Journal of Affective Elsevier
Disorders, 18 (1990) l-9
JAD 00660
Differences
in dichotic listening asymmetries according to symptomatology Jocelyn Wale 1 and Vaughan
in depression
Carr 2
’ Royal Adelaide Hospital, Adelaide, SA, Australia and ’ University of Newcastle, Newcastle, NS W, Australia (Received 25 October 1988) (Revision received 6 April 1989) (Accepted 18 April 1989)
Subjects suffering from major depressive disorder were compared to normal controls on two verbal dichotic listening tasks. Although there were no significant differences between the groups on a task primarily of language perception, significant differences were obtained on a task with an attentional component. Overall performance was lower for the depressed group and ear asymmetry was reduced. Within the depressed group ear asymmetry varied according to symptomatology; withdrawal-retardation was associated with a lack of asymmetry and anxiety with a normal right ear advantage. The results were interpreted in terms of an interaction between affect and attention, and possible underlying mechanisms of cerebral hemisphere function were discussed.
Key words: Depression;
Dichotic
listening;
Symptomatology;
Introduction Research into the possible relationship between psychopathology and atypical cerebral laterality has produced several studies suggesting involvement of the right hemisphere in affective disorders. These investigations have used a range of behavioural techniques for studying hemisphere function including dichotic listening. Despite inconsistencies in the findings, there appears to be a
Address for correspondence: cipline of Psychiatry, University South Wales, 2308, Australia. 0165-0327/90/$03.50
Professor Vaughan Carr, Disof Newcastle, Newcastle, New
0 1990 Elsevier Science
Publishers
Attention
convergence of evidence from dichotic listening data suggesting differences in ear asymmetry according to diagnostic subtype (see Bruder, 1983, for a review). This calls into question the simple hypothesis that affective disorder is associated with a non-specific abnormality in right hemisphere function. On verbal dichotic listening tasks the normal right ear advantage (REA) tends to be enhanced in persons with bipolar disorder (Lishman et al., 1978; Yozawitz et al., 1979) and this can be interpreted as reflecting a disturbance in right hemisphere function leading to a reduced level of responding to left ear items which are projected via the right hemisphere to the left hemisphere for
B.V. (Biomedical
Division)
2
processing. By contrast, patients with unipolar depression have been found to show no REA on dichotic tasks (Moscovitch et al., 1981; Johnson and Crockett, 1982). This finding suggests a decline in left hemisphere function, given the assumption that the normal REA reflects the predominance of the left hemisphere for language processing. However, different interpretations have been offered for the reduced or absent REA found in depression. Furthermore, the finding of increased ear asymmetry with recovery from depression (Moscovitch et al., 1981; Johnson and Crockett, 1982) indicates that reduced ear asymmetry is a transient state and hence any theoretical interpretations must take this into consideration. Moscovitch et al. (1981) found that whereas depressed persons requiring electroconvulsive therapy (ECT) tended to show no REA on a dichotic task, those treated with antidepressant drugs showed normal ear asymmetry. They suggested that the lack of a REA was due to a ‘strongly primed right hemisphere’, with a return to normal asymmetry following unilateral ECT to that hemisphere. Johnson and Crockett (1982) found a similar reduction in ear asymmetry in depressed persons with return to a normal REA following treatment with medication. They proposed that these changes in asymmetry were due to shifts in the relative activity of each hemisphere which, in turn, could be due to: (a) changes in interhemispheric inhibition via the corpus callosum, (b) intrahemispheric changes in response to medication, or (c) shifts in attentional bias or cognitive strategies by which processing capacity is allocated to the cerebral hemispheres. The suggestion of attentional bias is amenable to further investigation if different tasks of lateral asymmetry in processing are used. Tasks which differentiate between language processing and attentional bias may at least begin to clarify the underlying mechanisms which contribute to the processing asymmetries found with depression. The study by Moscovitch et al. (1981) also demonstrated that different groups of depressed subjects may vary in the ear asymmetries obtained with only the more severely depressed (i.e., those treated with ECT) showing a lack of normal asymmetry. Recent papers (Carr and Wale, 1988; Wale
