Interactions of visual and auditory mossy fiber inputs in the paraflocculus of the rat: a gating action of multimodal inputs.

We have demonstrated previously that visual and auditory inputs reach the rat paraflocculus via dorsolateral pontine gray from the secondary regions of the visual and auditory cortices. It has also been demonstrated that mossy fiber responses can be evoked to physiological acoustic stimuli in unanesthetized preparations. In this study, we investigated the interaction of auditory and visual inputs in the paraflocculus. Activity of parafloccular neurons was recorded in immobilized, locally anesthetized hooded rats. Selected images and tones were presented to animals. Orientation, position, and velocity of visual stimuli and different parameters of acoustic stimuli were controlled by a computer. Visual and auditory stimuli were also presented in combination or in different temporal sequences. We discovered that visual and auditory stimuli have coextensive termination zones in paraflocculus, and influence the same neurons in 60% of the cases. Combined auditory and visual stimuli produced synergistic responses in parafloccular neurons in comparison with single modality stimuli. Augmentation of responses could be observed even when one of the stimuli was subthreshold and did not alter per se the neuronal activity. Our findings suggest that within the cerebro-cerebellar system, subthreshold inputs are capable of powerful control over the neuronal activity and may alter responses to subsequent stimuli when properly biased by inputs from paired modalities.