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Abstract:
Prior studies have suggested that dopamine imbalances can
affect latent inhibition and learned irrelevance, two paradigms in
which prior exposure to cues retards subsequent learning about
those cues. For example, individuals with schizophrenia (elevated
brain dopamine) and subjects given the dopaminergic agonist
amphetamine both show abolition of latent inhibition. It might
therefore be expected that patients with Parkinson's disease
(decreased midbrain dopamine) might show the opposite effect:
exaggerated latent inhibition or learned irrelevance. Here, we
tested 13 medicated Parkinson's patients and matched controls on a
computer-based learned irrelevance task. In this task, normal
subjects are slower to learn that a color change predicts a screen
event if they have previously been exposed to the color
uncorrelated with the screen event, compared with a control
condition which was not exposed to the color. This learned
irrelevance effect was previously shown to be abolished in medial
temporal amnesia: amnesic subjects learned the association at the
same rate regardless of prior exposure. We found the opposite
effect in Parkinson's patients: while Parkinson's subjects were
slow to learn overall, they were actually faster to learn if the
color had been pre-exposed than if it was novel. This may represent
a perceptual learning effect, or a specific impairment in learning
to recognize and respond to novel cues.
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