and Carr. 1988) on perceptual asymmetries in schizophrenia and depression reported a reduced or absent REA for unipolar depressives and also for schizophrenic subjects rating high for negative symptoms and low for positive symptoms (hallucinations and delusions). This suggests that both of these disorders may have some process in common, perhaps reflected in the similar phenomenology of psychomotor retardation and negative symptoms, which is reflected in a decreased REA regardless of the primary pathology. Different studies tend to vary in the type of dichotic task used with little concern for the role that such factors as the attention and/or memory demands of the task may play in performance, or the low demonstrated reliability of some dichotic measures. Given the likely importance of attention to performance, task selection with these issues in mind would therefore seem to be just as important to research in this area as are questions of diagnostic subtype or symptomatology. The use of more than one dichotic test with different task demands, carefully chosen for demonstrated reliability, may help to clarify the mechanisms contributing to the perceptual asymmetries found in depression. Similarly, if reliable tasks are chosen according to specific task demands, questions regarding variations in performance according to symptomatology may be pursued. Two dichotic language tasks with demonstrably satisfactory levels of reliability and validity are the Fused Rhymed Words (FRW) test of Wexler and Halwes (1983) and the Dichotic Monitoring (DM) task of Geffen et al. (1978) and Geffen and Caudrey (1981). The former is primarily a language processing task of phonemic discrimination, similar to consonant-vowel syllable tasks in that only the initial stop consonants of each word pair differ. However, the task has been constructed to increase its power by minimising the effects of stimulus dominance and the opportunity for phonetic blend errors to occur. The second task, originally used for research into attention (Moray and O’Brien, 1967) uses phonemically distinct word pairs and has a large attentional component. It requires ongoing attention to a stream of information presented at a relatively rapid rate with the different items of each dichotic word pair being spatially separate. Thus the use of these two tasks
3
should enable questions regarding spatial attentional bias and/or left hemisphere language processing to be addressed. Using these two different tasks of lateral asymmetry in performance, the present study examined the ear asymmetries found in persons suffering from depression, and also attempted to determine whether or not changes in asymmetry were associated with particular types of symptomatology. If attentional factors are critical to the performance of depressed persons, then the asymmetries found on the DM task could be expected to vary from those of normals while the FRW scores would not be similarly affected. Alternatively, anomalies in language processing or overall hemispheric functioning should be reflected in both the FRW and the DM results. Subjects and methods Subjects There were 24 subjects (eight male, 16 female) who met DSM-III criteria (American Psychiatric Association, 1980) for major depressive disorder (unipolar). The distribution of diagnostic subtypes was as follows: without melancholia, 17; with melancholia, 3; with psychotic features, 4. Ages ranged from 15 to 63 years (mean = 42.5 years, SD = 16.5). They were matched for age and sex with 24 volunteer control subjects having no known psychiatric or neurological condition, drawn primarily but not exclusively from hospital staff and covering a range of occupations. The depressed subjects were recently admitted in-patients to a short-term psychiatric treatment unit in a general hospital. The clinical condition of each was considered sufficiently severe to preclude out-patient treatment. All subjects were assessed for handedness using the Annett (1970) handedness questionnaire and the distribution of hand preferences for each group did not vary significantly (depressed: 18 right, 2 left, 4 mixed; controls: 19 right, 4 left, 1 mixed). Of the depressed subjects, 83% were free of antidepressant medication; the remainder had been on antidepressant medication for an average of 2.75 days. One third of the depressed group were completely medication-free and, apart from those who had just commenced antidepressant therapy, a further five had
been given a night-time sedative hypnotic, three were on low-dose benzodiazepines, two on a neuroleptic agent and three were taking some combination of these. Subjects were not included if there was a history of recent alcohol or psychoactive drug abuse or neurological disorder. Apparatus Each of the dichotic tasks has been described in detail elsewhere (Geffen et al., 1978; Wexler and Halwes, 1983). The FRW test requires the subject to indicate which word is heard when two words, differing only in the first phoneme, are presented simultaneously one to each ear and fuse so that there is only one perception. Responses for eight lists of 30 word pairs were recorded manually by the test administrator and later entered onto an Apple IIe computer for analysis. The DM task requires the subject to press a response button whenever a target word (‘dog’) is heard. Lists of 120 word pairs containing 20 targets (10 for each ear), phonemic distracters and irrelevant words were presented simultaneously one to each ear at a rate of l/750 ms. Responses and stimuli were recorded on an Apple IIe computer as described by Wale and Geffen (1986) for further analysis. Stimuli for each task were presented using Maico auraldome headphones and a Uher two-channel tape recorder. Programmes were obtained from each of the relevant laboratories where the tests were developed for the scoring and analysis of individual subject results. Procedure The depressed subjects were interviewed and administered all research procedures within 72 h of hospital admission. The Brief Psychiatric Rating Scale (BPRS) (Overall and Gorham, 1962) was administered by one of the authors (V.C.) as part of a larger research study of schizophrenia. The interview on which the BPRS ratings were based was that of the Present State Examination (Wing et al., 1974). The BPRS has previously shown differences in dichotic listening performance in schizophrenic subjects according to symptom rat-. ing (Carr and Wale, 1988). Within 24 h of the clinical assessment, the two dichotic listening tasks were administered. Half of each subject group performed the FRW test prior to the DM test and
