~ Pergamon S0005-7967(96)00046-0

Behav. Res. Ther. Vol. 34, No. 9, pp. 695-705, 1996 Copyright © 1996 Elsevier Science Ltd

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E V I D E N C E F O R A T T E N T I O N T O T H R E A T E N I N G S T I M U L I

I N D E P R E S S I O N

A N D R E W M A T H E W S *t, VALERIE R I D G E W A Y I and D O N A L D A. W I L L I A M S O N 2

~Applied Psychology Unit, Medical Research Council, 15 Chaucer Road, Cambridge CB2 2EF, U.K. and "~Department of Psychology, Louisiana State University, Baton Rouge, LA 70803-0104, U.S.A.

(Received 8 May 1996)

Summary--A modified version of the attentional deployment task developed by MacLeod, Mathews and Tata (1986) [Journal of Abnormal Psychology, 95, 15-20] was used to examine two issues: first, whether there was any evidence of attentional bias in depressed subjects, rather than in anxious subjects alone; and second, whether attentional effects would occur in the location of stimuli that could not be identified. Subjects were presented with pairs of words, one above the other, and the extent to which attention favored threatening rather than neutral words was assessed from the latency to detect a dot in the same location of one them. These detection latencies showed that depressed, but not anxious subjects, were selectively attentive to socially threatening words. There was also evidence for attentional effects in the anxious subjects favoring physically threatening words. Furthermore, panic disorder patients were preferentially attentive to the location of physically-threatening stimuli that could not be accurately identified. Overall, the results provide further evidence that emotionally disturbed subjects tend to orient attention towards personally-relevant emotional stimuli. However, the previous hypothesis that this attentional bias occurs only in anxiety, and not in depression, was not supported. Copyright © 1996 Elsevier Science Ltd

I N T R O D U C T I O N

In previous work it has been found that Ss with a diagnosis of Generalized Anxiety Disorder (GAD) selectively attended to emotionally threatening words, in preference to simultaneously presented neutral words (MacLeod, Mathews & Tata, 1986; Mogg, Mathews & Eysenck, 1992). In these experiments, two words were displayed simultaneously and on some trials one of them was then replaced by a small dot. The latency to detect this probe dot was used to index the extent to which Ss were attending preferentially to one type of word rather than another. Anxious Ss were faster to detect probes replacing threatening words, while non-anxious controls were faster when probes replaced neutral words (MacLeod et al., 1986) or failed to show any differences (Mogg et al., 1992). Only the former study included a depressed group, and the mean scores for these Ss lay between those for the anxious and control Ss, without differing significantly from either. On the basis of these and other findings, it has been proposed that attention to threatening information in the environment is characteristic of anxiety but not of depression (Williams, Watts, MacLeod & Mathews, 1988; Mathews, 1990).

The importance of this suggestion depends on the related possibility that attentional bias towards threatening information could play a causal (or at least a maintaining) role in anxiety disorders, but not in depression. According to this view, a tendency to selectively attend to information related to possible threat will tend to enhance anxious mood, and lead to the subjective impression of personal danger that is characteristic of anxiety disorders. It has proved more difficult to find convincing evidence for a memory bias that favors threatening information in anxious Ss, with many mixed and contradictory results (Mathews & MacLeod, 1994). As a result, we have further argued that the initial attentional bias in anxiety is followed by voluntary attempts to avoid further processing, thus sometimes leading to poorer than expected recall (Mogg, Mathews & Weinman, 1987).

It appears, therefore, that anxiety is associated with a fairly robust attentional bias, but variable

*Author for correspondence.

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696 Andrew Mathews et al.

effects on subsequent memory for the attended information. In apparent clear contrast with this pattern, depression is more strongly associated with memory effects favoring negative information about the self (Blaney, 1986; Teasdale, 1988; Mathews & MacLeod, 1994), but no evidence for any attentional bias. It is thus tempting to speculate that these distinctive patterns of processing emotional information reflect important differences between the two types of mood state or disorder. That is, anxiety is concerned with the anticipation of future threat and is hence associated with a vigilant attentional mode of processing, while depression is more concerned with past loss or failure, and is associated with a ruminative or elaborative processing mode.

