Head-eye co-ordination: Visual and nonvisual mechanisms of vestibulo-ocular reflex slow-phase modification

During normal head and body manoeuvres there is a continual requirement to modify the gain of the slow-phase component of the vestibulo-ocular response. The acquisition and fixation of earth-fixed targets requires a gain close to unity, whereas head-free tracking of moving targets necessitates a maximal suppression of the VOR in order to maintain visual acuity. Two mechanisms are available for the modification of VOR gain which appear to be quite distinct in their response to pseudorandom motion stimuli. Visual feedback forms the most important mechanism of suppression, but its effectiveness is governed in a nonlinear manner by the frequency composition of the stimulus. This nonlinear characteristic, which has been associated with stimulus predictability, appears to be inherent in the visual feedback and may also be observed in the pursuit reflex response. Modification of VOR gain can also be achieved by nonvisual means, but this is brought about by a separate mechanism to that used for pursuit, which is not affected by the frequency composition or predictability of the stimulus. Suppression of the VOR during voluntary head movements is no better and, indeed, may be worse than during whole-body rotation despite the fact that the motion is predetermined. This underlines the fact that the changes in gain associated with the predictability of stimulus motion probably arise as a function of the nonlinear visual feedback characteristics.