4
this order was reversed for the other half of the group. There were rest periods between each of the tasks and whenever the depressed subjects requested a break in the procedure. The tasks were always administered in the afternoon to offset possible effects of diurnal mood variation on the results of the depressed group. Two practice lists preceded the FRW task. The DM test was preceded by a brief test of hearing consisting of two tones which should be heard centrally if hearing is equal in each ear (Geffen et al., 1978) and a shaping procedure consisting of monaural lists. Since audiometric testing was not readily available, subjects were excluded if these preliminary procedures revealed any evidence to suggest reduced hearing in one ear compared to the other. After two dichotic practice lists, four test lists were administered. The hand of response was counterbalanced in a R/L/R/L order, and the headphones were reversed after alternate lists. Results The analysis programme for the FRW test corrects for stimulus dominance (Halwes, 1969) and corrected scores for each ear were used in statistical comparisons. On the DM task, performance between the groups was compared on the percentage of targets detected (i.e., hit rate, HR) and also on the reaction time (RT) to correct responses. Repeated measures ANOVAs, group (2) x ear (2) were carried out on each of these measures. Planned comparisons using f-tests were also done on ear differences for each group to determine whether these accorded with previous research findings. There were no main or interaction effects on the FRW measure. In the DM experiment the control group detected more targets than the depressed group (see Table l), F(l, 46) = 15.02, P < 0.001, and responses over both subject groups were greater to the right ear (mean = 83.8, SD = 13.9) than to the left ear (mean = 78.0, SD = 16.8) F(1,46) = 5.8, P -=z 0.05. The group by ear interaction was not significant. However, the planned comparison t-tests showed that the control group, as expected, had a higher right ear score compared to the left (t = 2.1, P < 0.05) while this was not so for the depressed group (f = - 1.26, P > 0.10).
TABLE
I
DICHOTIC MONITORING RESULTS FOR EACH SUBJECT GROUP ACCORDING TO EAR FOR EACH OF THE DEPENDENT VARIABLES Depressed LE
Control RE
LE
RE
HR
73
76
83
91
RT
(17) 618
(12) 605
(15) 600
(12) 562
(84) 0.92 (0.20) 0.43 (0.20)
(90) 0.91 (0.06) 0.26 (0.21)
P(A) log P
(136) 0.83 (0.09) 0.51 (0.19)
(117) 0.84 (0.06) 0.56 (0.31)
LE, left ear; RE, right ear; HR, percentage target detection; RT, reaction time (ms); P(A), target discrimination; log j3, response bias. Standard deviations are given in parentheses.
On the RT measure of the DM experiment, ANOVA revealed that the two groups did not vary significantly in the speed with which they responded to targets, but responses to right ear items (mean = 583 ms, SD = 106) were faster than to those of the left ear (mean = 609 ms, SD = 112) F(l, 45) = 4.95, P -c0.05. The group by ear interaction was not significant. Similar planned comparisons to those performed for HR showed that controls responded significantly faster to right ear targets compared to left (t = 2.1, P -c0.05) while depressed subjects showed no significant difference in RT between the ears (t = 0.93, P > 0.3). However, unlike the HR measure where the standard deviations were similar for both groups, the lack of a REA for depressed subjects on RT may have been due to comparatively greater variability in performance in depressed subjects as suggested by the standard deviation figures for RT in this group. In an attempt to determine the underlying basis for performance differences between the groups, similar ANOVAs were also carried out on the signal detection measures P(A) and /? (log-transformed scores) both of which take account of false-positive responses. Target discrimination as measured by P(A) was superior for control compared to depressed subjects, F(1, 46) = 8.26, P < 0.01, and the former group also showed a greater readiness to respond (p), F(l, 46) = 10.8, P
Disorders, 18 (1990) l-9
JAD 00660
Differences
in dichotic listening asymmetries according to symptomatology Jocelyn Wale 1 and Vaughan
in depression
Carr 2
’ Royal Adelaide Hospital, Adelaide, SA, Australia and ’ University of Newcastle, Newcastle, NS W, Australia (Received 25 October 1988) (Revision received 6 April 1989) (Accepted 18 April 1989)
Subjects suffering from major depressive disorder were compared to normal controls on two verbal dichotic listening tasks. Although there were no significant differences between the groups on a task primarily of language perception, significant differences were obtained on a task with an attentional component. Overall performance was lower for the depressed group and ear asymmetry was reduced. Within the depressed group ear asymmetry varied according to symptomatology; withdrawal-retardation was associated with a lack of asymmetry and anxiety with a normal right ear advantage. The results were interpreted in terms of an interaction between affect and attention, and possible underlying mechanisms of cerebral hemisphere function were discussed.
