Memory deficits in depression - Springer Link

Cognitive Therapy and Research, Vol. 12, No. 6, 1988, pp. 611-627

Memory Deficits in Depression Catherine A. Colby London Board of Education, London, Ontario

Ian H. Gotlib 2 University o f Western Ontario

Despite clinical reports of impaired memory functioning of depressed persons, the results of empirical studies in this area have been equivocal. The present study was designed to address a number of the methodological shortcomings of these earlier investigations, examining potential depressionassociated deficits in sensory and short-term memory processing in a single sample of subjects. Twenty depressed and 20 nondepressed subjects participated in two tasks, each examining one of these aspects of memory. The results of this study suggest that whereassensory memory is intact in depressed individuals, short-term memory processing is impaired. More specifically, the depressed subjects experienced difficulty with the retention of material, likely owing to impaired rehearsal ability. The results of this study are discussed with respect to their implications for cognitive models of depression, and suggestions for future research in this area are offered. KEY WORDS: depression; memory; cognitive functioning; information processing.

Impaired cognitive functioning is considered to be a prime symptom of clinical depression. In a review of research on depression, for example, Akiskal and McKinney (1975) indicate that "poor concentration, slow thinking or mixed-up thoughts" should be treated as signs or symptoms of depression (p. 287). Similarly, in a clinical description of depression, Mendels (1970) characterizes the thought process of depressives as becoming increasingly in~The authors wish to thank two anonymous reviewers for their helpful comments on an earlier version of this paper. 2Address all correspondence to Ian H. Gotlib, Department of Psychology, University of Western Ontario, London, Ontario, Canada N6A 5C2. 611

0147-5916/88/1200-0611506.00/0 © 1988PlenumPublishingCorporation

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efficient as the depression develops: "Loss of interest, decrease in energy, inability to accomplish tasks, difficulty in concentration, and erosion of motivation and ambition all combine to impair efficient functioning" (p. 7). Moreover, depressed persons themselves report the same kinds of phenomena; indeed, the anecdotal literature is replete with reports by depressed patients of memory impairment (cf. Breslow, Kocsis, & Belkin, 1980; Kahn, Zarit, Hilbert, & Neiderehe, 1975; Sternberg & Jarvik, 1976). Despite these clinical reports, the empirical evidence of memory impairment in depressed persons is equivocal. A number of investigators have, in fact, reported depression-associated deficits in memory functioning. For example, depressed subjects have been found to perform more poorly than have nondepressed subjects on the Wechsler Memory Scale (e.g., Breslow et al., 1980; Stromgren, 1977; Stromgren, Christensen, & Fromholt, 1976; Walter, 1958) and on a synonym-learning task (Kendrick, Parboosingh, & Post, 1965). Moreover, depressed persons have been reported to exhibit impairment in short-term memory (e.g., Glass, Uhlenhuth, & Weinreb, 1978; Sternberg & Jarvik, 1976), in long-term memory (e.g., Whitehead, 1974), and in the ability to shift information from short-term to long-term memory (e.g., Henry, Weingartner, & Murphy, 1973). While these studies have reported depression-associated memory deficits, other investigators have failed to differentiate depressed and nondepressed subjects with respect to their memory functioning. For example, Kendrick and Post (1967), Whitehead (1973), Miller and Lewis (1977), and Hart, Kwentus, Taylor, and Harkins (1987) all found no differences in memory performance between groups of elderly depressed and nondepressed subjects. Other investigators have reported nonsignificant differences between depressed and nondepressed subjects with respect to both short-term (Koh & Wolpert, 1983) and long-term (Weingartner, Cohen, Murphy, Martello, & Gerdt, 1981) memory functioning. Finally, Kahn et al. (1975) and Popkin, Gallagher, Thompson, and Moore (1982) both found that although the depressed subjects in their samples reported experiencing significantly greater memory problems than did the nondepressed subjects, the two groups did not differ with respect to objective performance on a variety of memory tasks. There is controversy, therefore, concerning the existence of memory deficits in depressed individuals. Moreover, if there is in fact a memory deficit in depressed persons, it is unclear what aspects of memory are impaired. For example, whereas some investigators have found a deficit due to attention (e.g., Glass et al., 1978), others have not (e.g., Miller & Lewis, 1977). Similar disagreements revolve around potential deficits in short-term memory functioning (e.g., Koh & Wolpert, 1983; Sternberg & Jarvik, 1976; Whitehead, 1973), in long-term functioning (e.g., Hart et al., 1987; Sternberg & Jarvik, 1976; Stromgren, 1977; Stromgren et al., 1976), and in the transfer of information from short-term to long-term store (e.g., Henry et al., 1973).