These speculations depend critically on the reliability of the observed differences in attentional and memory bias between depressed and anxious samples. However, these two groups have very rarely been included within the same design, and conclusions have usually been drawn on the basis of comparisons across different studies. Because the present experiment is concerned with attention, we will focus specifically on attentional bias, and the issue of whether it is specific to anxiety rather than depression.

Several authors have documented Stroop-type interference when color-naming emotional words either in anxious (Mathews & MacLeod, 1985; Mogg, Mathews & Weinman, 1989; McNally, Riemann & Eunsil, 1990; Mogg, Bradley, Williams & Mathews, 1993) or in depressed samples (Gotlib & McCann, 1984; Gotlib & Cane, 1987; Williams & Nulty, 1986). Since color-conflict interference is usually thought of as an attentional phenomenon (MacLeod, 1991), emotional interference might equally be taken as evidence that both depression and anxiety are associated with selective attention to emotionally congruent information. However, the source of emotional interference with color-naming has not been systematically investigated, let alone unequivocally established. While differential attention to emotional information is certainly a viable hypothesis, many other explanations seem possible, including post-attentional elaboration on emotional meaning.

There have been very few studies that directly assessed attention to threatening words in depressed Ss (MacLeod et al., 1986; Gotlib, McLachlan & Katz, 1988; Hill & Dutton, 1989). As indicated earlier, MacLeod et al. concluded that their depressed sample did not show any evidence of bias, since no significant effects were established in either direction for this group. However, the depressed group was not well matched in age or clinical status with the other two, and consisted of older in-patients, in contrast to the younger anxious Ss who were being seen as out-patients.

The study by Gotlib et al. (1988) involved a modified task that differed in several possibly important respects from that used by MacLeod et al. (1986). Instead of using a probe dot, the two words were replaced by color bars, producing the illusion that the bar in the attended location appeared first. The words used were self-descriptive adjectives rather than threat descriptors, and the Ss were mildly depressed college students, rather than being clinically depressed. Despite these differences, the results again indicated no evidence of attentional bias in depressed Ss, although they were significantly distinguished from non-depressed controls, who attended more to the positive words. In a follow-up study of the same method with another student sample, Mogg, Mathews, May, Grove, Eysenck and Weinman (1991) concluded that this paradigm was a rather fragile one, and that when effects were found, they depended more on state anxiety (not measured by Gotlib et al.) than on depression. Finally, Hill and Dutton (1989), using the original paradigm, failed to find differences between students with high or low scores on the Beck Depression Inventory, in attention to self-esteem threatening words. Thus, although the evidence is not conclusive, it is consistent with the view that selective deployment of attention to threatening words is associated with anxiety, but not depression.

A final issue is that of whether attentional shifts are initiated by nonconscious "preattentive" processes, as argued by Williams et al. (1988), or whether they occur only after Ss become aware of the word meaning. The former possibility is important to evaluate, because if true it might help to explain how anxiety episodes could sometimes arise without the individual being aware of the stimuli that initiated them. Broadbent and Broadbent (1988) successfully replicated the findings ofMacLeod et al. (1986) with a large student sample varying in anxiety level, but noted that vigilance effects in high trait anxious Ss were larger in the second half of the experiment. As a result they suggested that their Ss may have become increasingly aware of the categories of word being presented, and that attentional effects may depend on such awareness. One purpose of the present

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study was to investigate this issue by presenting some stimuli in such a way as to allow Ss to be fully aware of the words prior to the onset of attentional probes, but presenting others so as to severely restrict such awareness. If awareness of word meaning is necessary before attentional shifts are initiated, then biases should not be apparent in the more restricted condition.

Mogg, Bradley and Williams (1995) have recently reported a related study of attentional bias in both depressed and anxious Ss, using a modified version of the visual probe task (MacLeod et al., 1986). In this modified version, some of the critical words were displayed for 1 sec before being probed for attention, and others for only 14 msec before being masked, to reduce awareness. Contrary to previous evidence, Mogg et al. reported evidence for attentional bias in depressed (as well as anxious) Ss, but only when words were displayed for the longer duration. The present study overlaps with that of Mogg et al. (1995) in several respects, but because the former was completed prior to their report, a full consideration of similarities and differences will be postponed for later discussion.

