SLOW EYE-MOVEMENTS

Monkeys and humans are able to perform different types of slow eye mov ements. The analysis of the eye movement parameters, as well as the in vestigation of the neuronal activity underlying the execution of slow eye movements, offer an excellent opportunity to study higher brain fu nctions such as motion processing, sensorimotor integration, and predi ctive mechanisms as well as neuronal plasticity and motor learning. As an example, since there exists a tight connection between the executi on of slow eye movements and the processing of any kind of motion, the se eye movements can be used as a biological, behavioural probe for th e neuronal processing of motion. Global visual motion elicits optokine tic nystagmus, acting as a visual gaze stabilization system. The under lying neuronal substrate consists mainly of the cortico-pretecto-olivo -cerebellar pathway. Additionally, another gaze stabilization system d epends on the vestibular input known as the vestibule-ocular reflex. T he interactions between the visual and vestibular stabilization system are essential to fulfil the plasticity of the vestibule-ocular reflex representing a simple form of learning. Local visual motion is a nece ssary prerequisite for the execution of smooth pursuit eye movements w hich depend on the cortico-pontino-cerebellar pathway. In the wake of saccades, short-latency eye movements can be elicited by brief movemen ts of the visual scene. Finally, eye movements directed to objects in different planes of depth consist of slow movements also. Although the re is some overlap in the neuronal substrates underlying these differe nt types of slow eye movements, there are brain areas whose activity c an be associated exclusively with the execution of a special type of s low eye movement. (C) 1997 Elsevier Science Ltd.