Investigations of executive function with neuropsychology and electrophysiology in young, aged and Parkinsonian adults

Experiment one investigated the effect of a warning signal on voluntary and reflexive reactions when the foreperiod varied from 1.0 to 6.5 seconds. Foreperiods concluded with an imperative stimulus (white noise or airpuff) to which the participant responded. The imperative stimulus also triggered a startle-blink reflex. Latency of the electromyographic activity to the blink and the manual response was analyzed. Aging was associated with smaller and delayed blinks. Patient's blinks were not significantly slower than aged adults and amplitude was intermediate between aged and young. Equivalent foreperiod effects were observed across groups in blink. Foreperiod effects diverged for voluntary responses across groups with no effect of foreperiod for PD patients. The dissociation between foreperiod effects on reflexive and voluntary reactions in PD suggests that higher level preparatory processes are dependent upon intact dopaminergic pathways. Experiment two investigated response selection processes in young, aged and Parkinsonian adults. Previously, in a response-conflict task an exaggerated error dip (indicative of wrong-hand activation) was observed in PD patient's lateralized readiness potential (LRP) suggesting that changes in non-motoric processing might contribute to their delayed RTs (Praamstra et al., Brain, 1998). Participants responded with the hand corresponding to the direction of a centrally presented arrow. Flanking distractor arrows were either compatible or incompatible with the target arrow. Analyses did not confirm the exaggerated error dip found by Praamstra et al. on incompatible trials. The time interval from stimulus onset until LRP onset was delayed in aged relative to young adults with no effect of PD. Conversely, the time interval from LRP onset until keypress was delayed by PD, but not by age. The results indicate that slowed responding by PD patients in a conflict task is primarily due to impaired motor processes. Results from experiment one suggest slowed processing due to aging at both reflexive and voluntary levels of behavior and a loss of temporal information dependent on dopaminergic pathways. Results from experiment two suggest that slowed processing in aging and PD are due to dissimilar mechanisms.