Both clinically anxious and depressed groups were studied in the present experiment, as well as a matched control group, to determine whether attentional effects in either full or restricted awareness conditions were present in depressed as well as anxious Ss. Different types of threatening words were used to match the likely concerns of each clinical group (see Table 1). Half of the threatening words used referred to physical threats, such as death and disease, that have already been found to be relevant to the concerns of patients with GAD and Panic Disorder (Mathews & MacLeod, 1985; McNally et al., 1990). The remaining half referred to social threats such as failure or incompetence, known from previous work to be more relevant to the concerns of depressed patients than are physical threats (Watkins, Mathews, Williamson & Fuller, 1992). The experimental design used thus involved three groups of Ss, who were presented with different types of emotional word, each paired with a neutral word, and displayed under two different conditions of awareness.

METHOD

Overview

A total of 67 anxious, depressed or control Ss were tested using a modified form of the attentional probe task developed by MacLeod et al. (1986). On critical trials, an emotionally threatening and a neutral word were displayed one above the other on a computer screen, and Ss were required to read a number centrally located between them. After half a second the words were removed, and a small dot appeared in the location previously occupied by one of the words. Latency to detect the dot was used as an index of attention to that word. On half of the trials that were probed in this way the word pairs were unmasked and easy to read, and for half they were masked to restrict the ability of Ss to read them. The prediction that anxious or depressed Ss would selectively attend to threat cues could thus be tested by comparing latencies when the dot position did or did not coincide with the location of a threatening word, under conditions of relatively high or low awareness.

Subjects

Anxious and depressed University, in response to

Ss attended the Psychological Services Center at Louisiana State newspaper articles announcing the availability of treatment. After

Table 1. Sample of words used in the experiment: Physical-threat/ household and social-threat/household pairs, presented in random

order and position

Physical threat/household Social threat/household

choking/cooking hopeless/hatstand coffin/carpet inept/item

disease/diapers humiliated/humidifier ambulance/airblower indecisive/illustration

funeral/funnel lonely/living suffocate/sweeping useless/utensil

surgery/sleeper friendless/furniture injury/inside stupid/steps

698 A n d r e w M a t h e w s et al.

Table 2. Mean subject characteristics by group (SDs in parentheses)

Anxious (25) Depressed (20) Control (22)

Age 36 (12.3) 35 (8.8) 34 (12.5) Education 13.4 (2.9) 15.1 (2.0) 15.6 (2.2) BDI 16.9 (I 1.6) 25.4 (8.3) 6.0 (5.9) STAI state 46.0 (13.6) 52.9 (13.1) 36.3 (7.3) STAI trait 52.8 (13.8) 63.2 (9.9) 39.5 (10.3)

telephone screening, some were offered an initial assessment interview with a clinical graduate student under the supervision of the Clinical Director (DW). Assessment interviews were structured according to the Schedule for Affective Disorders and Schizophrenia (Endicott & Spitzer, 1978) to confirm depressive disorder diagnoses, and the Anxiety Disorders Interview Schedule--Revised (DiNardo, Barlow, Cerny, Vermilyea, Vermilyea, Himadi & Waddell, 1985) in the case of anxiety disorder diagnoses.

To be included in the study, anxious Ss had to have a diagnosis of GAD (11) or of Panic Disorder (14) with or without agoraphobia. Subjects not meeting these criteria, such as those with only simple phobias, were excluded, together with those who also had any other primary psychiatric diagnoses. Depressed Ss met criteria for either Dysthymia (13) or Major Depressive Disorder (7), without other primary diagnosis. After collection of clinical Ss was complete, an additional sample of 22 normal controls was obtained, matched by sex and age with the other two groups. These Ss were either mature students at LSU, volunteers from a local health club, or were friends and relatives of staff members. None of the controls was diagnosed as having any psychiatric disorder, or was known to have been so diagnosed in the past. However, three control Ss were excluded on the grounds that their score on the Beck Depression Inventory (Beck, Ward, Mendelson, Mock & Erbaugh, 1961) fell into the clinical range (see Table 2 for means).

Materials

Two types of emotional words were used, designated as socially threatening (24 words), or physically threatening (24), selected from previous work in our laboratory (Table 1). Each of these emotional words was paired with a neutral word that began with the same letter and was of the same length (plus or minus one letter). These neutral words were all related to the home, and were typically the name of a common household object. There were thus 48 pairs, each consisting of one emotional and one categorized neutral word, and these pairs constituted the critical presentations for collection of attentional probe data.

