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

The way a rabbit moves its eyes ill response to a stimulus consisting of tw o moving random dot patterns largely depends on the relative luminances of the two patterns. Concurrent rotation of the animal enhances the response t o the visual pattern that represents the same head movement as the vestibul ar stimulation. In this paper we investigate tho role that the flocculus pl ays in this behaviour. We injected the non-selective acetylcholine agonist carbachol into the flocculus. These injections are known to increase the ga in of the optokinetic reflex, but have a smaller effect on the vestibule-oc ular reflex. We investigated the effect on the oculomotor response to (vest ibulo-) transparent stimuli, where one pattern oscillated sinusoidally and the other pattern was stable with respect to the head. We found that the in jections caused a higher response gain at a lower luminance of the oscillat ing pattern. Furthermore the influence of concurrent vestibular stimulation decreased. These findings agree with a role of the flocculus that is downs tream of the visual normalisation, but upstream of the visual-vestibular in teraction.