Key words: Depression;
Dichotic
listening;
Symptomatology;
Introduction Research into the possible relationship between psychopathology and atypical cerebral laterality has produced several studies suggesting involvement of the right hemisphere in affective disorders. These investigations have used a range of behavioural techniques for studying hemisphere function including dichotic listening. Despite inconsistencies in the findings, there appears to be a
Address for correspondence: cipline of Psychiatry, University South Wales, 2308, Australia. 0165-0327/90/$03.50
Professor Vaughan Carr, Disof Newcastle, Newcastle, New
0 1990 Elsevier Science
Publishers
Attention
convergence of evidence from dichotic listening data suggesting differences in ear asymmetry according to diagnostic subtype (see Bruder, 1983, for a review). This calls into question the simple hypothesis that affective disorder is associated with a non-specific abnormality in right hemisphere function. On verbal dichotic listening tasks the normal right ear advantage (REA) tends to be enhanced in persons with bipolar disorder (Lishman et al., 1978; Yozawitz et al., 1979) and this can be interpreted as reflecting a disturbance in right hemisphere function leading to a reduced level of responding to left ear items which are projected via the right hemisphere to the left hemisphere for
B.V. (Biomedical
Division)
2
processing. By contrast, patients with unipolar depression have been found to show no REA on dichotic tasks (Moscovitch et al., 1981; Johnson and Crockett, 1982). This finding suggests a decline in left hemisphere function, given the assumption that the normal REA reflects the predominance of the left hemisphere for language processing. However, different interpretations have been offered for the reduced or absent REA found in depression. Furthermore, the finding of increased ear asymmetry with recovery from depression (Moscovitch et al., 1981; Johnson and Crockett, 1982) indicates that reduced ear asymmetry is a transient state and hence any theoretical interpretations must take this into consideration. Moscovitch et al. (1981) found that whereas depressed persons requiring electroconvulsive therapy (ECT) tended to show no REA on a dichotic task, those treated with antidepressant drugs showed normal ear asymmetry. They suggested that the lack of a REA was due to a ‘strongly primed right hemisphere’, with a return to normal asymmetry following unilateral ECT to that hemisphere. Johnson and Crockett (1982) found a similar reduction in ear asymmetry in depressed persons with return to a normal REA following treatment with medication. They proposed that these changes in asymmetry were due to shifts in the relative activity of each hemisphere which, in turn, could be due to: (a) changes in interhemispheric inhibition via the corpus callosum, (b) intrahemispheric changes in response to medication, or (c) shifts in attentional bias or cognitive strategies by which processing capacity is allocated to the cerebral hemispheres. The suggestion of attentional bias is amenable to further investigation if different tasks of lateral asymmetry in processing are used. Tasks which differentiate between language processing and attentional bias may at least begin to clarify the underlying mechanisms which contribute to the processing asymmetries found with depression. The study by Moscovitch et al. (1981) also demonstrated that different groups of depressed subjects may vary in the ear asymmetries obtained with only the more severely depressed (i.e., those treated with ECT) showing a lack of normal asymmetry. Recent papers (Carr and Wale, 1988; Wale
and Carr. 1988) on perceptual asymmetries in schizophrenia and depression reported a reduced or absent REA for unipolar depressives and also for schizophrenic subjects rating high for negative symptoms and low for positive symptoms (hallucinations and delusions). This suggests that both of these disorders may have some process in common, perhaps reflected in the similar phenomenology of psychomotor retardation and negative symptoms, which is reflected in a decreased REA regardless of the primary pathology. Different studies tend to vary in the type of dichotic task used with little concern for the role that such factors as the attention and/or memory demands of the task may play in performance, or the low demonstrated reliability of some dichotic measures. Given the likely importance of attention to performance, task selection with these issues in mind would therefore seem to be just as important to research in this area as are questions of diagnostic subtype or symptomatology. The use of more than one dichotic test with different task demands, carefully chosen for demonstrated reliability, may help to clarify the