SENSORY INFORMATION CODING IN FROG SEMICIRCULAR CANALS: HOW SPIKE FIRING DEPENDS ON EPSP RATE

We have previuosly demonstrated, by fluctuation analysis of the posterior canal summated EPSP discharge, that the cyto-neural junction of the hair cells is the source of asymmetry, adaptation and phase lead of the afferent response. To characterize the role of postsynaptic mechanisms in signal processing a computation of EPSP and spike rates was attempted in the same fibres. Sensory activity was recorded from the posterior nerve at rest and during sinusoidal canal rotation at 0.1 Hz. Spikes were counted and then digitally cancelled from the records to allow the statistical analysis of the EPSP discharge. It resulted that: 1) the resting EPSP emission rate was considerably higher than the corresponding spike frequency; 2) while the EPSP rate rose during excitation, the encoder gain appeared to increase leading to an even faster increase in spike rate; 3) the decline in spike number, after the excitatory peak, anticipated that of the EPSPs; 4) contrary to the EPSP behaviour, complete silencing of spike firing during inhibition (rectification) was observed. It follows that the relation between EPSP rate and spike firing is complex both in terms of encoder gain (non-linearity and rectification resulting in amplification of the response asymmetry) and in terms of phase (apparent lead)