LONG-TERM DEFICITS IN OTOLITH, CANAL AND OPTOKINETIC OCULAR REFLEXES OF PIGMENTED RATS AFTER UNILATERAL VESTIBULAR NERVE-SECTION

Static and dynamic otolith, horizontal vestibular and optokinetic ocul ar reflexes were investigated in pigmented rats 1-6 and more months af ter unilateral vestibular nerve (UVN) section. Evoked responses were c ompared with published data from control rats studied under identical conditions. Static lateral tilt of UVN rats in the Light evoked a vert ical deviation in static eye position that was as large as in controls . In darkness, the evoked responses in UVN rats 6 months after the les ion were consistently smaller than in controls, Linear horizontal acce leration in darkness evoked vertical and torsional response components in UVN rats that were parallel-shifted towards lower gains and larger phase lags. Off-vertical axis rotation on a platform provoked respons es that differed markedly from those recorded in intact rats with resp ect to the bias velocity component. These results suggest a permanent deficiency in the static and dynamic otolith-ocular reflex performance of UVN rats. Ocular responses to horizontal table velocity steps in d arkness exhibited a direction-specific asymmetry in UVN rats. Step res ponses evoked by acceleration towards the intact side were larger in g ain and longer in duration than responses evoked by acceleration towar ds the operated side. When compared with control data, responses to ei ther side were reduced in UVN rats and the velocity store mechanism wa s barely activated by velocity steps towards the operated side. Respon ses evoked by horizontal optokinetic stimulation with constant pattern velocities were below control values in either direction. Slow-phase eye velocity saturated at much lower values than in intact rats, parti cularly during pattern motion towards the intact side. The duration of the optokinetic afternystagmus was asymmetrically reduced with respec t to control data. Practically identical reductions in duration were f ound for vestibule-ocular responses in the opposite directions. Behavi ng animals exhibited no obvious impairment in their spontaneous locomo tory or exploratory activities. However, each UVN rat was impaired, ev en 2 years after the lesion, in its postural reaction to being lifted by the tail in the air. This observation suggests the presence of a pe rmanent deficit in static and dynamic otolith-spinal reflexes that may be substituted on the ground by proprioceptive inputs.