We examined covert orienting attention in twenty patients with Parkinson's disease using Posner's reaction time (RT) method. Patients were divided according to the grade of severity (Hoehn and Yahr), the P1 group was grade I-II and P2 group grade II-IV. Each group of patients was compared with an equal number of age-matched controls. In controls, and in the less disabled younger P1 group, a significant RT difference was shown between "valid" and "crossed" conditions, that is, when the cue and target appeared in the same square or in equivalent squares in opposite visual fields. In the P2 group, however, there was no advantage of the "valid" over "crossed" conditions. Furthermore, the RT difference seen in controls between "valid" and "towards the fovea" or "away from the fovea" (conditions where cue and target were in adjacent squares) disappeared in the Parkinsonian groups. These results suggest that the covert shift of attention from initial focus to the cue is very weak, or does not occur in more disabled Parkinsonian patients, resulting in only a shift of attention to the target. In considering the elementary mental operations involved in covert orienting attention, this deficit may be attributable to a disturbance of moving attention.
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