Head movement co-ordination in the African chameleon

Abstract

Chameleon head movement was studied to learn how information from more than one sensory system can be co-ordinated to produce a single motor behavior. In this study, the co-ordination of visual, vestibular and kinesthetic senses was examined by: (1) moving a cricket back and forth in front of a hungry chamelon, (2) moving the body of an alert chameleon or the body of an anesthetized chameleon, and (3) moving both a cricket and the body of an alert chameleon. During cricket movement, the chameleon locked both eyes straight forward in their orbits and followed the cricket movement with a visually guided head movement. During movement of the body of an alert chameleon, the vestibulo-collic reflex kept the chameleon's head relatively stationary in space. During movement of the body of an anesthetized chameleon, there was no measurable movement of the head relative to the body. Thus, during the body movements used in this study, passively induced kinesthetic stimulation was negligible. It was hypothesized that during movement of both a cricket and the body of an alert chameleon, the visually guided head movement and the vestibulo-collic reflex were additive. This hypothesis was supported by demonstrating that in this situation, the vestibulo-collic reflex was not suppressed. It is suggested that the vestibulo-collic reflex compensated for the passive head movement produced by body movement without canceling active, visually guided head movement. This may have been accomplished by the use of an efference copy system associated with the motor commands that produce visually guided head movement.