MEMORY AND LEARNING IN DEPRESSION 1 Learning

2.3.4.1.1 Clinical samples

Many studies have reported that depressed patients show impairment on measures of learning. Depressive deficits have been reported on word-leaming tasks (Coughlan & Hollows, 1984; Friedman, 1964; Henry, Weingartner & Murphy, 1973; Sternberg & Jarvik, 1976), with the exception of Rohling and Scogin (1993). Depressed patients have also shown deficits on the Wechsler Memory Scale (WMS; Wechsler, 1945) (Breslow, Kocsis & Belkin, 1980; Danion, Willard-Schroeder, Zimmerman, Grange, Schlienger & Singer, 1991; Stromgren, 1977; Williams, Little, Scates & Blockman, 1987), and on recall of nonsense syllables (Cohen, Weingartner, Smallberg, Pickar & Murphy, 1982), and tests of visual retention (Elliott, Sahakian, McKay, Herrod, Robbins & Paykel, 1996; Friedman, 1964; Robertson & Taylor, 1985).

Several studies have compared learning of verbal and nonverbal material, with conflicting findings. Peselow, Corwin, Fieve, Rotroson and Cooper (1991) reported depressive deficits for recall of both words and pictures, while Steif, Sackeim, Portnoy, Decina and Malitz (1986) found depressive impairment in recognition of both words and faces. Richards and Ruff (1989), using a battery of verbal and nonverbal tasks, found depressed Ss were differentiated from controls by some of the nonverbal tasks, but by none of the verbal tasks. A similar pattern was reported by Boone, Lesser, Miller, Wohl, Berman et al. (1995), with depressed and control Ss differing significantly on a composite score of visual memory, but not on a composite score of verbal memory. Calev, Korin, Shapira, Kugelmass and Lerer (1986) used verbal and nonverbal tasks matched for difficulty, and found depressed Ss equally impaired on both relative to controls. Deptula, Manevitz and Yozawitz (1991) found evidence of differential depressive impairment on both free recall and recognition of nonverbal relative to verbal serial learning tasks. Although the tasks in the latter study were not matched in the same way as those of Calev et al. (1986), the authors reported the control Ss performed at a similar level on both. They suggested the inconsistency in the findings might result from their use of a serial learning task compared with the single trial task used by Calev et al. (1986), since the differential deficit was revealed only on later trials. An alternative explanation is that the tasks used by Deptula et al. (1991) were not sufficiently sensitive to detect differences in the control group.

2.3.4.2 Free Recall Versus Recognition Memory 2.3.4.2.1 Clinical samples

Several studies have included a within-subject comparison of performance on tasks measuring different types of retrieval process, with conflicting results. Some studies reported depressed Ss showed impairment on measures of free recall but not recognition memory when compared with normal controls (Beatty et al., 1990; Brand, Jolles & Gispen-de Wied, 1992, Experiment 1; Ilsley et al., 1995; Watts & Sharrock, 1987; Williams et al., 1987), suggesting free recall is a more sensitive measure of depressive deficits. Other studies found depressed Ss to be impaired on both recognition and free recall tasks (Austin et al., 1992; Backman & Forsell, 1994; Brown, Scott, Bench & Dolan, 1994; Silberman, Weingartner, Laraia, Bymes & Post, 1983; Wolfe, Granholm, Butters, Saunders & Janowsky, 1987). One study failed to find depressive deficits on either recognition or recall tasks (Davis & Unruh, 1980); however, there is evidence the Ss were less severely depressed than Ss in other studies, and the control group consisted of nondepressed psychiatric outpatients rather than normal controls.

Inconsistency in the findings is likely to reflect differences between the studies in terms of sample characteristics, and may also reflect variations between the tasks on dimensions other than recognition versus free recall, such as differences in difficulty. This is supported by the study carried out by Brand et al. (1992, Experiment 2) who manipulated the difficulty of the Rey Auditory Verbal Learning Test by varying the number of learning trials (1, 3, or 5) before testing both free recall and recognition. They found depressed Ss showed impaired recall in all three conditions, but deficits in recognition memory emerged only in the most difficult (1-trial) condition. Calev and Erwin (1985) attempted to overcome this problem by using recognition and free recall tasks matched for difficulty. They found while depressed Ss were impaired on both tasks relative to normal controls, the degree of impairment was differentially greater on the free recall task. The findings suggest depression is usually associated with deficits on tests of free recall, and less consistently with deficits on recognition memory tasks.

There are at least two possible explanations for the apparent differential depressive deficit on free recall versus recognition memory tasks. First, theories of recognition memory suggest it may rely on two separable processes: at retrieval Ss may base their response either on conscious recollection of the item, or on feelings of familiarity that may occur

in the absence of conscious recollection (for review see Gardiner & Java, 1993). The former is associated with effortful processes, while the latter is thought to result from automatic processes (see section 2.2.5.1.2). This means if conscious recollection fails during recognition memory, Ss may still be able to achieve the correct response by relying on the 'familiarity' response. If automatic responses are spared in depression, then this could explain why free recall is more sensitive to depressive deficits than recognition memory. This explanation is supported by a study carried out by Hertel & Milan (1994) using the process dissociation paradigm developed by Jacoby (1991). In standard recognition test paradigms, correct responding may be based on either familiarity or recollection, or a combination of both, and it is not possible to separate the two. Jacoby and his colleagues (e.g. Jacoby, 1991) developed a way of dissociating the two processes by putting them in opposition to each other during recognition by means of different task instructions. Hertel and Milan (1994) found dysphoric students were impaired on recollection but not familiarity compared with normal controls when these two processes were measured in opposition, but there was no group difference when both familiarity and recollection were working in the same direction, as they do in a standard recognition task.

