In the isolated frog labyrinth, inhibition or facilitation of the posterior canal sensory discharge can be achieved by antidromic electrical shocks to the anterior-horizontal nerves. The existence of a peripheral inhibitory efferent pathway is well established, but the efferent nature of facilitation is still controversial. To clarify the modalities of functioning of the efferent system following different activation patterns we have analysed the changes in posterior canal spike rate produced by various protocols of stimulation of the anterior nerve. Results may be summarized as follows: 1) the latency of any effect, estimated from the modifications in EPSP rate, is 15 ms; 2) the degree of inhibition or facilitation of the afferent discharge is positively related to the frequency of stimulation (10-50/s); 3) during long stimulation periods (10 s) a decrease of both responses (adaptation) is observed; 4) the rebound spike discharge at the end of the inhibitory period is a linear function of the preceding stimulation rate; 5) with short, high frequency stimulus trains (250-624 ms; 200-80/s) stimulation time, instead of frequency, is crucial in determining the magnitude of the response. Inhibition and facilitation do not coexist in the same unit and are equally dependent on thestimulation pattern; this suggests that the effects are sustained by two different groups of efferent fibres acting in an antagonistic way on the receptor