To these critical word pairs were added an additional 96 neutral/neutral pairs that did not include either an emotional word or a categorized household word. Of these, 24 were used in further attentional probe trials, so that equal numbers of social/neutral, physical/neutral or neutral/neutral pairs were probed (24 of each). The remaining 72 neutral/neutral word pairs were unprobed fillers, so that there was the same total number of unprobed as probed pairs. The total number of unique word pairs was thus 144; 72 of which were probed, and 72 of which were not.

Attentional deployment task

Each word pair was presented twice, once in each of two display conditions, referred to as long or short exposure trials. Long exposure trials consisted of the presentation of the randomly chosen word pair for 500 msec, one word 3 cm above the other in the middle of the screen, with a single digit number (between 1 and 9) centrally placed between them. Subjects were instructed to read this number aloud, to ensure that they were fixating a point midway between the displayed words. On half of all trials this was followed immediately by the presentation of a small dot (the probe) in the prior location of one of the words, that remained until the S detected it and responded. Otherwise, the next word pair followed after 1 sec.

Short exposure trials consisted of a rapidly alternating sequence of brief (50 msec) displays, starting with a pair of masks constructed from digits and other nonletter symbols (for example, @%5&9[!#), followed by the randomly chosen word pair, then a second mask display, and then a blank screen. Masks were matched for word position and length, so that they completely obscured the presented words. This sequence was repeated three times, except that the final blank was

Attention and depression 699

omitted. In this way, the total interval between onset of the first word pair and offset of the third display of the same words was 500 msec, matching the display time of the long exposure trials. The time available for attention shifts towards the location of a word was thus the same for both types of trial, although words were continuously displayed in long trials, and for three periods of 50 msec in short trials.

Awareness check task

Another set of 32 neutral/neutral word pairs were used under short exposure conditions to determine the accuracy with which they could be identified. In this task, a single randomly selected word was presented in both upper and lower position, for three masked 50 msec displays as described above, while Ss read the number centrally placed between them. After this, two more words appeared to the left and right of the prior location of the number, one of which was the word that had just been displayed, while the other was a different word of the same length and beginning with the same letter. Positions of the repeated and new words were randomized across trials. Subjects were required to decide which of these was the word that had been flashed up previously, and then rate their confidence on a 1-5 scale, from "no idea, just a guess" to "quite certain".

Procedure

Subjects first performed the awareness check task, which was described as an experiment on word perception. Both this and the attentional deployment task that followed were controlled and timed using the Micro Experimental Laboratory system (MEL: Schneider, 1988). Subjects were seated about 2 ft in front of a computer screen, and read self-paced instructions including two examples, before beginning the task. Most Ss responded to the practice trials by complaining that it was quite impossible to see the briefly presented words, and were reassured that they should just do the best they could. After making their choice between the two possible words by pressing a left ( < ) or right ( > ) arrow key, Ss were shown the 1-5 confidence scale, and asked to press a number key to rate how sure they were about their decision.

On completion of 32 awareness check trials, new instructions were presented that described the attentional deployment task. Sixteen practice trials were then given, one block of eight long exposure trials, and one of eight short exposure trials. Order of these and subsequent blocks was randomly determined by the MEL system. Half of both sorts of trial included the display of a probe dot, and Ss were instructed to keep their finger on the space bar, and to press it as soon as they saw the dot. They were also reminded as necessary to read the displayed number aloud.

After any questions had been answered, Ss proceeded with the main task, which was divided into six blocks of 48 trials each. The first block of 48 trials was again randomly allocated either to the short or long exposure condition, and the second to the alternative condition. After two blocks of trials Ss were given the opportunity to rest, after which the randomization procedure was repeated with another two blocks, and so on. Half of all trials was probed within each block, and the location probed as well as the position of the critical words was balanced within blocks. There was thus a total of 288 trials, 144 of each duration, with each pair of words being presented twice, once in each condition.

After completion of the attention deployment task, the awareness check task was administered for a second time. Finally, Ss filled out the State form of the State Trait Anxiety Inventory (Spielberger, Gorsuch, Lushene, Vagg & Jacobs, 1983); and, if they had not already done so in the previous assessment session, the Trait form of the same questionnaire and the Beck Depression Inventory (Beck et al., 1961).

