BASIC RESPONSE CHARACTERISTICS OF AUDITORY-NERVE FIBERS IN THE GRASSFROG (RANA-TEMPORARIA)

Responses to free-field sound of 401 fibers from the VIIIth nerve of t he grassfrog, Rana temporaria, are described. The spontaneous activiti es of the fibers ranged from 0 to 75 spikes/s, showing only weak corre lation with frequency or sensitivity of the fibers. The highest sponta neous activities were approximately twice as high as reported previous ly for frogs. Best frequencies ranged from 100 to 1600 Hz and threshol ds ranged from 21 to 80 dB SPL. The median dynamic range was 20 dB and the slopes of the rate-level curves ranged from 5 to 20 spikes/(s.dB) . Most of the units showed post-excitatory suppression (PS) of their s pontaneous activity. The duration of PS increased with sound level, al so in fibers showing a decrease in firing rate at high intensities. Mo st fibers showing one-tone suppression did not show PS at their best s uppression frequencies. Strong suppression was observed also in very p hasic cells giving one spike per stimulation. Therefore, the mechanism underlying PS is probably different from that underlying adaptation. The sharpening of the neural encoding of temporal parameters and the s trong encoding of sound offset as well as onset caused by PS very like ly is biologically important. (C) 1998 Published by Elsevier Science B .V.