| |
Abstract:
Several paradigms have been used for assessing cognitive
flexibility, which may reflect the functioning of distinct
segregated cortico-striatal 'loops' in a dopamine-dependent way. We
examined the effects of L-Dopa medication withdrawal in patients
with Parkinson's disease (PD) on 3 tests that assess different
forms of flexibility. The intradimensional/extradimensional shift
(ID/ED) task (Downes et al., 1989) measures flexibility at the
level of attentional set (a learned predisposition to attend to one
dimension of multi-dimensional stimuli) in a visual discrimination
paradigm. Task-set switching (Rogers et al., 1998) examines
switching at a similar level as the ID/ED paradigm, but is not
confounded by learning as is the case for the ID/ED paradigm.
Reversal learning requires flexibility at the level of
stimulus-reward associations. Withdrawal of dopaminergic medication
had a detrimental effect on task-set switching, but a beneficial
effect on reversal learning. The ID/ED paradigm was less sensitive
to medication withdrawal. These results agree with the hypothesis
that the tasks may rely on different cortico-striatal 'loops' that
are differentially depleted of dopamine in PD (Kish et al., 1988).
The orbitofrontal 'loop' that is thought to play a role in reversal
learning is not depleted of dopamine in PD and an impairment in
patients 'on' medication concurs with a dopamine 'overdose'
hypothesis for the orbitofrontal-ventral striatal 'loop' (Swainson
et al., 2000).
|