mechanisms contributing to the perceptual asymmetries found in depression. Similarly, if reliable tasks are chosen according to specific task demands, questions regarding variations in performance according to symptomatology may be pursued. Two dichotic language tasks with demonstrably satisfactory levels of reliability and validity are the Fused Rhymed Words (FRW) test of Wexler and Halwes (1983) and the Dichotic Monitoring (DM) task of Geffen et al. (1978) and Geffen and Caudrey (1981). The former is primarily a language processing task of phonemic discrimination, similar to consonant-vowel syllable tasks in that only the initial stop consonants of each word pair differ. However, the task has been constructed to increase its power by minimising the effects of stimulus dominance and the opportunity for phonetic blend errors to occur. The second task, originally used for research into attention (Moray and O’Brien, 1967) uses phonemically distinct word pairs and has a large attentional component. It requires ongoing attention to a stream of information presented at a relatively rapid rate with the different items of each dichotic word pair being spatially separate. Thus the use of these two tasks
3
should enable questions regarding spatial attentional bias and/or left hemisphere language processing to be addressed. Using these two different tasks of lateral asymmetry in performance, the present study examined the ear asymmetries found in persons suffering from depression, and also attempted to determine whether or not changes in asymmetry were associated with particular types of symptomatology. If attentional factors are critical to the performance of depressed persons, then the asymmetries found on the DM task could be expected to vary from those of normals while the FRW scores would not be similarly affected. Alternatively, anomalies in language processing or overall hemispheric functioning should be reflected in both the FRW and the DM results. Subjects and methods Subjects There were 24 subjects (eight male, 16 female) who met DSM-III criteria (American Psychiatric Association, 1980) for major depressive disorder (unipolar). The distribution of diagnostic subtypes was as follows: without melancholia, 17; with melancholia, 3; with psychotic features, 4. Ages ranged from 15 to 63 years (mean = 42.5 years, SD = 16.5). They were matched for age and sex with 24 volunteer control subjects having no known psychiatric or neurological condition, drawn primarily but not exclusively from hospital staff and covering a range of occupations. The depressed subjects were recently admitted in-patients to a short-term psychiatric treatment unit in a general hospital. The clinical condition of each was considered sufficiently severe to preclude out-patient treatment. All subjects were assessed for handedness using the Annett (1970) handedness questionnaire and the distribution of hand preferences for each group did not vary significantly (depressed: 18 right, 2 left, 4 mixed; controls: 19 right, 4 left, 1 mixed). Of the depressed subjects, 83% were free of antidepressant medication; the remainder had been on antidepressant medication for an average of 2.75 days. One third of the depressed group were completely medication-free and, apart from those who had just commenced antidepressant therapy, a further five had
been given a night-time sedative hypnotic, three were on low-dose benzodiazepines, two on a neuroleptic agent and three were taking some combination of these. Subjects were not included if there was a history of recent alcohol or psychoactive drug abuse or neurological disorder. Apparatus Each of the dichotic tasks has been described in detail elsewhere (Geffen et al., 1978; Wexler and Halwes, 1983). The FRW test requires the subject to indicate which word is heard when two words, differing only in the first phoneme, are presented simultaneously one to each ear and fuse so that there is only one perception. Responses for eight lists of 30 word pairs were recorded manually by the test administrator and later entered onto an Apple IIe computer for analysis. The DM task requires the subject to press a response button whenever a target word (‘dog’) is heard. Lists of 120 word pairs containing 20 targets (10 for each ear), phonemic distracters and irrelevant words were presented simultaneously one to each ear at a rate of l/750 ms. Responses and stimuli were recorded on an Apple IIe computer as described by Wale and Geffen (1986) for further analysis. Stimuli for each task were presented using Maico auraldome headphones and a Uher two-channel tape recorder. Programmes were obtained from each of the relevant laboratories where the tests were developed for the scoring and analysis of individual subject results. Procedure The depressed subjects were interviewed and administered all research procedures within 72 h of hospital admission. The Brief Psychiatric Rating Scale (BPRS) (Overall and Gorham, 1962) was administered by one of the authors (V.C.) as part of a larger research study of schizophrenia. The interview on which the BPRS ratings were based was that of the Present State Examination (Wing et al., 1974). The BPRS has previously shown differences in dichotic listening performance in schizophrenic subjects according to symptom rat-. ing (Carr and Wale, 1988). Within 24 h of the clinical assessment, the two dichotic listening tasks were administered. Half of each subject group performed the FRW test prior to the DM test and