RESULTS

Subject characteristics

The three groups were closely matched for age, and one-way analysis of variance (ANOVA) of ages across groups was not significant, F < 1.0. Years of education were similar for depressed and control groups, but the anxious group had about 2 yr less on average than the other two groups,

700 Andrew Mathews et al.

F(2,59) = 4.56, P < 0.02. All questionnaire scores differed across the three groups with P < 0.001 in each case. For the Beck Depression Inventory, Tukey HSD tests showed that the depressed group scored higher than the anxious group (P < 0.01), who in turn were higher than the controls (P < 0.01). The same pattern was repeated for the Spielberger Trait anxiety score, with the depressed group being highest and controls lowest (all Ps < 0.01). Speilberger State scores were not significantly different across the two clinical groups, but both were higher than for controls (P < 0.05). Thus it appears that, on the basis of self-report measures, the depressed group was generally more disturbed than the anxious group, while both were more depressed and anxious than the normal control group (Table 2).

Probe detection times

Latencies in milliseconds for all trials in which one threatening and one neutral word were presented constituted the main data for analysis. Preliminary overall analysis revealed no main effect of group, so the data was pooled for the purpose of eliminating outliers. Latencies of less than 100 msec were removed as probable anticipation errors, and latencies greater than 1500 msec (approximately three standard deviations above the mean) were likewise removed as atypical and probably representing lapses of concentration. Mean latencies were then calculated for each S under each of 16 possible conditions, representing all possible combinations of long vs short exposure conditions, type of threa.tening word present, location of this word in the upper or lower position, and whether the upper or lower position was probed.

A repeated measure ANOVA was carried out on cell means for each S, with one between S factor (group) and four within S factors (exposure, type of threat, word position and probe position). As before, this revealed no main effect of group, but more rapid responses were reliably associated with the short exposure condition, F(1,64) = 8.45, P < 0.005, and with probes presented in the upper position, F(1,64) = 11.55, P < 0.002. These latter main effects can probably be attributed to reduced distraction effects when Ss could not identify the words that were displayed in short exposure trials, and the normal reading pattern of beginning at the top.

There were also significant two-way interactions between threat position and threat type, F(1,64) = 5.68, P < 0.02, and more importantly, between threat position and probe position, F(1,64) = 5.91, P < 0.02. This latter interaction reflects an overall tendency for Ss to be faster when detecting probes in the same location as a threatening word, implying a general trend towards attentional vigilance for threat. However, this effect must be interpreted in the context of a four-way interaction between group, threat position, probe position, and threat type, that closely approached significance, F(2,64) = 2.78, P < 0.07. Examination of cell means suggested that this interaction reflected the fact that depressives were more vigilant than the other groups for socially-threatening words, and the anxious group were more vigilant for physically-threatening words (Table 3).

To clarify the above pattern of results, further analyses were carried out to test for group differences associated with the presence of particular types of threatening word. An ANOVA of latencies for socially-threatening word trials alone revealed the same pattern of main effects and two-way interactions as before, but also a clearly significant three-way interaction of group, threat position and probe position, F(2,64) = 3.74, P < 0.03. Inspection of cell means makes it clear that this interaction is attributable to greater vigilance on the part of depressed Ss for the socially-threatening words. To test this interpretation, further ANOVAs were completed for each group separately. The threat position by probe position interaction was significant for the depressed

Table 3. Mean detection latencies when threatening words were present, divided by group, probe position and threat position. (Vigilance scores are calculated by subtracting times for trials when probe and threatening word coincided, from trials when they did not)

Probe: Upper Lower Vigilance

Threat: Upper Lower Upper Lower

(A) Socially-threatening words Controls 528 528 562 559 1.5 Anxious 603 589 625 612 -0.5 Depressed 578 607 614 573 35.0 (B) Physically-threatening words Controls 530 545 544 556 1.5 Anxious 592 610 624 618 12.0 Depressed 583 593 583 591 1.0

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Table 4. Mean detection latencies in anxious subjects when physically-threatening words were present, divided by threat position and probe position

Probe: Upper Lower Vigilance

Threat: Upper Lower Upper Lower

(A) Short exposure trials GAD subjects 613 604 614 656 -26.0 Panic subjects 538 565 588 555 30.0 (B) Long exposure trials GAD subjects 638 681 684 674 27.0 Panic subjects 592 605 621 606 14.0

group, F(1,19) = 6.01, P < 0.03; but not for the anxious group or for the normal controls, both Fs < 1.0. Since the depressed group was made up of 13 patients with a primary diagnosis of dysthymia, and 7 with that of major depressive disorder, a subsidiary analysis was carried out that included this division by diagnosis as a between S factor. However, this ANOVA only confirmed the overall threat position by probe position interaction as before, P < 0.02. Thus, when considering only social threat words, depressed, but not anxious patients, showed attentional vigilance effects.

