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Impaired implicit sequence learning in depression: a probe for frontostriatal dysfunction?

Published online by Cambridge University Press:  19 December 2005

SHARON L. NAISMITH
Affiliation:
Brain & Mind Research Institute, University of Sydney, Sydney, Australia
IAN B. HICKIE
Affiliation:
Brain & Mind Research Institute, University of Sydney, Sydney, Australia
PHILIP B. WARD
Affiliation:
School of Psychiatry, University of New South Wales, Sydney, Australia
ELIZABETH SCOTT
Affiliation:
Brain & Mind Research Institute, University of Sydney, Sydney, Australia
CRAIG LITTLE
Affiliation:
School of Psychiatry, University of New South Wales, Sydney, Australia

Abstract

Background. Implicit learning through motor sequencing tasks is sensitive to basal ganglia dysfunction. Consequently, it is ideally suited for testing elements of the frontostriatal model of major depression and performance can be related to key clinical, neuropsychological, vascular and biochemical data.

Method. Twenty-one subjects with moderate to severe unipolar depression and 21 age-, sex- and education-matched controls were recruited. Clinical, vascular and biochemical data were recorded. Subjects were administered a battery of neuropsychological tests that assessed speed of processing, working memory, learning, memory, language, perceptual organization and executive functioning. Additionally, subjects were administered a motor sequencing implicit learning task. Implicit learning is assumed when reaction times improve during the sequenced condition as compared to the pseudo-random baseline condition.

Results. The rate of implicit learning in persons with depression was only half that of control subjects (3·6% v. 7·3%). Lower rates of implicit learning in patients were associated with poorer performance on neuropsychological tests of visuomotor speed and mental flexibility, longer duration of depressive episode and severity of acute stress. In a small number of subjects, poorer performance was also related to past suicide attempt.

Conclusions. Impaired implicit learning in persons with depression is consistent with frontostriatal dysfunction. Performance is related to some clinical characteristics and to neuropsychological functioning on tests of visuomotor speed and mental flexibility.

Type
Original Article
Copyright
2005 Cambridge University Press

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