SIGNAL PROCESSING IN FROG SEMICIRCULAR CANALS: THE RELATION BETWEEN EPSP RATE AND SPIKE FIRING.

Computations of posterior canal EPSP amplitude and frequency carried out under TTX treatment by fluctuation analysis have revealed that asymmetry, adaptation and phase lead in the afferent response are of presynaptic orígin. Contrary to the spike discharge behavíour, complete silencing of the EPSP rate during inhibition (rectificatíon) was never observed. Therefore, it seemed appropriate to attempt a simultaneous analysis of EPSP and spike activities ín the same fibres. Summated EPSPs and spikes were intraaxonally recorded at rest and during sinusoidal canal rotation at 0.1 Hz. Spikes were counted in the tracings and digítally cancelled to allow the statistical evaluation of the EPSP rate. The absolute values of EPSP and spike rates varied from one unit to the other but the same behaviour was qualitatively observed in all the units examined. The main features of the afferent response may be summarized as follows: 1) the resting spike rate was low (0.5-3 spikes/s); 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 following the excitatory peak anticipated that of the EPSP rates; 4) fibres with low resting spíke activity showed marked rectification. It is concluded that the relation between EPSP occurrence and spike discharge 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).