The main effect of Word Valence also was significant (F[I,30] = 11.21, P < 0.01), reflecting the higher number of correct identifications made for Threat Words (11.92) than for Non-Threat Words (10.70). Additionally, a trend towards a significant main effect of Subject Group was observed (F[1,30] = 3.09, 0.05 < P < 0.1, with the Control Subjects showing a slightly higher number of correct identifications (12.41) than was shown by the Anxiety Patients (10.22).

Of most importance, however, was the finding that all of these main effects were subsumed within a significant higher-order interaction involving Presentation Status, Word Valence and Subject Group (F[1,30] = 4.25, P < 0.05), which was the only other effect to emerge from the ANOVA. In order to facilitate the presentation of this interaction, we calculated an Implicit Memory Index for each word type by subtracting the number of correctly identified New Words from the number of correctly Old Words, for each group of Ss and for each valence of words. A high score on this index will reflect a high level of implicit memory. Figure 2 illustrates the nature of the above three-way interaction, by plotting this Implicit Memory Index as a function of both Word Valence and Subject Group.

As can be seen, the nature of the interaction is fully consistent with the experimental hypothesis. Relative to the Control Subjects, the Anxiety Patients did show a disproportionately high level of implicit memory for the Threat Words relative to the Non-Threat Words, as predicted. In fact, the control Ss obtained a lower implicit memory score for the Threat Words (5.89) than for the Non-Threat Words (7.13). In contrast, this pattern was reversed for the Anxiety Patients, who

Table 4. Mean number of words identified in the implicit memory task (standard deviations in parentheses)

Threat Words Non-Threat Words Old Words New Words Old Words New Words

Control Subjects 15.75 9.88 15.56 8.44 (3.94) (4.47) (3.98) (4.42)

Anxiety Patients 14.44 7.63 12.13 6.69 (3.48) (3.90) (3.83) (4.51)

10 COLIN MACLEOD and KARl MCLAUGHLIN

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WORD VALENCE

Fig. 2. Implicit memory scores for Threat and Non-Threat Words.

obtained a higher implicit memory score for Threat Words (6.81) than for the Non-Threat Words (5.44).

DISCUSSION

The pattern of results obtained in this study provide clear support for our experimental predictions. Although the recognition task revealed the presence of explicit memory for stimulus words, there was no tendency whatsoever for the anxiety patients to show any relative explicit memory advantage for the threat-related items. The tachistoscopic identification task revealed that Ss also showed substantial implicit memory for the stimulus words. However, the degree of implicit memory depended upon both the emotional valence of these words, and upon the S group. Control Ss tended to demonstrate a lesser degree of implicit memory for the threat words than for the non-threat words. In contrast, the anxiety patients tended to show a greater degree of implicit memory for the threat words than for the non-threat words. These findings are fully consistent with Williams et al.'s hypothesis that elevated anxiety should be associated with an implicit memory advantage for threat-related information, but with no explicit memory advantage for such information.

By demonstrating that anxiety patients show a relative memory advantage for threat-related stimuli on an implicit memory task, but not on an explicit memory task, the current experiment has provided a conceptual replication of Mathews et al.'s (1989) earlier experimental findings. Furthermore, the current study confirms that this dissociation generalises to measures of explicit and implicit memory that are quite different to those employed by Mathews et al., and occurs even when the initial encoding task does not encourage elaborative processing. Given the clear success of this present replication attempt, it is interesting to consider the possible reasons why both Nugent and Mineka (1994), and Mathews himself (1994) have been unable to replicate Mathews et al.'s (1989) earlier finding, using the same encoding task and memory tasks as were employed in that original study.

