Single unit recordings were carried out in the reticularis thalamic nucleus (RT) and the ventral lateral geniculate body (LGv) of chronically prepared alert cats under sinusoidal vestibular stimulation in the horizontal plane. Optokinetic stimulation was also used. Of the 57 recorded neurons, 12 present vestibular modulation in the dark, analogous to Duensing's and Schaefer's (1958) type I response in the vestibular nuclei. Responses of 26 cells are similar to response of type II vestibular neurons and 14 units have a type III response; the 5 remaining cells were activated by vestibular stimulation in the vertical sagittal plane. The majority of these cells does not present detectable direct visual responses, but 50% can be driven by optokinetic stimulation. 74% of types I, II and III neurons show saccadic resonses to vestibular nystagmic saccades in the dark. About 60% present similar saccadic modulations during optokinetic nystagmus and 55% keep this response for spontaneous saccades in the dark or in front of a striped background. The saccadic responses are constant for a given neuron in all cases of stimulation with latencies ranging from 30 msec prior to the beginning of the saccade to 120 msec after its onset. The histological localization of these units falls on one hand into the caudal part of the RT nucleus (type III neurons) above the dorsal lateral geniculate nucleus and on the other hand within the internal subdivision of the LGv and its rostral limit (all other types). The significance of this new, saccadic and vestibular focus in the feline thalamus is discussed in relation with the two previously known vestibular thalamic relays in terms of interrelations between the vestibular and the visual systems.
A photographic technique was used to study the evolution of lateral head-tilt following hemilabyrinthectomy in adult cats. Animals were maintained post-operatively in normally lit conditions (LM cats), in total darkness (DM cats), or in stroboscopic light. In LM casts, the head tilt peaked at 45 degrees (with the lesionned side down) on the second post-operative day, and decreased to about 0 degree within about 10 days. This evolution was followed by rebounds of head-tilt to large angles before a stable compensated head postion could be maintained (approximately at the end of the third post-operative month). In DM cats the head remained tilted by a large angle throughout the duration of the dark period. Re-exposure to light was followed by a rapid decrease of head-tilt. In stroboscopic light, the evolution of head-tilt was found to be closely similar to that in the normally lit condition. Finally, when put back in the dark at a late post-operative stage, already compensated animals were found to loose their symmetrical head position, and to re-acquire a strong head tilt. This effect resumed on re-exposure to light. It is inferred that static visual input is a necessary condition for compensation of the postural deficits of hemi-labyrinthctomy in the cat. Maintenance of a stable head posture also depends upon continuous availability of visual input.
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