Visual Target Velocity Coding through Ocular Muscle Proprioception

Abstract

Considering the eyes as contributing only to visual function is extremely restrictive in that the eyes are also involved, with the head as their carrier, in target position and velocity coding, a vital function for our daily activities. Indeed, to follow a visual object with the hand, the brain has first to code both the object position and velocity with respect to the body and second to program and control the appropriate commands to the arm and hand muscles. As compared to visual function per se and to eye movement control function (saccade, smooth pursuit), the target position and velocity coding with respect to the body is still not elucidated and some basic observations are still controversial. The position of the object with respect to the body has to be computed from the distance of the object image to the fovea and the angular position of the eye in the orbit (with respect to a head-centric reference). Moreover, since the head carries the eyes in space, determination of the target position with respect to the body frame of reference also requires computation of the head angular position with respect to the body. In binocular viewing, both eyes must be aligned on the target and both eye positions must be properly sensed to provide accurate target position coding. We shall limit our analysis of the overall problem of eye-hand tracking of visual target to the coding of a visual target velocity with respect to the body in terms of what is commonly referred to as retinal and extra-retinal coding.