Neural Representations of Complex Temporal Modulations in the Human Auditory Cortex

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American Physiological Society

RESUMO

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, fFM ≈ 40 Hz, and amplitude modulation (AM) at a slow rate, fAM <15 Hz. Magnetoencephalography recordings show fast FM and slow AM stimulus features evoke two separate but not independent auditory steady-state responses (aSSR) at fFM and fAM, respectively. The power, rather than phase locking, of the aSSR of both decreases with increasing stimulus fAM. The aSSR at fFM is itself simultaneously amplitude modulated and phase modulated with fundamental frequency fAM, showing that the slow stimulus AM is not only encoded in the neural response at fAM but also encoded in the instantaneous amplitude and phase of the neural response at fFM. Both the amplitude modulation and phase modulation of the aSSR at fFM are most salient for low stimulus fAM but remain observable at the highest tested fAM (13.8 Hz). The instantaneous amplitude of the aSSR at fFM is successfully predicted by a model containing temporal integration on two time scales, ∼25 and ∼200 ms, followed by a static compression nonlinearity.

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