Psychological Medicine



Original Article

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


SHARON L. NAISMITH a1c1, IAN B. HICKIE a1, PHILIP B. WARD a2, ELIZABETH SCOTT a1 and CRAIG LITTLE a2
a1 Brain & Mind Research Institute, University of Sydney, Sydney, Australia
a2 School of Psychiatry, University of New South Wales, Sydney, Australia

Article author query
naismith sl   [PubMed][Google Scholar] 
hickie ib   [PubMed][Google Scholar] 
ward pb   [PubMed][Google Scholar] 
scott e   [PubMed][Google Scholar] 
little c   [PubMed][Google Scholar] 

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.

(Published Online December 19 2005)


Correspondence:
c1 Brain & Mind Research Institute, 100 Mallett St, Camperdown NSW 2050, Australia. (Email: snaismith@med.usyd.edu.au)


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