Memory Deficits in Depression

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While a number of diverse factors have contributed to this confusing state of affairs, perhaps the most important involve methodological shortcomings of previous studies in this area, and the questionable relationship between various memory tasks and memory processes. For example, in studies in which psychotic geriatric depressives are compared with normal controls, the reported deficits could be due to the general effects of psychosis (e.g., Cronholm & Ottosson, 1961) or to the combined effects of hospitalization and psychosis (e.g., Kendrick et al., 1965). In other studies in which depressed subjects serve as their own controls, no efforts have been made to control for effects of statistical regression to the mean (e.g., Stromgren, 1977; Stromgren et al., 1976). Finally, not only do investigators fail to link their memory tasks to specific memory processes (e.g., Friedman, 1964), but information regarding the procedural use of the tasks is often lacking (e.g., Calev & Erwin, 1985; Henry et al., 1973), making it difficult for the reader to judge the specific memory processes that the task may or may not be assessing. The present study was conducted to address methodological limitations of previous investigations in this area. Mildly to moderately depressed university students participated in two tasks, designed specifically to assess the two memory processes most likely to be impaired in depressed individuals. The first task examined the possibility that depressed persons will exhibit impair-, ment in a memory process labeled variously as the "visual information store" (Sperling, 1960), "iconic memory" (Neisser, 1967), and "sensory memory" (Atkinson & Shiffrin, 1968). Tasks assessing this memory process essentially measure attention to a stimulus display. Assessment of iconic, or sensory, memory is particularly important because it is considered to be the first stage in virtually all information-processing models (Atkinson & Shiffrin, 1968; Long, 1980); a deficit in iconic memory (i.e., attentional impairment), therefore, could affect all subsequent processing. Compared with their nondepressed counterparts, depressed individuals may be less efficient in registering information, perhaps owing in part to their elevated levels of selffocus (cf. Ingram, Lumry, Cruet, & Seiber, 1987; Lewinsohn, Hoberman, Teri, & Hautzinger, 1985). To test this hypothesis, a modified Sperling (1960) task was employed, in which subjects were presented with a series of stimuli consisting of arrays of digits and letters. Either immediately or shortly following display offset, subjects were reuqired to reproduce either a specific row of the display or the entire array. The second task was selected to assess retrieval and short-term memory functioning. Although Sternberg and Jarvik (1976) reported depressionassociated deficits in short-term memory, the nature of this deficit is unclear. For example, depressives may be less able than nondepressives to encode information efficiently either because of inefficient coding strategies or because they lack the energy or resources required to develop such strategies (cf. Cohen, Weingartner, Smallberg, Pickar, & Murphy, 1982). Alternatively,

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the increased self-focus of depressed individuals might interfere with their storage and retention capacity. The purpose of this task was to examine in greater detail the hypothesis that depressed individuals demonstrate deficits in the processes involved in short-term memory functioning. Depressed and nondepressed subjects participated in a digit span task in which strings of digits of varying length were presented and recalled after a variable unfilled interval. This task, therefore, permitted an assessment of storage, retention, and retrieval performance. 3 By examining two aspects of memory functioning in a single sample of subjects, the present study attempted to address the problem of the lack of cross-study comparabilitY in previous research resulting from the use of diverse populations, definitions and measures of depression, and memory tasks. In addition, to control for possible memory distortions due to the affective valence of the stimuli (cf. Blaney, 1986; Gotlib, 1983; Gotlib, McLachlan, & Katz, 1988; Johnson & Magaro, 1987), neutral verbal stimuli were used in all tasks.