Lack of a clearly significant interaction involving duration makes it strictly inappropriate to break the data down by exposure condition. None the less, a separate analysis after division into long and short exposure trials revealed a nearly significant vigilance effect in depressed Ss for long exposure trials alone (P = 0.06), but not for short exposure trials alone (P = 0.29). The trend towards vigilance for social threat was thus stronger (albeit non-significantly so) under conditions of greater awareness, in depressed Ss.

A similar analysis of only those trials in which physically threatening words were presented gave a different pattern of results. Apart from a main effect of exposure, the only other finding was that the interaction of group, exposure and threat position approached significance, F(2,64)= 2.95, P < 0.06. Analysis by groups revealed only a non-significant trend towards a threat position by probe position interaction in the anxious group, F(1,24) = 2.88, P = 0.10, that was absent in the other two groups.

However, like the depressed Ss, the anxious group could be considered heterogenous, as it included Ss with diagnoses of both GAD (11) and Panic Disorder (14). For this reason, a final analysis of this data set was performed after breaking down the anxious group by specific diagnosis. This revealed a clearly significant four-way interaction between diagnostic sub-group, exposure, threat position, and probe position, F(1,23) = 6.44, P < 0.02. Inspection of cell means (Table 4) suggested that panic disorder Ss were vigilant for physical threat words, irrespective of exposure duration, whereas generalized anxiety patients were vigilant only for long exposures. Thus on ANOVA of data from panic patients alone, the overall threat position by probe position interaction was significant, F(1,13) = 5.27, P < 0.04; while for generalized anxiety patients the only significant interaction was of threat position by probe position by exposure, F(1,10)= 5.99, P < 0.04. Separate analyses of short or long exposures produced a clearly significant threat position by probe position interaction only for panic patients in short exposure trials (P < 0.04). Thus, whenever significant effects do emerge in the anxious Ss, they reflect greater vigilance for physically threatening words, across both exposure durations for panic patients, but only for long exposures in GAD patients

Awareness check task

A secondary consideration was that of how aware Ss were of the words presented under short exposure conditions. Two types of data were analyzed: ratings made by Ss of how confident they were about their guesses as to which word they had seen, and the actual accuracy of their guesses. Overall, Ss rated their confidence as being extremely low, with a mean of 1.7 on the 15 scale, just below 2.0 (labelled as "hardly at all"). These confidence ratings were subjected to ANOVA with one between S factor (group) and two within S factors, session (before or after the attentional task), and accuracy (correct vs incorrect trials). Trials on which Ss actually gave correct responses were associated with slightly higher confidence ratings, 1.9 vs 1.5, F(1,40) = 19.1, P < 0.001.

In general therefore, Ss believed that their responses were not much more than guesses, although

BRT 34/9--B

702 Andrew Mathews et al.

ratings were slightly higher when they were in fact correct. The mean number of correct responses was 37.6 out of a total of 64 trials, only slightly above chance level (32). ANOVA of the number of correct responses revealed no main effects, although there was a group by session interaction, F(2,40) = 4.31, P < 0.02, apparently due to anxious Ss becoming slightly less accurate over time.

D I S C U S S I O N

The clearest finding of the present study is that, contrary to most previous findings, the depressed patients were selectively attentive to threatening information. Indeed, from the present results there is stronger evidence for vigilance for socially threatening words in depressed than in anxious Ss, although anxious patients did show vigilance for physically-threatening words. On the second main question, of whether long exposures are necessary, the data are less clear. However, there were some indications of attentional effects in panic patients even for masked words. If these findings are considered reliable they suggest some important conclusions: first, that attentional vigilance to emotionally congruent (negative) information is not specific to anxiety but may occur in other mood disorders, although the type of threat attracting attention may differ; and second, that such attentional effects are not always dependent on full subjective awareness of the nature of the stimuli involved. Before considering the theoretical implications that would follow from such conclusions, it is necessary to consider two potential problems of interpretation: the high anxiety scores in depressed patients, and the question of whether Ss were aware of the briefly exposed words.