A second, related possibility is that free recall may benefit more than recognition memory from the use of strategies, such as attempts to make semantic links between words or the use of imagery techniques, during either encoding or recall. Hertel and her colleagues (e.g. Hertel, 1994; Hertel & Hardin, 1990; Hertel & Rude, 1991a,b) have postulated depression may be associated with reduced initiative, leading to failure to use strategies spontaneously or engage in elaborative thinking (see section 2.3.9.2). If true, then it could have a differential impact on free recall relative to recognition memory performance in depressed Ss.

2.3.4.3 Response Bias

It has been suggested that depressive deficits on memory tasks may be due to a conservative response style. That is, depressed Ss may have the information available to them but, because of an overly cautious response strategy, may be unwilling to report the information. A conservative response bias of this type would be expected to result in errors of omission rather than commission on free recall tasks, and reduced rates of hits and false alarms on recognition memory tasks.

2.3.4.3.1 Clinical samples

On tests of free recall, two studies (Henry et al., 1973; Whitehead, 1973) have shown depression to be associated with errors of omission. However, four studies (Brand et al., 1992; Manschreck, Maher, Rosenthal & Berner, 1991; Dalla Barba et al, 1995; Peselow et al., 1991) have reported depressed patients do not differ from normal controls in the number of intrusion errors made.

On tasks of recognition memory, reduced rates of both hits and false alarms would indicate a conservative response style, or, if signal detection analysis is used, depression would be expected to lead to an effect on Beta (an index of response bias) rather than d' (an index of sensitivity). Both Miller and Lewis (1977) and Dunbar and Lishman (1984) found depression was associated with lower levels of both hits and false alarms. Furthermore, when signal detection analyses were carried out, depressed Ss were found to differ from controls on Beta but not d', indicating depressed Ss showed a more cautious response criteria rather than a true memory deficit. The study reported by Lamer (1977) has sometimes been quoted in support of a conservative response style in depression. He found depressed Ss did not differ from physically ill controls on d', and both groups showed an equally cautious response style. However, some of the 'physically ill' group had disorders associated with cognitive deficits (e.g. two cardiovascular attacks, one parkinsonism), and therefore did not constitute an adequate control group.

Other studies contradict the conservative response bias hypothesis. Silberman, Weingartner, Laraia, Bymes and Post (1983) and Calev and Erwin (1985) both reported that depressed Ss achieved significantly fewer hits than controls, but did not differ on false alarms. Several studies (Channon, Baker & Robertson, 1993 a; Brown et al, 1994; Deijen, Orlebeke & Rijsdijk, 1993; Watts, Morris & MacLeod, 1987) have carried out signal detection analyses and found depressed Ss to be impaired relative to controls on d', but not to differ significantly on Beta. Two studies (Backman & Forsell, 1994; Corwin, Peselow, Feenan, Rotrosen & Fieve, 1990) reported impairment on d', but also found depressed Ss to be significantly more conservative than controls on Beta. Another two studies found evidence of a more liberal response bias in depressed Ss (Deptula et al., 1991; Wolfe et al., 1987), although the study carried out by Wolfe et al. (1987) combined the results for hits and misses, making interpretation difficult. Overall the evidence suggests depressed Ss may be more likely than controls to show a conservative response

criteria, but they also show a reduction in memory sensitivity which cannot be explained in terms of a response bias.

2.3.4.3.2 Nonclinical samples

Two studies (Henriques, Glowacki & Davidison, 1994; Hertel & Milan, 1994), carried out with nonclinical samples, may shed light on the findings in relation to recognition memory test performance. Henriques et al. (1994) studied response bias in three pay-off conditions: neutral, punishment and reward. In both the reward and punishment conditions, earnings were maximised by a liberal response criterion. Dysphoric students were not impaired on d' in any of the experimental conditions; nor on Beta in the neutral or punishment conditions. However, dysphoric Ss were significantly more conservative than controls in the reward condition, and there was a nonsignificant tendency for them to show a more liberal bias in the punishment condition. This suggests dysphoric Ss were more sensitive to punishment than reward, with the converse true for the controls. Although in this study payment was used to create the different conditions, the findings may explain the conflicting findings described above. It is possible that procedural variations between the studies resulted in some appearing more threatening or encouraging than others, thereby potentially influencing Ss' response criteria in different ways.

The second study was carried out by Hertel and Milan (1994) using a process dissociation paradigm to test recognition memory. The study is described in detail in section 2.3.4.2.1 above. In essence, Hertel and Milan (1994) found that when the processes of remembering by recollection versus familiarity were put into opposition during retrieval on a recognition task, dysphoric Ss were impaired on recollection but not familiarity compared with normal controls. There was no group difference when both familiarity and recollection were working in the same direction (as they do in a standard recognition task). This finding suggests variations between studies in the degree to which both depressed and control Ss rely on the two processes (recollection and familiarity) during task performance could be a factor in the variable findings outlined above.