An extreme supernormal period in cerebellar parallel fibres.

1. The electrical responses produced by stimulation at the surface of the cerebellar cortex have been studied in anaesthetized cats.2. The propagation of the underlying activity is attributed to parallel fibres, but the mode of generation of the potential changes is less certain.3. The threshold for a second response is up to 40% lower following a conditioning stimulus, and the responses to test stimuli less strong than the conditioning stimulus are correspondingly potentiated.4. A second stimulus given 22 msec after the first produces a response which propagates 15-20% faster along the folium.5. The reduction of the threshold and the increase of the propagation velocity are as large with small (but above threshold) conditioning shocks as with large shocks.6. Both after-effects are present from 5 to 100 msec after a conditioning stimulus, with maximal values at about 10-30 msec.7. The fibre conduction velocity inferred from collision experiments agrees with that inferred from the propagation velocity of the responses, and shows a similar increase after conditioning.8. A second shock does not recruit a new and faster population of parallel fibres.9. Under the conditions of electrical stimulation the after-effects are probably restricted to those fibres which are active during conditioning.