Sensitivity to AM incoherence is affected by center frequency and modulation rate

Abstract

Fine-grained sensitivity to frequency modulation (FM) at slow rates and low-frequency carriers is thought to be due to auditory-nerve phase locking (time code). Alternatively, a unitary code for FM at all rates and carrier frequencies could be based on cochlear conversion of FM to amplitude modulation (AM) (place code). One weakness of the place-coding theory is it cannot readily explain rate- and carrier-dependent trends in FM sensitivity. This study asked whether FM trends could potentially be explained by sensitivity to two AM envelopes that are out of phase (incoherent AM) at separate cochlear locations, thereby simulating the effects of FM. AM discrimination was assessed for two-component complexes centered at low (500 and 1500 Hz) and high (7000 Hz) frequencies, spaced 2/3 or 4/3 octaves apart, and modulated at slow (2 Hz) and fast (20 Hz) rates. Coherent and incoherent two-component AM detection was assessed for the same conditions. Preliminary results show that sensitivity to AM incoherence is best at low center frequencies and slow rates, consistent with trends traditionally found in FM detection that have been attributed to time coding. Findings suggest time coding may not be necessary to explain trends in FM sensitivity. [Work supported by NIH Grant R01DC005216.]