Sound localization cues in the marmoset monkey

The most important acoustic cues available to the brain for sound localization are produced by the interaction of sound with the animal's head and external ears. As a first step in understanding the relation between these cues and their neural representation in a vocal new-world primate, we measured head-related transfer functions (HRTFs) across frequency for a wide range of sound locations in three anesthetized marmoset monkeys. The HRTF magnitude spectrum has a broad resonance peak at 6-12 kHz that coincides with the frequency range of the major call types of this species. A prominent first spectral notch (FN) in the HRTF magnitude above this resonance was observed at most source locations. The center frequency of the FN increased monotonically from ∼12 to 26 kHz with increases in elevation in the lateral field. In the frontal field FN frequency changed in a less orderly fashion with source position. From the HRTFs we derived interaural time (ITDs) and level differences (ILDs). ITDs and ILDs (below 12 kHz) varied as a function of azimuth between ±250 μs and ±20 dB, respectively. A reflexive orienting behavioral paradigm was used to confirm that marmosets can orient to sound sources.