It perhaps could be argued that Nugent and Mineka's failure to replicate Mathews et al.'s (1989) results may reflect the fact that these researchers contrasted high and low trait anxious students, whereas Mathews et al. compared clinically anxious patients with non-anxious control Ss. However, the level of trait anxiety reported by Nugent and Mineka's high trait anxious students

Implicit and explicit memory bias in anxiety 11

on the STAI (50.4 in exp. 1; 52.9) were very close to those reported by Mathews et al.'s (1989) clinically anxious patients (56.5), and were as high as the level reported by our own clinical patients (51.94). Therefore, any attempt to attribute Nugent and Mineka's replication failure to their use of a non-clinical population must rest on the argument that clinical status per se, not simply extreme levels of anxiety, serves to mediate the presence of this implicit memory bias. Of course, this argument cannot serve to explain Mathews (1994) own recent failure to replicate their original finding, using a clinically anxious population. In the interest of parsimony, therefore, we suggest that these replication failures may reflect the relative insensitivity of the experimental methodology employed by Mathews et al. (1989), Mathews (1994), and Nugent and Mineka (1994), to the pattern of anxiety-linked memory biases predicted by Williams et al. (1988). Perhaps this may be due to the specific nature of the implicit memory task employed in those earlier experiments.

If elevated anxiety is associated with facilitated implicit, but not explicit, memory for threat- related information, then this anxiety-linked memory bias would be expected to appear most reliably on those tasks that provide the purest measure of implicit memory. However, as has been noted, Perruchet and Baveux (1989) claim to have demonstrated that the word stem completion task, employed by Mathews et al. (1989), Mathews (1994) and by Nugent and Mineka (1994), provides a particularly impure measure of implicit memory which is "strongly contaminated by explicit . . . remembering" (p. 85). This may explain both the marginal nature of Mathews et al.'s (1989) original support for Williams et al.'s predicted implicit memory bias, and the difficulties encountered in later replication attempts employing this particular index of implicit memory. In contrast, according to Perruchet and Baveux, the tachistoscopic identification task employed in the current experiment provides the most "direct . . . uncontaminated and ubiquitous indicator of implicit memory" (p. 77). Perhaps this explains why the anxiety-linked implicit memory bias, predicted by Williams et al., was so clearly demonstrated using this index of implicit memory in the current study.

Of course, it remains for future research to determine whether the support currently provided for Williams et al.'s predictions, through the use of these alternative measures of explicit and implicit memory, proves easier to replicate than the original supportive findings obtained by Mathews et al. (1989) using word stem completion tasks. Such future research also might usefully attempt to establish which emotional dimension is most directly associated with the currently observed pattern of memory biases. Although the clinical patients considered in the present study were suffering from generalised anxiety disorder, and reported elevated levels of both trait and state anxiety, it must be recognised that they also reported higher levels of depression than did the control Ss. Consequently, it could be argued that the observed implicit memory bias may have been mediated by depression, rather than by anxiety. Such an account would be quite at odds with Williams et aL's model, according to which depression (unlike anxiety) is associated with the disproportionate elaborative processing of emotionally negative information, but not with the disproportionate integrative processing of such information. Indeed, Williams et al.'s account leads clearly to the prediction that elevated depression will result in an explicit memory advantage for emotionally negative stimuli, but no implicit memory advantage for such stimuli (i.e. a direct reversal of the predictions Williams et al. make concerning anxiety). In view of the differential predictions made for anxiety and depression, Williams et al.'s model would be subjected to a stronger test if future research using the current paradigm were designed to distinguish the patterns of implicit and explicit memory associated with anxiety and with depression.

Although it is well established that depression indeed is associated with facilitated explicit memory for negative stimuli (e.g. Derry & Kuiper, 1981; Clark & Teasdale, 1982, 1985; McDowell, 1984; Mathews & Bradley, 1983; Bradley & Mathews, 1983; Dobson & Shaw, 1987), only two recent experiments have contrasted depressed Ss' implicit and explicit memory for emotional information (Denny & Hunt, 1992; Watkins, Mathews, Williamson & Fuller, 1992). In each of these studies Williams et al.'s prediction, that depressed Ss would show facilitated explicit, but not implicit, memory for negative information, was supported. However, the index of implicit memory considered in both experiments was provided by the same word stem completion task employed by Mathews et al. (1988), Mathews (1994), and Nugent and Mineka (1994). Given Peruchet & Baveux's criticisms of this measure of implicit memory, it may be appropriate for future research to examine the performance of depressed Ss on the explicit and implicit memory

12 COLIN MAcLEOD and KARl MCLAUGHLIN

tasks used in the current experiment. Furthermore, the prediction that anxiety and depression will be associated with quite different patterns of implicit and explicit memory for emotional stimuli would be most directly addressed if the performance of depressed Ss, anxious Ss and control Ss were contrasted within a single experimental study.

