Directional sensitivity of auditory neurons in the superior colliculus of the bat,Eptesicus fuscus,using free field sound stimulation

Summary Electrophysiological recordings were made from 130 single neurons of the superior colliculus (SC) of big brown bats,Eptesicus fuscus, in order to test their general as well as directional auditory response properties. Bursts of constant frequency and frequency modulated signals were broadcasted through a condenser loudspeaker, which could be placed at any azimuth and elevation on a hemisphere in front of the bat's head. The SC units responded equally well to both types of signals. The best frequencies of SC neurons ranged from 25 up to 82 kHz, and their tuning curves appeared to be broad (compared to those of the main auditory nuclei of the same and other bats) with Q_10dB values of 1.8–17.5. All units encountered revealed directional sensitivity and were classified in two groups: the majority of them had a constant best angle of response (BA) whatever the sound intensity was (unidirectional type); the others showed a shift in their BA towards the center of the stimulating hemisphere as the intensity decreased (pluridirectional type). In both response types, the BA results are corroborated by the properties of the receptive fields. The presence of an auditory space map in the horizontal plane is not obvious for the superior colliculus of this bat.Abbreviations BA best angle - BF best frequency - CF constant frequency - FM frequency modulated - FTC frequency threshold curve - IC inferior colliculus - MLD nucleus mesencephalicus dorsalis - MT minimum threshold - RF receptive field - RP reference point - SC superior colliculus - SPL sound pressure level re. 20 Pa·cm^–2