Impaired visual search mechanisms in Parkinson's disease (PD): A psychophysical and event-related potentials study

There are two limiting types of visual search: parallel and serial. 'Parallel' search ('pop-out') means that the time taken to locate a target is independent of the complexity of the scene, whereas 'serial' search entails a scanning process of each element in turn, thus causing an increase in response time. It is well established that the normal visual system operates in a parallel mode for simple tasks such as picking out a vertical bar in a field of horizontal bars. Troscianko and Calvert suggested that visual search in PD operated in a serial manner in a simple task (in which normal subjects perform parallel search). We performed parallel and conjunction search experiments with PD patients and included measurements of event-related potentials (ERPs), in particular the latency and amplitude of the P300. We found no difference in the slopes of the function relating P300 latency to set size between controls and PD. However, we discovered in parallel search in controls a higher P300 amplitude compared with Parkinsonian patients. Psychophysical (reaction time) data in this experiment confirmed the findings of Troscianko and Calvert, showing significantly less parallel search slopes in the PD groups. In a conjunction search study which was carried out on a different group of subjects, P300 amplitude as a function of set size was constant in both PD patients and controls. These results indicate that there is a deficit in parallel search mechanisms in PD patients, whereas there is no evidence For such a deficit in conjunction search. There may be deficient pre-attentive mechanisms in PD patients which is revealed by an abnormally low P300 parietal amplitude.