Electrophysiological analysis of cognitive slowing in Parkinson's disease

To analyze chronometrically the evidence for possible cognitive slowing in Parkinson's disease, we measured visual event-related potentials (ERPs) and reaction times (RTs) in 29 patients with nondemented Parkinson's disease and 19 age-equivalent normal controls during the performance of semantic discrimination tasks. The components of the N1, P2, NA, N2 and P3 and simple and GO/NOGO RTs were observed. The N2 was measured from difference waveforms, subtracting the ERPs to frequent stimuli from those to infrequent stimuli in the discrimination task. Difference waveforms were also derived to delineate NA by subtracting the ERPs in the simple RT task from those of the frequent stimuli of the discrimination task. The N2 and P3 latencies and GO/NOGO RT in patients with Parkinson's disease were significantly longer than those in the controls, although there were no differences in N1, P2 and NA latencies or simple RT between the two groups. The results are interpreted as electrophysiological signs of cognitive slowing, particularly with respect to stimulus classification and attention processes in Parkinson's disease, independent of sensory problems. As for the automatic/controlled processes, the present results suggest that the automatic processing stage associated with NA may be less impaired than the attention-controlled processing reflected by N2 in patients with Parkinson's disease.