METHOD

Subjects Twenty depressed and 20 nondepressed undergraduates at the University of Western Ontario served as subjects. Two measures were used to assign subjects to these two groups. The Beck Depression Inventory (BDI; Beck, Ward, Mendelson, Mock, & Erbaugh, 1961) was used as the primary screening and group assignment device. The BDI is a 21-item self-report instrument that provides a measure of the individual's current level of depression. It has been found to correlate highly with other self-report measures of depression (Davies, Burrows, & Poynton, 1975; Gotlib, 1984) and with psychiatrists' ratings (Bumberry, Oliver, & McClure, 1978; see Gotlib & Cane, in press). A self-report modification of Feighner's Research Diagnostic Criteria for depression (RDC; Feighner et al., 1972) was used as the second screening measure. The Feighner criteria have been found to have both acceptable predictive validity (Morrison, Winokur, Crowe, &Clancy, 1973) and concurrent validity (Helzer, Clayton, Pambakian, & Woodruff, 1978) and have been used in research investigating various psychological and biological processes in samples of depressed subjects (e.g., Coryell, Lowry, & Wasek, 1980; Mathew, Largen, & Claghorn, 1979). The self-report version of the Feighner aSubjectsalso participatedin an additional task. Becauseit is not relevantto the presentdiscussion, however, it will be not described in this paper.

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RDC, developed by Breiter (1979), was identical to the original interviewerrated format, with the sole exception o f the elimination of the original Feighner criterion of requiring a psychiatric illness of 1 month's duration in order for a classification of depression to be made. Subjects were classified as depressed in the present study if they met two criteria: (a) They obtained BDI scores of 12 or greater, and (b) they endorsed at least one of four mood-related symptoms (Feighner A) and four or more secondary symptoms (Feighner B) on the Feighner RDC. Subjects who obtained BDI scores of 6 or less and who did not meet the Feighner criteria outlined above were classified as nondepressed.

Experimental Tasks Two tasks were used in the present study, assessing processes involved in sensory and short-term memory. Sensory Memory. A modified Sperling (1960) partial and whole report procedure was used to examine sensory memory functioning. Sixty-four unique 3 x 4 character arrays of digits and uppercase letters were each displayed twice, in random order. To prevent subjects from encoding row and column characters as pronounceable units, only consonants and digits were used in the arrays. Each of the three rows was composed of two consonants and two digits selected by means of a random number table, with the restriction that no digit or letter could be repeated in the same row. The stimuli were displayed in one field of a two-field Gerbrands tachistoscope. The area circumscribed by the array of characters, each ½ in. (1.3 cm) in height and subtending a vertical visual angle of 1.2 °, was 3V2 by 2½ in. (8.9 by 6.4 cm), subtending horizontal and vertical visual angles of 8.3 ° and 5.9 ° , respectively. Subjects depressed a telegraph key to initiate each trial. In order to assess the Miller and Lewis (1977) Contention that the greater self-preoccupation of depressed individuals would impair their attentional performance, a delay of 0, 10, 20, or 30 sec was introduced, after which the display appeared and remained illuminated for 150 msec. Offset of the display was followed either immediately or after 300 msec by a tone of low, medium, or high pitch. Each tone signaled the subjects to recall a different row of the display (i.e., top, middle, or bottom). In addition, on one-quarter of the trials a bell, rather than a tone, was sounded, signaling the subjects to report the whole display. After the report cue was given, subjects immediately recorded the salient items on a piece of paper containing a grid reproducing the rectilinear structure of the stimulus array. To control for the possibility of a conservative response style in the depressed subjects, subjects were instructed to write a response for each appropriate location (i.e., to fill in all appropriate spaces with some letter or digit), guessing if necessary.

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Subjects received four blocks of trials, each composed of 32 stimuli representing all combinations of four ready intervals (0, 10, 20, 30 sec), two report delays (0, 300 msec), and four report methods (top, middle, and bottom rows; whole report), for a total of 128 trials. The 64 unique arrays were presented in two blocks of 32 (A and B) in an ABAB order. Presentation of trials was randomized within blocks. Short-Term Memory. A digit span task was used to assess short-term memory functioning. Consistent with the visual presentation of the Sperling (1960) procedure, strings of digits composed of 6, 8, 10, or 12 items were projected onto a screen 2 ft. (61 cm) in front of the subject by a Kodak Carousel projector. The strings were ½ in. (1.3 cm) in vertical length, subtending a visual angle of 1~2°, and ranged from 1 ½ in. (3.8 cm) to 5¼ in. (13.3 cm) in horizontal length, subtending visual angles of 3.6 ° and 12.3 °, respectively. Within each string, digits were arranged such that no two consecutive numbers immediately followed each other (i.e., strings such as 3 5 8 9 were excluded). Furthermore, in strings of lengths 10 or less, no digit appeared twice, while in strings of length 12, although two randomly selected digits were repeated, the repetitions were not successive. Strings were displayed for a period equivalent to 1 sec per digit (e.g., six-digit strings were displayed for 6). After stimulus offset, a tone signaling the subject to verbally report the digit string was presented either immediately or after delays of 20 or 30 sec. The experimenter recorded the response and reset the timers for the next display presentation, and a cueing tone was presented to direct the subject's attention to the viewing screen. Each combination of string length and retention interval was randomly presented once within a block of trials. Subjects completed four blocks of 12 trials, for a total of 48 trials. Subjects also received 10 practice trials, randomly sampled from the set of stimuli representing the 12 combinations of four digit sizes and three delays.

