Eye movements and spatial working memory in Parkinson's disease

Mechanisms of spatial working memory and eye movement control were investig ated in eight mild to moderate Parkinson's disease patients (PDs). Subjects were presented with a sequence of four targets which had to be memorized a nd then recalled by moving their eyes to fixate the four locations in the c orrect order. Two variations on this procedure were used in which either a different sequence of lights was presented on each trial, or an identical s equence of lights was repeated on each trial. In both conditions subjects m ade memory-guided eye movements in the dark, without any visual cues to eye movement accuracy or the locations of the previously illuminated lights. A nalysis of the amplitude of the primary eye movement and final eye position for each step in the sequence showed that PDs made several discrete saccad ic eye movements of reduced amplitude before reaching the final eye positio n (multi-stepping). When a novel target sequence had to be memorized on eac h trial, the final eye position reached by PDs for each location was also f ound to undershoot relative to controls. In contrast, when an identical seq uence of targets was repeated on each trial, PDs' final eye position was fo und to be normal, although primary movement amplitudes were still reduced. PDs showed no multi-stepping and normal final eye position gain under condi tions for which the target lights in the sequence were illuminated during m ovement execution. PDs also made an increased proportion of overt errors in target sequence recall. Parallel neuropsychological testing in PDs and con trols revealed that error rates in the sequential memory-guided saccade tas k were significantly correlated with performance in a task thought to be se nsitive to spatial working memory dysfunction. The findings suggest that sh ort-term spatial memory representations are disrupted in the early stages o f PD. (C) 1999 Elsevier Science Ltd. All rights reserved.