Effects of cholinergic neuromodulation in cerebellar flocculus on transparent motion processing in the rabbit

The way a rabbit moves its eyes in response to a stimulus consisting of two moving random dot patterns largely depends on the relative luminances of the two patterns. Concurrent rotation of the animal enhances the response to the visual pattern that represents the same head movement as the vestibular stimulation. In this paper we investigate the role that the flocculus plays in this behaviour. We injected the non-selective acetylcholine agonist carbachol into the flocculus. These injections are known to increase the gain of the optokinetic reflex, but have a smaller effect on the vestibulo-ocular reflex. We investigated the effect on the oculomotor response to (vestibulo-) transparent stimuli, where one pattern oscillated sinusoidally and the other pattern was stable with respect to the head. We found that the injections caused a higher response gain at a lower luminance of the oscillating pattern. Furthermore the influence of concurrent vestibular stimulation decreased. These findings agree with a role of the flocculus that is downstream of the visual normalisation, but upstream of the visual-vestibular interaction.