Effect of Target Predictability on the Initiation of Smooth Pursuit in Healthy Subjects and Patients with Cerebellar Lesion

Abstract

Repeated presentation of a continuously moving visual target can elicit two types of smooth eye movements that can be identified by their time of onset (Kao and Gellman 1994). One type builds up well before the target motion appears and represents an anticipatory smooth eye movement (ASEM) that is solely driven by predictive expectations derived from prior target presentations (Kowler et al. 1984; Kao and Morrow 1994, Barnes et al. 1997). ASEM are more frequent and faster during presentation of highly predictable stimuli, but they are not necessarily extinguished by making the target movement unpredictable (Kowler et al. 1981). Smooth pursuit eye movements (SP), the second type of slow eye movements, begin with a certain delay after target onset (Tychsen and Lisberger 1986, Carl and Gellman 1987). If SP is elicited by an unexpected target motion, the first 100–125 ms of the smooth eye movement (initiation phase) are directly controlled by the target motion that was perceived before SP onset (Tychsen and Lisberger 1986). This way, SP initiation operates as an open-loop system. After this, visual feedback and extraretinal input of eye and head movements provide an internal signal that controls steady-state pursuit, the so called maintenance phase of SP (Tychsen and Lisberger 1986, Carl and Gellman 1987). Although SP is predominantly controlled by visual input, pursuit maintenance during longer periods of tracking is optimized by expectations of the target motion. For example, during tracking of sinusoidal stimuli, prediction of target motion significantly contributes to the adjustment of gaze velocity and phase lag (van den Berg 1988, Barnes and Asselman 1991).