Perception of frequency modulation is mediated by cochlear place coding

Abstract

Natural sounds convey information via frequency and amplitude modulations (FM and AM). Humans are acutely sensitive to the slow rates of FM that are crucial for speech and music. This sensitivity has been thought to rely on precise stimulus-driven auditory-nerve spike timing (time code), whereas a coarser code, based on variations in the cochlear place of stimulation (place code), represents faster FM. Here we test this longstanding theory in listeners with normal and impaired hearing, resulting in widely varying place-coding fidelity. Contrary to predictions, FM detection thresholds at slow and fast rates are highly correlated and closely related to the fidelity of cochlear place coding. We support this conclusion with additional data showing that place-based coding degrades at high modulation rates and in high spectral regions in ways that were previously interpreted as reflecting the limits of fine neural timing. The results suggest a unitary place-based neural code for FM.