4
this order was reversed for the other half of the group. There were rest periods between each of the tasks and whenever the depressed subjects requested a break in the procedure. The tasks were always administered in the afternoon to offset possible effects of diurnal mood variation on the results of the depressed group. Two practice lists preceded the FRW task. The DM test was preceded by a brief test of hearing consisting of two tones which should be heard centrally if hearing is equal in each ear (Geffen et al., 1978) and a shaping procedure consisting of monaural lists. Since audiometric testing was not readily available, subjects were excluded if these preliminary procedures revealed any evidence to suggest reduced hearing in one ear compared to the other. After two dichotic practice lists, four test lists were administered. The hand of response was counterbalanced in a R/L/R/L order, and the headphones were reversed after alternate lists. Results The analysis programme for the FRW test corrects for stimulus dominance (Halwes, 1969) and corrected scores for each ear were used in statistical comparisons. On the DM task, performance between the groups was compared on the percentage of targets detected (i.e., hit rate, HR) and also on the reaction time (RT) to correct responses. Repeated measures ANOVAs, group (2) x ear (2) were carried out on each of these measures. Planned comparisons using f-tests were also done on ear differences for each group to determine whether these accorded with previous research findings. There were no main or interaction effects on the FRW measure. In the DM experiment the control group detected more targets than the depressed group (see Table l), F(l, 46) = 15.02, P < 0.001, and responses over both subject groups were greater to the right ear (mean = 83.8, SD = 13.9) than to the left ear (mean = 78.0, SD = 16.8) F(1,46) = 5.8, P -=z 0.05. The group by ear interaction was not significant. However, the planned comparison t-tests showed that the control group, as expected, had a higher right ear score compared to the left (t = 2.1, P < 0.05) while this was not so for the depressed group (f = - 1.26, P > 0.10).
TABLE
I
DICHOTIC MONITORING RESULTS FOR EACH SUBJECT GROUP ACCORDING TO EAR FOR EACH OF THE DEPENDENT VARIABLES Depressed LE
Control RE
LE
RE
HR
73
76
83
91
RT
(17) 618
(12) 605
(15) 600
(12) 562
(84) 0.92 (0.20) 0.43 (0.20)
(90) 0.91 (0.06) 0.26 (0.21)
P(A) log P
(136) 0.83 (0.09) 0.51 (0.19)
(117) 0.84 (0.06) 0.56 (0.31)
LE, left ear; RE, right ear; HR, percentage target detection; RT, reaction time (ms); P(A), target discrimination; log j3, response bias. Standard deviations are given in parentheses.
On the RT measure of the DM experiment, ANOVA revealed that the two groups did not vary significantly in the speed with which they responded to targets, but responses to right ear items (mean = 583 ms, SD = 106) were faster than to those of the left ear (mean = 609 ms, SD = 112) F(l, 45) = 4.95, P -c0.05. The group by ear interaction was not significant. Similar planned comparisons to those performed for HR showed that controls responded significantly faster to right ear targets compared to left (t = 2.1, P -c0.05) while depressed subjects showed no significant difference in RT between the ears (t = 0.93, P > 0.3). However, unlike the HR measure where the standard deviations were similar for both groups, the lack of a REA for depressed subjects on RT may have been due to comparatively greater variability in performance in depressed subjects as suggested by the standard deviation figures for RT in this group. In an attempt to determine the underlying basis for performance differences between the groups, similar ANOVAs were also carried out on the signal detection measures P(A) and /? (log-transformed scores) both of which take account of false-positive responses. Target discrimination as measured by P(A) was superior for control compared to depressed subjects, F(1, 46) = 8.26, P < 0.01, and the former group also showed a greater readiness to respond (p), F(l, 46) = 10.8, P
