IMPAIRED PARALLEL VISUAL-SEARCH MECHANISMS IN PARKINSONS-DISEASE - IMPLICATIONS FOR THE ROLE OF DOPAMINE IN VISUAL-ATTENTION

1. There are two limiting types of visual search. ''Parallel'' search (''pop-out'') means that the time taken to locate a target is independ ent of the number of distractor items, while ''serial'' search entails an increase in search time as the number of distractors is increased. 2. There are various theories about how visual search mechanisms may operate, but they are all in agreement that the normal visual system o perates in a parallel mode for simple tasks such as picking out a vert ical bar in a field of horizontal bars. 3. Since it is known that earl y vision in patients with Parkinson's disease (PD) is impaired, possib ly due to abnormal function of the neurotransmitter dopamine, the effe ct of this impairment on visual search was investigated. 4. The findin g was that search in PD operates in a mainly serial manner for the abo ve task (on which normal subjects perform parallel search). It was sho wn that this was due to an abnormal visual search process rather than due to an inability to see the target or to slow response times. 5. Th e findings point to a possible role of dopamine in mediating attention al mechanisms, and may be relevant to some reported perceptual abnorma lities in PD and in schizophrenia.