Visual-vestibular interaction during transparent optokinetic stimulation in the rabbit

 Transparent motion is a visual stimulus condition that generates multiple motion vectors on the retina that can differ in speed, direction, and/or luminance. Transparent motion creates a conflict for retinal stabilization. In this study we investigated the effect of transparent visual motion on the oculomotor reflexes that provide retinal stabilization in the rabbit. In the first experimental condition, the animals were stationary. We presented one stationary and one oscillating visual pattern to the animals while varying the luminance of the patterns. We found that the optokinetic eye movement responses were fully determined by the luminance of the individual visual inputs, weighted for the total luminance. No effect of absolute stimulus intensity was found. In the second experimental condition we oscillated the animals, while using an identical visual stimulation paradigm. The contribution of the vestibulo-ocular reflex enhanced the response to the visual pattern, which was in agreement with the vestibular stimulus. This effect of vestibular stimulation was independent of the absolute intensity of the visual stimuli. From this result we conclude that the weighting process of the transparent visual patterns occurs upstream from the site of the visual-vestibular interaction. Both the visual weighting and the visual-vestibular interaction were dependent on stimulus frequency. In line with the properties of the visual and vestibular stabilization reflexes in isolation, the contribution of the vestibular system increased, whereas the influence of the optokinetic system decreased with increasing stimulus frequency.