Though the results of the current study serve to confirm Williams et al.'s predicted anxiety-linked implicit memory advantage for threat-related information, cognitive theorists remain divided concerning the issue of how best to conceptualise the distinction between explicit and implicit memory performance. The traditional view, upon which Williams et al.'s predictions originally were based, has been that implicit and explicit memory tasks tap quite distinct information storage systems (e.g. Mandler, 1980; Graf & Schacter, 1985; Schacter & Graf, 1986; Schacter, 1987). According to this account, implicit memory is facilitated by integrative processing alone, whereas elaborative processing serves to facilitate retrieval from explicit memory (e.g. Graf & Mandler, 1984; Mandler et al., 1986). Within this framework, the current findings offer support for Williams et al.'s hypothesis that elevated anxiety is associated with the facilitated integrative, but not elaborative, processing of threatening information.

However, more recently, several theorists have challenged the view that implicit and explicit memory exist as separate stores within the cognitive system, and have suggested instead that dissociations between implicit and explicit memory performance may better be construed as examples of transfer-appropriate processing (e.g. Blaxton, 1989; Roediger, 1990; Roediger & McDermott, 1992). Specifically, such theorists have argued that performance on any memory task will be facilitated when that memory task assesses a similar type of cognitive processing to that initially carried out on the original stimuli during encoding. They suggest that most explicit memory tasks require conceptual processing of the information that must be retrieved, and hence performance is improved when the initial encoding task also required conceptual processing rather than simply perceptual processing of the originally presented stimuli. Conversely, since most implicit memory tests (including the current tachistoscopic identification task) plausibly require perceptual rather than conceptual processing, performance on these tasks should be best when the stimuli presented in the encoding task originally were processed at a perceptual rather than a conceptual level. According to this account, therefore, the current finding that anxiety patients showed facilitated implicit, but not explicit, memory for threat-related information suggests that elevated anxiety is associated with a tendency towards increased perceptual processing of threatening stimuli, but not with any increased conceptual processing of such stimuli.

Assuming that the emotion of anxiety originally developed to prepare an organism to deal with some proximate, physically dangerous, threat in the external environment, such as an aggressive predator, then it is not difficult to identify the possible evolutionary value of this proposed type of cognitive bias. Enhanced perceptual processing of a stimuli that has been identified as threatening may well increase survival chances more than would the enhanced conceptual processing of that stimuli. For example, it may well be more adaptive to ascertain the physical location and trajectory of an attacking predator's teeth and claws than to discriminate the particular genus to which that species of predator belongs, at least during the period of immediate confrontation. Of course, such primordial adaptive benefits may be reduced or lost in contempo- rary society, where threats tend to be more social in nature. Indeed, if anxiety patients do tend to selectively process social threat cues perceptually, but not conceptually, then this may result in an increased tendency to perceive such potential threat cues but a limited ability to conceptualise their (possibly innocuous) social function.

As yet, it remains uncertain whether the currently observed group differences in the patterns of selective processing shown on implicit and explicit memory tasks should be considered to reflect the independent operation of two separate memory systems, or the differential transfer of perceptual and conceptual processing from the encoding task to each type of memory task. Nevertheless, the present results clearly confirm that elevated anxiety indeed is associated with a memory advantage for threat-related information when implicit retention is assessed using a perceptual identification task, but not when explicit retention is assessed using a recognition task. These findings provide a conceptual replication of Mathews et al.'s (1989) findings, and are fully consistent with the specific experimental predictions derived from Williams et al.'s model of anxiety and cognition.

Implicit and explicit memory bias in anxiety 13

Acknowledgement--This research was supported by Australian Research Council Grant A79030216 to Colin MacLeod.

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