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

Summary

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.

Long lasting stimulus influences (aftereffects) were often noticed during experimentation with awake horseshoebats (Fig. 2). Long lasting inhibitory influences lasted on the average 250–500 ms after cessation of the stimulus appearing at frequencies within the inhibitory areas, whereas long lasting facilitation, lasting 250–500 ms, occurred only in the frequency range lacking inhibition (Fig. 1 B).

A stimulus program simulating hearing of pure-tone components of the emitted sound and echo revealed further adaptations of auditory neurons to echolocation in flying horseshoebats (Figs. 3–5), extending results of two-tone stimulation with continuous tone: a) the off-response to the first tone (emitted sound) disappeared for frequencies of the second tone above RF, depending on time interval between both stimuli (Fig. 4 C); b) the enhancement to second tones, caused by frequencies where the gap of inhibition occurred, was confined to RF (Fig. 4 B), as well as a simultaneous threshold decrease of about 20 dB for second stimuli. Realisation of these neuronal adaptations and their significance in terms of active echolocation is discussed.