Procedure Potential subjects were recruited from classes of undergraduates who participated in a mass testing session conducted 2 to 4 months prior to their participation in the study. At this time students completed the BDI. Those students who met BDI criteria for assignment to either the depressed or the nondepressed groups were requested to return at a later date to participate in the experimental session. Upon arriving at the designated room, subjects were readministered the BDI and were also asked to complete the Feighner Questionnaire. Those subjects whose group assignment status was altered by these test scores were dismissed; only those subjects who were classified consistently as depressed or nondepressed on both assessment occasions were


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included in the study (cf. Kendall, Hollon, Beck, Hammen, & Ingram, 1987; Sacco, 1981). Subjects were also required to complete the WAIS-Clarke Vocabulary Scale (Paitich & Crawford, 1971) in order to control for possible confounding due to group-related differences in intelligence. This 40oitem multiplechoice instrument was chosen for use in the present study because it correlates .92 with the Vocabulary subscale of the WAIS and has the advantage of being self-administered. After completing these three questionnaires, subjects began the experiment proper. Half of the subjects in each group participated in the sensory memory task followed by the short-term memory task, while this order was reversed for the remaining subjects. To minimize carry-over effects, subjects were given a 10- to 15-rain break between these two tasks.

RESULTS S u b j e c t Characteristics

Means and standard deviations for the depressed and nondepressed subjects' ages, reported grade point averages, and scores on the WAIS-Clarke Intelligence Test, BDI, and Feighner A and B criteria are presented in Table I. Because of the group selection criteria, the two groups of subjects differed as expected with respect to their BDI scores (t(38) = 19.28) and their Feighher A and B scores (t(38) -- 10.4, t(38) = 8.16, respectively, all p's < .001). Subjects in the two groups did not differ, however, with respect to age, WAISClarke scores, or grade point averages (all t's(38) < 1). The results of these analyses suggest, therefore, that any observed group differences in memory functioning cannot be attributed to differences in these latter variables. Table I. Means and Standard Deviationson Demographicand

Group SelectionMeasuresa Depressed subjects (20)

Nondepressed subjects (20)

Variable M SD M Age 19.5 1.4 20.0 Reported GPA (in %) 74.2 9.0 76.3 WAIS-Clarke 116.4 5.7 116.8 BDI 15.7 2.6 1.7 Feighner A 2.9 1.2 0.0 Feighner B 4.6 1.6 1.1 aNumbers in parentheses indicate group sizes, GPA = point average, BDI = Beck Depression Inventory.

