Binaural responses in the auditory midbrain of chicken (Gallus gallus).

The auditory midbrain is the location in which neurons represent binaural acoustic information necessary for sound localization. The external nucleus of the midbrain inferior colliculus (IC) of the barn owl is a classic example of an auditory space map, but it is unknown to what extent the principles underlying its formation generalize to other, less specialized animals. We characterized the spiking responses of 139 auditory neurons in the IC of the chicken (Gallus gallus) in vivo, focusing on their sensitivities to the binaural localization cues of interaural time (ITD) and level (ILD) differences. Most units were frequency-selective, with best frequencies distributed unevenly into low-frequency and high-frequency (> 2 kHz) clusters. Many units showed sensitivity to either ITD (65%) or ILD (66%) and nearly half to both (47%). ITD selectivity was disproportionately more common among low-frequency units, while ILD-only selective units were predominantly tuned to high frequencies. ILD sensitivities were diverse, and we thus developed a decision tree defining five types. One rare type with a bell-like ILD tuning was also selective for ITD but typically not frequency-selective, and thus matched the characteristics of neurons in the auditory space map of the barn owl. Our results suggest that generalist birds such as the chicken show a prominent representation of ITD and ILD cues in the IC, providing complementary information for sound localization, according to the duplex theory. A broadband response type narrowly selective for both ITD and ILD may form the basis for a representation of auditory space.