SHORT-TERM CHANGES IN NECK MUSCLE AND EYE-MOVEMENT RESPONSES FOLLOWING UNILATERAL VESTIBULAR NEURECTOMY IN THE CAT

The purpose of this study was to investigate changes in neck muscle an d eye movement responses during the early stages of vestibular compens ation (first 3 weeks after unilateral vestibular neurectomy, UVN). Ele ctromyographic (EMG) activity from antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles and eye movements were re corded during sinusoidal visual and/or otolith vertical linear stimula tions in the 0.05-1 Hz frequency range (corresponding acceleration ran ge 0.003-1.16 g) in the head-fixed alert cat. Preoperative EMG activit y from the splenius and longus capitis muscles showed a pattern of alt ernate activation of the antagonist neck muscles in all the cats. Afte r UVN, two motor strategies were observed. For three of the seven cats , the temporal activation of the individual neck muscles was the same as that recorded before UVN. For the other four cats, UVN resulted in a pattern of coactivation of the flexor and extensor neck muscles beca use of a phase change of the splenius capitis. In both subgroups, the response patterns of the antagonist neck muscles were consistent for e ach cat independently of the experimental conditions, throughout the 3 weeks of testing. Cats displaying alternate activation of antagonist neck muscles showed an enhanced gain of the visually induced neck resp onses, particularly in the high range of stimulus frequency, and a gai n decrease in the otolith-induced neck responses at the lowest frequen cy (0.25 Hz) only. By contrast, for cats with neck muscle coactivation , the gain of the visually induced neck responses was basically unaffe cted relative to preoperative values, whereas otolith-induced neck res ponses were considerably decreased in the whole range of stimulation. As concerns oculomotor responses, results in the two subgroups of cats were similar. The optokinetic responses were not affected by the vest ibular lesion. On the contrary, otolith-induced eye responses showed a gain reduction and a phase lead. Deficits and short-term changes afte r UVN of otolith-and semicircular canal-evoked collic and ocular respo nses are compared.