Elsevier

Neuropsychologia

Volume 40, Issue 9, 2002, Pages 1586-1590
Neuropsychologia

Sustained attention deficit in bipolar disorder is not a working memory impairment in disguise

https://doi.org/10.1016/S0028-3932(02)00019-2Get rights and content

Abstract

Euthymic patients with bipolar disorder have been reported to show persistent deficits in sustained attention. However, the sustained attention task which was used also placed demands on working memory. Bipolar disorder patients in the euthymic state were therefore compared with healthy controls on two measures of sustained attention with and without a working memory component. Signal detection methodology was applied to the results. Euthymic patients with bipolar disorder were particularly impaired at detecting targets in the sustained attention task without a working memory component. This deficit was still apparent in a sub-group of patients who were not currently receiving lithium medication. By contrast, performance in the sustained attention task involving working memory task was not significantly different in the two groups. Sustained attention deficits apparent during the euthymic period of bipolar disorder cannot be explained in terms of working memory impairment and represents a reduced inherent capacity rather than a change in response bias. Deficits in sustaining attention may help explain the difficulties in psychological and occupational functioning in bipolar disorder patients during remission.

Introduction

Bipolar disorder is characterised by recurrent episodes of mania and depression, interspersed with periods of apparent recovery. However, recovery is often not complete, there remains a high incidence of occupational, psychological and social difficulties during remission [6], [11], [23]. Perhaps consistent with these observations, independent neuroimaging investigations have reported stable structural abnormalities and MRI signal hyperintensities in the brains of bipolar patients which may be expected to have functional consequences even in the euthymic state [9], [24], [28].

There has also been growing interest in the cognitive impairments apparent during periods of remission in bipolar disorder. Deficits in executive function [10], [31], memory [3], [10], [27], [31] and sustained attention have been reported to occur during these periods [3]. We have recently found that of a large number of neuropsychological tests given to bipolar patients in an euthymic phase of their illness, only performance on a sustained attention task was impaired after controlling for low levels of affective symptoms [3]. This impairment was also present in a subgroup of younger patients, selected to be early in their illness course, suggesting that impaired sustained attention may be related to vulnerability to the disorder. The task used to assess sustained attention was the rapid visual information processing (RVIP) task from the CANTAB which requires subjects to detect three digit sequences (3–5–7, 2–4–6 and 4–6–8) in a stream of digits over a 7 min period [22]. As well as sustaining attention over a period of time, this task also requires the subject to hold in mind the three digit sequences. Hence, it remains possible that a deficit in working memory rather than sustained attention is at the heart of the persistent impairment in RVIP performance, especially given the impaired memory processes also reported to occur in this group [3], [10], [27], [31].

The present investigation, therefore, aimed to clarify whether the observed deficit in vigilance was also apparent in a task without a working memory component. Two tasks requiring sustained attention were developed, based on the Mackworth clock task [16], which placed either no demand upon working memory (WM− task) or contained a relatively high working memory load (WM+ task).

Section snippets

Subjects

The study was approved by Oxfordshire Psychiatric Research Ethics Committee. Thereby, 19 patients were recruited for this study from current registers of patients who had been admitted as in-patients and meeting DSM-IV criteria for Bipolar Disorder (American Psychiatric Association, 1994). All had been diagnosed with bipolar affective disorder by the treating physician for at least 2 years. Confirmation of diagnosis was made using the structured clinical interview for DSM-IV (SCID) axis I

Results

Group characteristics for the euthymic bipolar group and controls are displayed in Table 1. The two groups were matched for age, years of education, NART IQ and WAIS Block Design raw score. Despite very low scores on the HAM-D and Young scales, the euthymic bipolar group still scored significantly higher than controls on both measures.

The number of targets correctly detected was affected by group and task (group×task interaction: F(1,36)=4.5, P<0.05). Therefore, performance on the WM− and WM+

Discussion

We have previously found that euthymic patients with bipolar disorder showed persistent impairments in a sustained attention task with an additional small working memory component [3]. The results found in the present sample suggest that this deficit cannot be explained in terms of impaired working memory capacity. Euthymic patients with bipolar disorder were found to show robust impairments only on the sustained attention task based on a perceptual discrimination without working memory

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