The first problem arises because the depressed group in this study scored as or more highly than the anxious group on the STAI measure of trait anxiety (as did the depressed Ss in the related study of Mogg et al., 1995). Thus it could be argued that the anxiety component in these depressed Ss was responsible for any attentional vigilance effects. However, in several previous studies using these measures, we and others have not been able to find clinically depressed Ss who do not also score highly on the STAI. In fact, despite its name, the STAI is not a pure measure of anxiety. It contains items that appear to overlap with depression (for example, "I wish I could be as happy as others seem to be"), and it is correlated with questionnaire measures designed to measure depression. In this study for example, the BDI was significantly correlated with both state anxiety (r = 0.79), and with trait anxiety (r = 0.88) on the STAI.

Despite this high degree of overlap on questionnaire measures, those with diagnoses of depression often perform quite differently from those with anxiety disorders on certain cognitive tasks. For example, Bradley and Mathews (1983) found that diagnosed depressives have a memory bias favouring negative words, while patients reporting depressed mood, but with a diagnosis of anxiety disorder, did not. Conversely, Bradley, Mogg, Millar and White (1995) found Stroop interference due to emotional words in anxious patients, but not if they also had a diagnosis of depression. Thus, for whatever reason, diagnosis of an emotional disorder can have a more specific relationship with processing bias than do questionnaire measures of mood. Subjects in the present study were selected to have a primary diagnosis of anxiety disorder, or of depression, but not both. The finding that depressed Ss were actually more vigilant for the socially threatening words is thus taken as clear evidence against the hypothesis that attentional bias is specifically associated with anxiety disorders.

A second potential problem is the difficult question of knowing whether Ss were partly aware of the words presented under short exposure conditions. The position adopted here is based on the distinction made by Cheesman and Merikle (1985) between subjective and objective awareness. Subjective awareness refers to the Ss' belief that they cannot identify a stimulus, whereas objective awareness refers to the ability of Ss to perform at above chance levels when forced to make a choice about that stimulus. Cheesman and Merikle have shown that stimuli presented at just above objective awareness threshold, such that Ss forced choice judgements are better than chance, but may none the less be below subjective threshold, in that Ss remain convinced that they are guessing at random. Furthermore, such Ss fail to adopt helpful strategies that depend on conscious knowledge about the presented stimuli.

In the present experiment, with very rare exceptions, all Ss believed that they were guessing when forced to choose between two possible words, and had very low levels of confidence in the accuracy

Attention and depression 703

of their decisions. Furthermore, most Ss were at chance levels of accuracy in the awareness check trials, and the mean level of accuracy (58%) was only slightly above chance, even when Ss were actively trying to identify the word. This pattern of results is consistent with virtually no subjective awareness (that is, all Ss believed that they could not see the words), but a small degree of objective awareness (that is, Ss' behaviour was influenced, despite their belief to the contrary).

Putting the above two potential problems aside, we can thus summarize the main findings as follows. Depressed Ss showed significant attentional vigilance effects for socially-threatening words and, despite the lack of a clearly significant interaction with exposure, this was apparent only for long exposure durations. In anxious patients, exposure condition interacted significantly with attentional vigilance for physically-threatening words, and the only component to remain significant on post hoc testing was the vigilance effect for short exposure trials in Ss with panic disorder. The results therefore allow two main conclusions. First, depressed patients can show attentional vigilance effects (contrary to our own previous published results) but this is restricted to socially-threatening words, and is probably more reliable when words are clearly visible. Second, panic patients show vigilance effects, for physically-threatening words, even at durations when awareness is severely limited.

