ALTERNATING SWITCH COSTS IN PARKINSON'S DISEASE: BACKWARD INHIBITION
IN BASAL GANGLIA DISEASE.
Christina L. Fales and Barbara J. Knowlton.
University of California at Los Angeles.
Task switching is a cardinal executive function that is often impaired in patients with frontostriatal disorders. Task switching costs are measured as the difference in time to perform a task in a switch trial versus a repeat trial. Alternating switching is a special case of task switching proposed to be an important contributor to switch costs.
Alternating switch costs reflect "backward inhibition," the presumed suppression applied to the task set being left during a switch. These costs are measured as the difference between switch costs in alternating situations (sequence ABA, for two tasks A and B) versus non-alternating situations (sequence CBA, for three tasks A, B, and C).
Disproportionate costs in alternating switching may explain the switching
deficits often found in patients with Parkinson's disease. We measured alternating
switch costs in early and middle stage Parkinson's patients, using three
classification tasks. Patients and controls showed equivalent response times
to perform tasks in repeat, alternating-switch and non-alternating-switch
trials. However, the patients showed decreased accuracy in the alternating-switch
versus non-alternating switch trials, suggesting that Parkinson's disease
entails a reduced ability to overcome backward inhibition when reactivating
a recently-abandoned task set.