What is pathological with gaze shift fragmentation in Parkinson's disease?

Oculomotor dysfunction in Parkinson's disease (PD) is mainly characterized by a fragmentation of memory-guided gaze shifts (target is reached by several hypometric saccades). Since this phenomenon can also be observed in normal subjects, we scrutinized its pathophysiological significance in PD patients. We recorded horizontal eye movements in eleven mildly- or moderately-affected PD patients and eleven control subjects. A quantitative assessment of gaze shift fragmentation was made possible by increasing its incidence over a sequence of two visually- and two subsequent memory-guided gaze shifts. Basic saccade measures (latency, velocity, etc.) were similar in the two subject groups as well as in fragmented versus non-fragmented gaze shifts. Fragmentation probability is increased in the second memory-guided gaze shift, and this clearly more so in patients than in controls. The fragmentation shows a typical gain pattern (uniform increase of gain of saccadic amplitudes across correction saccades towards 1.0 with the last saccade of the gaze shift) independent of subject group, stimulus mode, and fragmentation degree. Gaze shift fragmentation represents a physiological phenomenon, which has thus far been overlooked. It reflects a robust correction mechanism, which assures that target is reached even if the pre-oculomotor drive through the basal ganglia to the superior colliculus becomes abnormally weak or under inadequately strong inhibition – as is postulated for PD. Thus, only the abnormally high incidence of fragmentation, and of the associated amplitude reduction of the primary saccades, rather than the phenomenon per se, can be used as a diagnostic criterion in early stages of PD.