Abnormalities of habituation and task switching in patients with Parkinson's disease: an ERP study with frequent nontarget stimuli

To evaluate the effects of preceding stimuli, event-related potentials (ERPs) elicited by frequent nontarget stimuli (FNT) were studied in nondemented patients with Parkinson's disease (PD) and healthy control subjects (Controls). Each stimulus, composed of two Japanese phonetic characters, was presented. Identical 'hirakana' characters (9%) were designated as target stimuli (Targets). FNT (73%) were composed of different 'hirakana' characters, and remaining stimuli were category-deviant nontarget stimuli (CDNT) composed of different 'katakana' as well as phonetically task-relevant but category-deviant nontarget stimuli (RCDNT) composed of identical 'katakana.' Trials elicited by FNT were divided based on the preceding four stimulus types and averaged, respectively. ERPs elicited by FNT among the preceding four stimulus conditions were compared. Controls showed significant differences in N2 amplitudes, depending upon the preceding four stimulus conditions, whereas the PD patients did not show such differences. Regardless of the preceding stimulus conditions, the P3 components showed frontal maximal distribution, with larger P3 amplitudes for the PD patients than Controls. These findings suggest abnormalities of habituation and task-switching deficits resulting from depletion of attentional resources in PD patients.