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Abstract:
The effect of endogenous dopamine on the learning of simple
and complex category structures was investigated in several
experiments. The stimuli in all tasks were colored geometric
figures presented on a colored background, which varied across
trials on four binary-valued dimensions. In the simple tasks, each
set of contrasting categories was separated by a unidimensional
explicit rule, whereas the complex tasks required integrating
information from three stimulus dimensions and resulted in implicit
rules that were difficult to verbalize. The effects of reductions
in endogenous dopamine were investigated in an experiment that
compared the category learning ability of young adults, healthy
elderly people, and Parkinson's disease patients. On the explicit
unidimensional task, the Parkinson's patients were significantly
impaired relative to the healthy elderly, and the elderly were
significantly impaired relative to the young adults. For the
complex implicit task, there was no difference between the elderly
and Parkinson's patients, but both groups were slightly impaired
relative to the young adults. To investigate the effects of
increases in endogenous dopamine, positive affect was induced in
two groups of healthy young adults. Both positive affect groups
showed a facilitation in category learning relative to a neutral
affect control group, and the facilitation was greater for the
explicit unidimensional category structures. These results support
the hypothesis that category learning is mediated by separate
explicit and implicit systems, and that dopamine facilitates the
working of these systems. The results are interpreted within the
framework of a recent neuropsychological theory of category
learning called COVIS.
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