ANTICIPATORY SMOOTH EYE-MOVEMENTS OF HIGH-VELOCITY TRIGGERED BY LARGETARGET STEPS - NORMAL PERFORMANCE AND EFFECT OF CEREBELLAR DEGENERATION

We studied frequencies and dynamic characteristics of anticipatory smo oth eye movements (ASEM) in humans who were tracking step target movem ents of 20-70 deg amplitude, During presentation of periodic steps of constant amplitude healthy subjects showed frequent high velocity ASEM reaching maximal peak velocities of 5-40 deg/sec, There was no effect of ASEM on the frequency of anticipatory saccades, Randomization of t arget step amplitude or onset reduced the frequency of ASEM but did no t completely abolish fast ASEM, In patients with cerebellar degenerati on who exhibited impaired smooth pursuit, fast ASEM were absent and th e number of slow ASEM was minimal, In conclusion, large sequential tar get steps can elicit much higher ASEM velocities than typically descri bed in the literature, Similar to slow ASEM triggered by small steps, these fast ASEM do not require specific training and are not canceled by unpredictable step target motion, However, fast ASEM depend on the intact function of the cerebellum which gives further evidence of thei r generation by the smooth pursuit oculomotor subsystem.