GAIN CHANGES OF THE CATS VESTIBULOOCULAR REFLEX AFTER FLOCCULUS DEACTIVATION

Motor learning can be demonstrated in the vestibule-ocular reflex (VOR ) by changing its gain (eye velocity/head velocity) with goggles and o ptokinetic (OK) drums. It is known that the flocculus is essential for this plasticity but there is controversy about whether the modifiable synapses mainly responsible are in the flocculus. To investigate this further we utilized the known reciprocal relationship between complex spikes and simple spikes in Purkinje cell discharges. By stimulating climbing fibers from the olive to the flocculus at 7 Hz, the simple sp ike rate of almost all recorded floccular cells could be driven to zer o. This was termed floccular shutdown and is felt to effect a function al, reversible flocculectomy. Sixty single units in the flocculi of fo ur cats were recorded. Stimulation of the climbing fibers at 7 Hz caus ed the discharge rate to decrease to zero in 95% of these cells. The g ain of the horizontal VOR in three cats was driven repeatedly to twice or half its normal value by rotation within a moving OK drum and also by wearing magnifying or fixed-field goggles; this process required 3 days. If, on the 4th day, the cat was exposed to an OK drum rotating in the opposite direction, the gain was driven back to normal in 30 mi n. If, however, the climbing fibers were stimulated at 7 Hz during the se 30 min, the gain did not return - learning was blocked. This verifi ed that loss of floccular activity by this method abolishes VOR gain p lasticity. Moreover, when 7 Hz stimulation first began, after 3 days o f adaptation, the adapted gain remained at its adapted value, either h alf or twice normal, even in the face of floccular shutdown. This resu lt appears incompatible with the hypothesis that the modifiable synaps es are in the flocculus.