Response characteristics of inferior colliculus neurons of the awake CF-FM batRhinolophus ferrumequinum

Abstract

In horseshoebats two-tone stimulation of inferior colliculus neurons with the first tone pulsed and fixed at the best excitatory frequency (BEF) and 10–30 dB above threshold, and a second continuous tone with variable frequency and intensity within the given range, led to a reduction of the response to the fixed tone in nearly all cases, i.e., inhibitory areas are widespread in inferior colliculus neurons. Furthermore, this two-tone stimulation disclosed, that best inhibitory frequencies (BIF) were often nearly the same as the BEF, with BIF-thresholds more frequently being below that one of BEF. In “filter”-neurons (i.e. neurons with BEF in the frequency range of the reference frequency (RF)) there was no inhibition discernable if the continuous tone was 500–4000 Hz below BEF and from individual RF to 3500 Hz below RF (Fig. 1). This gap of inhibition ranged from 78–83,5 kHz in all horseshoebats tested. The vicinity of this gap of inhibition to the BIF in the same neuron disclosed high frequency specifity of inhibitory influences. The frequency range, where this gap of inhibition occurred was moreover the same as that causing enhancement to second stimuli (Fig. 1 B), as well as that causing off-responses and showing non-monotonic spike count functions.