Long-term adaptive changes in primate vestibuloocular reflex. IV. Electrophysiological observations in flocculus of adapted monkeys.

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Abstract
Extracellular recordings of the simple spike activity of Purkinje cells (P-cells) in the flocculus were obtained from alert rhesus monkeys previously adapted to 2X telescopic spectacles or dove prism (left-right reversing) spectacles. The gain of the horizontal vestibuloocular reflex (VOR) was determined each day prior to unit recording using 0.2 Hz, .+-. 20.degree. sinusoidal chair oscillations. Mean horizontal VOR gain for the animals adapted to 2X telescopic spectacles was 1.50 (range, 1.38-1.70) and for the prism adapted, 0.18 (range, 0.05-0.36). The latter showed progressively increasing phase lags during the recording period, which lasted several wk in each animal (mean lag, 81.degree.; range, 6-160.degree.). P-cells [160] were identifed as horizontal gaze velocity cells on the basis of their simple spike responses during ocular pursuit and/or vestibular stimulation and were subjected to quantitative analysis to determine their sensitivities to head velocity, eye velocity, eye position and passive oscillation in the dark. In animals adapted to 2X spectacles these P-cells were usually significantly more sensitive to head velocity than normal. In the animals adapted to prisms, they were usually significantly less sensitive to head velocity and more sensitive to eye velocity than normal. Changes in the responsiveness of these P-cells apparently do not mediate the adaptive changes in the VOR. Their effects on eye movements are probably a secondary consequence of the latter. Following sudden disturbances of the head, the modified vestibular response of the adapted animals are apparent at short latencies before any changes are evident in the discharges of the P-cells.