Macrosaccadic oscillation, saccadic dysmetria and motor error in spinocerebellar degeneration

The macrosaccadic oscillation was analyzed quantitatively in a 31-year-old man with spinocerebellar degeneration, using DC electrooculogram. Over a follow-up period of one year the macrosaccadic oscillation changed into saccadic dysmetria where square oscillations were suppressed during fixation and occurred only after visually guided saccades. The initial movements in the oscillation followed the main saccades with a mean latency of 147.6 msec. When a saccade showed an overshoot, a fast movement occurred in the opposite direction, and with undershoot of the saccade, the fast movement occurred in the same direction as that of the saccade. Furthermore, the amplitudes of the oscillation had a close correlation with those of the motor error, ie, the difference in the amplitudes between the target displacement and saccade. The results suggest that square oscillations in the macrosaccadic oscillation and saccadic dysmetria may be caused by an elevation of the gain in the corrective saccadic system.