The role of integrators in maintaining actively assumed abnormal postures

Abstract

1. Geckos adapt their head-position to the spatial orientation of substrates they are actively moving on. Furthermore an adaptation of head-position is carried out in response to structures in the animal's vicinity. We found a tendency to keep the ventral side of the head orientated towards such objects. Thus, in geckos, visual guidance overrides vestibular mechanisms inducing compensatory head-movements during abnormal body postures in many other species. 2. Compensatory head-movements during optokinetic, vestibular, and combined optokinetic and vestibular stimulation in restrained geckos also revealed a dominant role of vision in guiding postural control in these animals. Vestibular influences on head-position are weak as long as optokinetic cues are absent. With optokinetic stimulation present vestibular afferents significantly increase the gain of the optokinetic response. This effect is the more obvious the higher the optokinetic stimulation speed is. 3. When a head-position deviating from the normal position has been actively assumed in response to either one of the above conditions it is not instantaneously abolished after the inducing stimulus ceases to be present. The head returns to a position normal with respect to the body by a slow drift that takes several minutes. This drift is faster when visual stabilization is interrupted by darkness as compared to a situation with the experimental room illuminated. 4. We conclude that geckos have integrators through which postural change commands (e.g. from the visual system) are fed before interacting with postural control circuits. These mechanisms maintain actively assumed abnormal postures after cessation of the inducing command. Comparative investigations suggest that such mechanisms are present in many species of lizards and are especially well developed in animals that frequently assume different body orientations with respect to the gravity vector in their habitat.