Discharge properties of afferent fibres of the goldfish semicircular canal with high frequency stimulation.

The horizontal semicircular canals of goldfish were sinusoidally stimulated 0.07 and 63 Hz (about 3 decades). Single afferent fibre recordings showed sinusoidal modulation of discharges. Above 4 Hz the discharges became phase-locked to the stimulus. With increasing frequency the number of spikes per period decreased so that finally only one spike per period remained. At 63 Hz a stimulus of as little as 0.005 degrees was sufficient to drive the units to far above their spontaneous activity. As cupular deflection is less than the angle of body movement, the cupular deflection threshold for modulation of afferent discharges must be much less than 0.005 degrees. Transfer functions of afferent activity were determined. The simple pendulum model does not fit the data. Additional introduction of a third time constant and the low pass properties of the receptor cell membrane, the synaptic delay and the leaky integrator of the post synaptic afferent terminal improve the fit.