These findings can now be compared for consistency with the closely similar study of Mogg et al. (1995). In their study, Ss met criteria for GAD, or major depression, and scores on the STAI and BDI were very close to those reported here. The task used was almost identical, with only minor differences (e.g. long exposures were longer, at 1 sec; while short exposures were shorter, at 14 msec; the same probe was employed, but no central digit was displayed). The negative, word sets used by Mogg et al. were constructed differently in that physically- and socially-threatening words were combined into one anxiety-relevant set (e.g. embarrassed, cancer), while there was a separate set of words judged to be more relevant to depression (e.g. misery, discouraged). Their anxious patients showed evidence of attentional vigilance for both sets of negative words, and this effect was not modified by exposure duration. Depressed patients also showed vigilance for both word sets, but (despite a non-significant interaction with exposure) this was clearly significant only for the longer durations.

Despite some differences, the results of the present experiment bear striking similarities to those just described. Most importantly, both sets of results agree in showing clear vigilance effects in depressed patients, when negative words are clearly visible. The fact that this effect was confined to socially-threatening words in the present experiment is not inconsistent with Mogg et al., since their socially-threatening words were included as part of the supposedly anxiety-relevant set. It is quite possible, therefore, that depressed Ss attend selectively to all negative self-descriptive words, including both socially-threatening and other depressive state and trait descriptors. In both cases, the effect of masked words alone was weaker in depressives, and failed to reach significant levels.

Some differences were apparent in the respective anxiety groups, in that the GAD patients tested by Mogg et al. showed significant attentional effects over all conditions; whereas in the present study, although there was an overall interaction involving all anxious patients, when broken down this showed clear vigilance effects only for panic patients with physically-threatening words presented under conditions of restricted awareness. In this case the present findings for GAD patients are out of line with previously reported significant vigilance effects in this group (e.g. MacLeod et al., 1986; Mogg et al., 1992). The specific vigilance by panic patients for physically-threatening words is consistent, however, with previous findings in this S group (e.g. Asmundson, Sandler, Wilson & Walker, 1992), and with the clinical hypothesis that panic is associated with special concern about disease and bodily symptoms. We and others have previously argued (e.g. Mogg et al., 1992; Mathews & Klug, 1993; Mathews & MacLeod, 1994) that attentional effects depend on the match between presented stimuli and current emotional concerns of Ss. Thus, one possible explanation for the lack of clear effects in the present GAD sample is that the words we used did not match their concerns closely enough. Whatever the explanation of this one anomaly, the more interesting finding from both the present study, and that of Mogg et al. (1995), is that of clear vigilance effects in depressive patients.

This finding in depression raises further questions: why the effect was not found in previous studies; and why the clinical groups differed in the type of word and exposure conditions that attracted attention? As to why no significant effects have been found with depressed samples in

704 Andrew Mathews et al.

previous studies, there would seem to be a number of possibilities. We have already noted that the clinical groups were poorly matched in our first study of this issue (MacLeod et al . , 1986). Given the greater age and chronicity of the depressive in-patients tested, it now seems possible that their generally poorer concentration and motivation may have compromized the sensitivity of our measure of selective attention. Equally, the student groups used in other studies may have been less depressed than the present clinical sample, and attentional effects may not be apparent below some level of severity. Finally, different samples of words were used in each study, and the arguments above suggest the importance of matching content with the likely concerns of the sample tested.

This last point may also provide an explanation of the differences between groups in the type of word that attracted attention. Both clinical groups may be characterized by attention to threatening information provided this matches their dominant concerns: physical threats in the case of panic, and social threats in the case of depressed Ss. However, other interpretations are possible. Physically-threatening words describe feared stimulus events, either wholly external or related to bodily damage, such as "assault" or "disease". In contrast, most socially-threatening words have negative personal connotations, such as "stupid" or "pathetic". If depressed Ss ruminate a great deal about their own personal inadequacy, then they may attend to related words because they match these ruminations, rather than because they are perceived as threatening in the sense of representing a dangerous event. Thus it is possible to argue that depression is not associated with vigilance for cues associated with danger (as is the case with panic and GAD), but rather with attention to negative aspects of oneself. Finally, the failure to show effects in depression when words are masked, suggests that this self-related attention is strategic rather than automatic. Hence, the present results are not consistent with one part of the Williams et al. (1988) hypothesis (that depression is associated with a bias in memory, but not in attention). None the less, they remain consistent with another part of that hypothesis: the suggestion that bias in depression depends more on controlled rumination than on automatic perceptual vigilance.

Acknowledgements--Thanks are due to Lori Perin and Susan Barker for help in testing subjects.

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