Neural selectivity and tuning for sinusoidal frequency modulations in the inferior colliculus of the big brown bat, Eptesicus fuscus

Most communication sounds and most echolocation sounds, including those used by the big brown bat (Eptesicus fuscus), contain frequency-modulated (FM) components, including cyclical FM. Because previous studies have shown that same neurons in the inferior colliculus (IC) of this bat respond to linear FM sweeps but not to pure tones or noise, we asked whether these or other neurons are specialized for conveying information about cyclical FM signals. In unanesthetized bats, we tested the response of 116 neurons in the IC to pure tones, noise with various bandwidths, single linear FM sweeps, sinusoidally amplitude-modulated signals, and sinusoidally frequency-modulated (SFM) signals. With the use of these stimuli, 20 neurons (17%) responded only to SFM, and 10 (9%) responded best to SFM but also responded to one other test stimulus. We refer to the total 26% of neurons that responded best to SFM as SFM-selective neurons. Fifty-nine neurons (51%) responded about equally well to SFM and other stimuli, and 27 (23%) did not respond to SFM but did respond to other stimuli. Most SFM-selective neurons responded to a limited range of modulation rates and a limited range of modulation depths. The range of modulation rates over which individual neurons responded was 5-170 Hz (n = 20). Thus SFM-selective neurons respond to low modulation rates. The depths of modulations to which the neurons responded ranged from +/-0.4 to +/-19 kHz (n = 15). Half of the SFM-selective neurons did not respond to the first cycle of SFM. This finding suggests that the mechanism for selective response to SFM involves neural delays and coincidence detectors in which the response to one part of the SFM cycle coincides in time either with the response to a later part of the SFM cycle or with the response to the first part of the next cycle. The SFM-selective neurons in the IC responded to a lower and more limited range of SFM rates than do neurons in the nuclei of the lateral lemniscus of this bat. Because the FM components of biological sounds usually have low rates of modulation, we suggest that the tuning of these neurons is related to biologically important sound parameters. The tuning could be used to detect FM in echolocation signals, modulations in high-frequency sounds that are generated by wing beats of some beetles, or social communication sounds of Eptesicus.