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 independent 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 operates in a parallel mode for simple tasks such as picking out a vertical bar in a field of horizontal bars. 3. Since it is known that early vision in patients with Parkinson's disease (PD) is impaired, possibly due to abnormal function of the neurotransmitter dopamine, the effect of this impairment on visual search was investigated. 4. The finding was that search in PD operates in a mainly serial manner for the above task (on which normal subjects perform parallel search). It was shown 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. The findings point to a possible role of dopamine in mediating attentional mechanisms, and may be relevant to some reported perceptual abnormalities in PD and in schizophrenia.