SD

1.8 7.7 6.0 2.0 0.0 1.0 grade

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Memory Functioning A mixed-factorial design (combination of between- and within-group variables) involving a relatively large number of factors was employed to analyze the data. Because repeated measures and a large number of factors both have the effect of inflating the alpha values associated with the individual Ftests, the Geisser-Greenhouse conservative test of F was used to assess significance levels in all of the analyses reported here (Kirk, 1968). Sensory Memory Functioning. Whereas partial report data are assumed to reflect primarily sensory store processes, whole report data are assumed to reflect memory limitations of short-term store, and the transfer of information from sensory store to short-term memory. Consequently, these two types of data were analyzed separately. In both analyses, the number of items correctly reported was used as the dependent variable. For an item to be considered correct, the proper character must have been reported in the exact row and column location. The partial report data were analyzed by a fiveway analysis of variance (ANOVA) [2(Group: depressed, nondepressed) by 4(Ready Interval: 0, 10, 20, 30 sec) by 2(Delay: 0, 300 msec) by 3(Row: top, middle, bottom) by 4(Block" ABAB)], with repeated measures on the last four factors. The whole report data were analyzed in a similar fashion, with the exception that the Row factor was absent. Because of the large number of higher-order interactions, only significant interactions involving the group variable, the focus of this study, will be reported. In the analysis of the partial report data, no effects involving the group variable were obtained. In contrast, the analysis of the whole report data, which are indicative of the limits of immediate sensory memory, yielded a significant group by block interaction (F(1, 36) = 4.9, p < .05). This interaction is presented in Figure 1. Simple main effects analyses and post hoc Tukey tests indicated that while both the depressed and nondepressed subjects demonstrated improved performance over blocks (F(1, 108) = 4.2, p < .05; F(1, 108) = 50.1, p < .001, respectively), the two groups tended to differ in their performance on the final block (F(3, 144) = 2.6, p < .06). Thus, by the fourth block of trials, the depressed subjects demonstrated poorer performance on the whole report task than did the nondepressed subjects, suggesting a depression-associated attentional deficit on this task. Short-Term Memory Functioning. As is customary in digit span tests of short-term memory processes, the dependent variable in this study is the percentage of trials on which subjects gave correct responses (cf. Watkins, 1977). Subjects' scores were analyzed by a four-way ANOVA [2(Group) by 4(Digit String Size: 6, 8, 10, 12 items) by 3(Delay: 0, 20, 30 sec) by 4(Block: 1-4)], with repeated measures on the last three factors. Three significant effects involving the group variable were obtained. A significant main effect for group indicated that the depressed subjects performed more poorly on


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this task than did the nondepressed subjects (F(1, 36) = 5.2, p < .05). This main effect was qualified, however, by two significant interactions: group by digit string size (F(1, 36) = 4.26, p < .05) and group by delay (F(1, 36) = 3.82, p = .05). These two interactions are displayed graphically in Figures 2 and 3. With respect to the group by digit string size interaction, the results of simple effects analyses indicated that although the performance of the depressed subjects was generally poorer than that of the nondepressed subjects, these group differences attained statistical significance only for digit strings of 8 and 10 items (p < .01 and p < .001, respectively); no group differences were obtained for digit strings of 6 and 12 items (both p's > .05). The lack of significant group differences at the extreme string sizes of 6 and 12 is likely due to ceiling and floor effects. Furthermore, the effects at 8 and 10 digits account for virtually all of the interaction--the difference between the two groups at digit string size 8 is not significantly different from that at size 10 ( F < 1). Consequently, string size (at least for sizes 8 and 10, which are not affected by floor or ceiling effects) and group may most appropriately be considered as noninteracting variables.

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The group by delay interaction implies that the degree of deficit varies with the delay period differentially for subjects in the two groups. Tests of simple main effects demonstrated that whereas delay had no effect on the performance of the nondepressed subjects (p > .05), the depressed subjects performed significantly worse at 20- and 30-sec delays than they did at the 0-sec delay (p < .005). Consistent with these results, tests of simple main effects also showed that the depressed subjects performed more poorly than


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did the nondepressed subjects at delays of 20 and 30 sec (both p's < .01); the small difference observed at 0-sec delay was not significant (p > .05). 4 Thus, depressed subjects demonstrated deficits in short-term memory processing as a function of both the amount of information presented and the length of time allowed for processing.

DISCUSSION The results of this study suggest that whereas sensory memory functioning appears to be intact in mildly to moderately depressed individuals, short-term memory processing is impaired. More specifically, although depressed and nondepressed subjects were indistinguishable with respect to *This interaction, combined with the previous findings of floor and ceiling effects, suggests the presence of a triple interaction, in which the deficit observed at 20 and 30 sec should occur for string lengths of size 8 and 10, but not 6 and 12. While the conventional test for this effect was significant (F(6, 216) = 2.63, p < .02), the conservative test only approached significance (F(1, 36) = 2.63, p < . 11). Visual inspection of this triple interaction indicates a pattern consistent with expectation.

