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Harmonicity is a fundamental element of music, speech, and animal vocalizations. How the auditory system extracts harmonic structures embedded in complex sounds and uses them to form a coherent unitary entity is not fully understood. Despite the prevalence of sounds rich in harmonic structures in our everyday hearing environment, it has remained largely unknown what neural mechanisms are used by the primate auditory cortex to extract these biologically important acoustic structures. In this study, we discovered a unique class of harmonic template neurons in the core region of auditory cortex of a highly vocal New World primate, the common marmoset (Callithrix jacchus), across the entire hearing frequency range. Marmosets have a rich vocal repertoire and a similar hearing range to that of humans. Responses of these neurons show nonlinear facilitation to harmonic complex sounds over inharmonic sounds, selectivity for particular harmonic structures beyond two-tone combinations, and sensitivity to harmonic number and spectral regularity. Our findings suggest that the harmonic template neurons in auditory cortex may play an important role in processing sounds with harmonic structures, such as animal vocalizations, human speech, and music. marmoset | auditory cortex | music | harmonic | hearing

A target sound can become more audible and may ‘pop out’ from a simultaneously presented masker if the masker is presented first by itself, as a precursor. This phenomenon, known as auditory enhancement, may reflect the general perceptual principle of contrast enhancement, which facilitates adaptation to ongoing acoustic conditions and the detection of new events. Little is known about the mechanisms underlying enhancement, and potential confounding factors have made the size of the effect and its time course a point of contention. Here we measured enhancement as a function of precursor duration and delay between precursor offset and target onset, using two single-interval pitch comparison tasks, which involve either same-different or up-down judgments, to avoid the potential confounds of earlier studies. Although these two tasks elicit different levels of performance and may reflect different underlying mechanisms, they produced similar amounts of enhancement. The effect decreased with decreasing precursor duration, but remained present for precursors as short as 62.5 milliseconds, and decreased with increasing gap between the precursor and target, but remained measurable 1 second after the precursor. Additional conditions, examining the effect of precursor/masker similarity and the possible role of grouping and cueing, suggest multiple sources of auditory enhancement.

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