Classification of inferior collicular neurones of bats in terms of responses to pure tones, FM sounds and noise bursts.

1. Single unit activity in the inferior colliculus of bats was studied with pure tones, FM (frequency-modulated) sounds and noise bursts which are the most basic three components of the complex sounds produced by many different animals including man. Neurones were divided into three groups, (i) ;generalized' units responding to all three elements, (ii) ;deaf' units responding to two out of the three and (iii) ;specialized' units responding to only one of the three. Each of these was divided into three subgroups.2. Three subgroups of generalized units were called ;symmetrical', ;asymmetrical' and ;upper-threshold' units. The symmetrical unit had a wide excitatory area and responded to any sounds which had components falling in this area. The asymmetrical unit had a narrow excitatory area abutting on a large inhibitory one and showed different responses to FM sounds depending on the directions of frequency sweep. The upper-threshold unit had not only an excitatory area, but also an inhibitory one. The neurone failed to respond to strong sounds, i.e. these showed upper thresholds.3. Three subgroups of deaf units were called ;pure tone-deaf', ;FM-deaf' and ;noise-deaf' units. The pure tone-deaf unit did not respond to any pure tones but did respond to FM sounds and noise bursts. The noise-deaf unit did not respond to noise bursts but did respond to pure and FM tone pulses. An FM-deaf unit has not yet been confirmed.4. Three subgroups of specialized units were called ;pure tone-specialized', ;FM-specialized' and ;noise-specialized' units which responded exclusively to either pure tones, FM sounds or noise bursts, respectively. In the FM- and noise-specialized units, pure tone pulses caused only inhibitory processes.5. About 95% of neurones studied showed phasic on-responses to sound stimuli and almost no spontaneous discharges. Only a small percent of neurones showed spontaneous discharges higher than a few impulses per second. Response patterns of some of these neurones changed with frequency, intensity, duration and repetition rate of sound stimuli. These neurones responded to any of pure tones, FM sounds and noise bursts.6. All neurones except the symmetrical ones had inhibitory areas, in which sounds inhibited responses to excitatory ones when these are delivered simultaneously. Therefore the structure in complex sounds is very important in the excitation of these neurones.