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their short-term memory functioning at a delay of 1 sec, the depressed subjects exhibited poorer performance than did their nondepressed counterparts at delays of 20 and 30 sec. These results are consistent with the findings of Sternberg and Jarvik (1976) and Glass et al. (1978), who also obtained evidence of depression-associated deficits in short-term memory functioning. Although there are a number of possible explanations for this pattern of results, many of these can be eliminated on the basis of other findings of this study. Assuming that retention activity is minimal at immediate recall, for example, the finding of equivalent performance of the depressed and nondepressed subjects at the 1-sec delay reduces the viability of explanations of this memory deficit that implicate short-term store processes other than retention. One such account, for instance, might attempt to explain the deficit in terms of impaired storage capacity. If capacity was impaired, however, the least confounded condition for indexing capacity-percentage of trials correct at 1-sec delay-should have registered a difference between the depressed and nondepressed subjects; no such finding was obtained. Another explanation also rendered unlikely, at least for simple stimuli, involves encoding inefficiency on the part of the depressed subjects. Again, had depressed subjects been less adept than their nondepressed counterparts at encoding the digits, this difference should have introduced a deficit at 1-sec delay, but did not. The same reasoning also suggests that retrieval of information is not a problem in this task for depressed subjects. Finally, depressed subjects may simply demonstrate a conservative response style (cf. Miller & Lewis, 1977). It is important to note, however, that such a response style would be expected to affect performance generally (i.e., at all levels of delay), not just at the longer delay periods. Thus, this explanation, too, appears to be untenable. The results of this study do, however, support an explanation that suggests that the depressed subjects are experiencing difficulties with the retention of material (i.e., the ability to retain items in short-term memory for delays greater than 1 sec). Moreover, it is likely that these retention difficulties are a function of the impaired rehearsal ability of the depressed subjects (cf. Greene, 1987). Delay had no effect on the performance of the nondepressed subjects, a finding that suggests that these subjects used the unfilled retention interval to rehearse. Within the limits imposed by string size, these nondepressed subjects successfully maintained the trace of the to-berecalled items. In contrast, the 20- and 30-sec delays had deleterious effects on the performance of the depressed subjects, who demonstrated significantly fewer successful recall trials for string sizes of 8 and I0. The most likely explanation for these effects, although not directly investigated in this study, involves group differences in rehearsal ability. Because the stimuli used in this task were digit strings, it is unlikely that such processes as recoding or


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semantic elaboration were employed by the subjects. A more plausible explanation involves rehearsal, by which subjects either silently or overtly maintain verbal or visual representations of the digit strings. Some evidence from the present study, albeit indirect, provides a reasonable basis for these hypotheses. When recall is immediate, only minimal rehearsal is required. Thus, the obtained finding of no deficit at 1-sec delay is expected if rehearsal is the impaired process. Recall at 20 and 30 sec, however, does require rehearsal in order to maintain the information for short periods of time. The corresponding deficit at these two intervals, therefore, is consistent with the impaired rehearsal hypothesis. Other evidence from the empirical literature supports the postulation of rehearsal as the impaired process in the short-term memory functioning of depressed individuals. Berndt and Berndt (1980), for example, gave depressed and nondepressed subjects a Brown-Peterson task, in which rehearsal of consonant-vowel-consonant trigrams was purposefully prevented by having subjects count backward. Berndt and Berndt found that depressed and nondepressed subjects performed similarly on this task, indicating that when rehearsal is experimentally impaired in nondepressed subjects, they perform no differently from depressed subjects. The Hasher and Zacks (1979) research also provides evidence that depressives engage in less rehearsal than do nondepressed subjects. Subjects were given a list of 40 target words, followed by a recognition task consisting of a four-alternative, forced-choice test with distractors that were either associatively related, acoustically similar, or unrelated to the original target word. Although the depressed and nondepressed subjects did not differ from each other with respect to their total recognition scores, the depressed subjects did make fewer associative errors. Hasher and Zacks interpreted this finding in the context of multiattribute theories of word meaning. Essentially, the less actively an old item is rehearsed, the less likely its high frequency associates will be activated, and the less frequently such associates should be confused with the original item. Thus, because the depressed group made fewer associative errors than did the nondepressed group, Hasher and Zacks concluded that the depressed subjects engaged in less rehearsal activity than did the nondepressed subjects. Findings both from the present study and from other investigations, therefore, suggest that depressives' retention ability in short-term memory functioning is impaired and, further, that this impairment is likely due to rehearsal difficulties. The question of why rehearsal should be impaired in depressed individuals, however, is not answered by these data. Although this issue requires more direct exploration in future research, two possible explanations will be offered here. First, it is possible that depressed individuals engage in rumination, which interferes with the rehearsal of the to-beremembered material by competing for available cognitive resources (cf. Norman & Bobrow, 1975). Indeed, rumination, or self-focus, and its intru-

