BILATERAL LABYRINTHECTOMY IN THE CAT - EFFECTS ON THE POSTURAL RESPONSE TO TRANSLATION

1. This study examined the role of vestibular afferent information on the postural responses of four cats, evoked by movements of the suppor t surface during stance. Animals were exposed to linear translations o f the supporting surface in eight evenly spaced directions in the hori zontal plane, before and after bilateral labyrinthectomy. Postural res ponses were quantified in terms of the ground reaction forces under ea ch paw and the evoked activity in selected muscles. 2. The cats were a ble to stand on the platform within 1-3 days after labyrinthectomy and were able to maintain balance during all perturbations of stance, eve n when they stood in total darkness, completely deprived of visual inf ormation. After lesion, postural responses were characterized by norma l latency and normal spatial and temporal patterning of electromyograp hic (EMG) response. The pattern of force response showed the force con straint strategy that characterizes postural responses in the intact a nimal. 3. The only deficit in the postural response after lesion was a hypermetria, or active over-response that caused the animals to overb alance somewhat but did not impair their ability to remain upright. An alysis of the trajectory of the animal's center of mass during the tri als indicated that the hypermetria was due to an abnormally large, act ive response on the part of the animal and could not be attributed to changes in the passive stiffness of the musculoskeletal system. The hy permetria was transient, and response amplitude returned to control le vels after the rapid compensation phase of 10-15 days. 4. it is conclu ded that vestibular information is not essential for triggering the ra pid, automatic postural response to translations of the support surfac e, nor is it necessary for the selection or shaping of the evoked resp onse. Instead, somatosensory information appears to predominate in the se postural adjustments. However, vestibular afferent input does influ ence the scaling of the postural response.