RECOVERY OF HEAD POSTURAL CONTROL FOLLOWING UNILATERAL VESTIBULAR NEURECTOMY IN THE CAT - NECK MUSCLE-ACTIVITY AND NEURONAL CORRELATES IN DEITERS NUCLEI

Recovery of head postural control after unilateral vestibular neurecto my was investigated in the alert cat by chronically recording the spon taneous neck muscle EMG activity from splenius capitis on both sides a nd the vestibulocollic reflexes evoked during roll and pitch tilts. Ne uronal correlates occurring within the lateral (Deiters) vestibular nu clei (LVN) were also recorded during the time-course of recovery. Duri ng the acute phase (1-2 weeks), the cats exhibited strong imbalance in spontaneous neck muscle activity, characterized by increased muscular tone in the ipsilateral splenius capitis muscle and hypoactivity in t he contralateral one. At the same time, the mean resting activity of D eiters' neurons strongly decreased on the deafferented side, while a s light but significant decrease was observed on the intact side. Vestib ulocollic reflexes were totally lacking during the acute phase, whatev er the direction and the amplitude of tilt. Recovery developed in the following weeks, leading to complete rebalance of spontaneous EMG acti vity as well as near to normal static vestibulocollic reflexes 5 weeks after the lesion. However, compensation remained sub-normal during ro ll tilts while overcompensation was found during pitch tilts, suggesti ng that the intact labyrinth would play a leader role in the recovery process but that bilateral cooperation of the two labyrinths is requir ed for proper head postural control. Five weeks are also needed for a partial rebalancing of resting activity between both LVN. These result s indicate that changes in neck muscle activity observed in the acute cats and that recovery found in the compensated animals could result f rom modifications in neural networks controlling neck musculature, suc h as the LVN.