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sive influence on daily functioning is one of the more frequently assessed symptoms of depression (e.g., Ingram et al., 1987; Levitt & Lubin, 1975; Lewinsohn et al., 1985; Rippere, 1980). A number of investigators, for example, have found that both depressed patients (Beck, 1976; Fogarty & Hemsley, 1983; Teasdale & Rezin, 1978) and depressed students (Teasdale & Fogarty, 1979) report the intrusion of frequent unpleasant thoughts. In the present study, one item from secondary questionnaires required subjects to report the rate at which they experienced intrusive thoughts during the short-term memory task. A Fisher Exact Probability Test indicated that depressed subjects reported a higher rate of intrusive thoughts than did nondepressed subjects (p < .05). On a second question, 80% of the depressed subjects reported that they experienced their intrusions as interfering with task performance, compared with only 25% of the nondepressed subjects (z = 3.48, p < .001). A second possible explanation of why rehearsal processes may be impaired in depressed individuals is provided by the Hasher and Zacks (1979) model of memory performance. Hasher and Zacks postulate that cognitive operations vary from effortful (requiring considerable attentional capacity) to automatic (requiring little capacity). Cognitive operations that are automatic (e.g., encoding, sensory memory) are assumed to require minimal attention and not to interfere with other ongoing cognitive activities. In contrast, more effortful operations such as rehearsal require considerable attentional capacity and interfere with other capacity-requiring operations. The Hasher and Zacks model also postulates that attentional capacity varies between individuals and, more specifically, that depressives have reduced attentional capacity, likely as a result of rumination. Jointly, these two postulates predict performance decrements in depressed individuals only on tasks requiring the most effortful processing. The Hasher and Zacks (1979) model, therefore, can readily account for the Group by Delay interaction for intermediate string sizes. This interaction was interpreted as evidence of impaired retention, likely due to interference with rehearsal. Because retention at 20 and 30 sec requires more effort than does retention at 1 sec, where rehearsal is minimal, and because depressives are assumed to have diminished capacity for effortful processing, a performance deficit would be much more likely at these later periods. Thus, the outcome obtained in the present study is consistent with the Hasher and Zacks framework, in which rehearsal is assumed to be an effortful process. The Hasher and Zacks (1979) model is also instructive in understanding the Group by Block interaction obtained on the whole report task. Depressed subjects performed more poorly than did nondepressed subjects on the final block of trials on a task that required them to report 12 stimuli presented in a 3 by 4 array. This whole report task assesses the transfer of information from sensory memory into short-term memory, and the


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depression-associated deficit on the final trials of this task is likely a result of a diminishing effort by the depressed subjects (cf. Cohen et al., 1982). As Johnson and Magaro (1987) note, the inability of depressed individuals to sustain cognitive effort may have a particularly negative effect on memory performance. In sum, the finding in a single sample of subjects of a retention-based deficit in short-term memory functioning in the presence of intact sensory memory processes has shed considerable light on the existence, location, and possible processes implicated in the memory functioning of mildly to moderately depressed individuals. Nevertheless, further work is necessary to more fully explore the rehearsal hypothesis proposed earlier, as well as to examine a number of issues not addressed in the present study. Investigations need to be conducted examining both the generalizability of the short-term memory deficit reported in the present study to a more severely depressed sample, and the specificity of the deficit to depression, as opposed to other disorders, such as anxiety (Wine, 1971), dementia (Hart et al., 1987), and schizophrenia (Koh & Wolpert, 1983). In addition, the cognitive mechanisms underlying the deficit must be clarified, and the ways or conditions under which such a deficit interferes with everyday tasks involving short-term memory need to be identified. The present investigation adds to a body of literature indicating the presence of specific memory impairments in depressed individuals; it remains for future work to delineate more specifically the parameters of this deficit and its temporal relationship with depressive symptomatology.

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