Neuronal coding of linear motion in the vestibular nuclei of the alert cat. III. Dynamic characteristics of visual-otolith interactions.

In the present study we have investigated in the awake cat the response dynamics of vestibular nuclei neurons to visual or/and otolith stimulation elicited by vertical linear motion. Of the 53 units tested during sinusoidal motion at 0.05 Hz (9.1 cm/s), 1 (1.9%) was responsive to the otolith input only, 13 (24.5%) were influenced by the visual input only and 23 (43.4%) responded to both modalities. Neurons were excited either during upward or downward animal or visual surround movement. Most units displayed a firing rate modulation very close to motion velocity. All the neurons receiving convergent visual and otolith inputs (0.05 Hz, 9.1 cm/s) exhibited synergistic patterns of response. Motion velocity coding was improved in terms of input-output phase relationship and response sensitivity when visual and otolith signals were combined. Depending on the units, visual-otolith interactions in single neurons could follow a linear or a nonlinear mode of summation. The dynamic characteristics of visual-otolith interactions were examined in the 0.05 Hz-0.50 Hz frequency bandwidth. Visual signals seemed to predominate over otolith signals at low stimulus frequencies (up to 0.25 Hz), while the contrary was found in the higher frequency range of movement (above 0.25 Hz). The effects of visual stabilization (VS: suppression of visual motion cues) was observed in a small sample of units. As a rule, VS induced a reduction in the amplitude of unit response as compared to visual + otolith stimulation, the lower the motion frequency, the more pronounced the attenuation. VS also decreased the amplitude of the otolith-dependent component of response. The possible modes of visual-vestibular interactions in single cells are discussed. The present study supports the hypothesis that visual and vestibular motion cues are weighted according to their internal relevance.