Task-set switching deficits in early-stage Huntington's disease: implications for basal ganglia function

Executive functions are likely mediated by interconnected circuits including frontal lobe and basal ganglia structures. We assessed the executive function of task switching in patients with early-stage Huntington's disease (HD), a neurodegenerative disease affecting the basal ganglia. In two experiments, the HD patients had greater difficulty when switching than when repeating a task than matched controls, and this was true even when scaling for the overall slowing of the patients. In the first experiment, HD patients had a switching deficit even in a "pure" condition where they had to switch, predictably, and with substantial preparation time, between stimuli having only one possible response, indicating a switching deficit different from that for patients with Parkinson's disease or frontal lobe trauma, and possibly relating to inadequate activation of stimulus-response links or "response set." In the more elaborate second experiment, we could not account for the switching deficit of the patients in terms of inadequate preparation in advance of a switch, deficient suppression of task-set processing from the preswitch trial, or impaired suppression of interference due to the presence of a competing task set. Instead, we found that part of the switching deficit was due to elevated reaction time and errors on switch trials for a repeated response (same button press as on preswitch trial) relative to an alternated response (different button press from preswitch trial). We argue that this elevated "repetition effect" for the HD patients is due to excessive inhibition of the just-performed response in advance of a switch. Alterations in the "response-setting" process alone (Experiment 1) and both the response-setting and "response inhibition" process (Experiment 2) probably arise from striatal pathology in HD, thus accounting for the task-switching deficits and showing how basal ganglia implemented response processes may underpin executive function.