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RESEARCH-ARTICLE

Neural Representations of Complex Temporal Modulations in the Human Auditory Cortex

¹Departments of Electrical and Computer Engineering and ²Biology, University of Maryland, College Park, Maryland

Submitted 16 June 2009; accepted in final form 18 August 2009

ABSTRACT

Natural sounds such as speech contain multiple levels and multiple types of temporal modulations. Because of nonlinearities of the auditory system, however, the neural response to multiple, simultaneous temporal modulations cannot be predicted from the neural responses to single modulations. Here we show the cortical neural representation of an auditory stimulus simultaneously frequency modulated (FM) at a high rate, f_FM 40 Hz, and amplitude modulation (AM) at a slow rate, f_AM <15 Hz. Magnetoencephalography recordings show fast FM and slow AM stimulus features evoke two separate but not independent auditory steady-state responses (aSSR) at f_FM and f_AM, respectively. The power, rather than phase locking, of the aSSR of both decreases with increasing stimulus f_AM. The aSSR at f_FM is itself simultaneously amplitude modulated and phase modulated with fundamental frequency f_AM, showing that the slow stimulus AM is not only encoded in the neural response at f_AM but also encoded in the instantaneous amplitude and phase of the neural response at f_FM. Both the amplitude modulation and phase modulation of the aSSR at f_FM are most salient for low stimulus f_AM but remain observable at the highest tested f_AM (13.8 Hz). The instantaneous amplitude of the aSSR at f_FM is successfully predicted by a model containing temporal integration on two time scales, 25 and 200 ms, followed by a static compression nonlinearity.