Human auditory cortical dynamics during perception of long acoustic sequences: phase tracking of carrier frequency by the auditory steady-state response.

We recorded human auditory cortical activity during the perception of long, changing acoustic signals and analyzed information provided by dynamic neural population measures over a large range of time intervals (approximately 24 ms-5 s). Participants listened to musical scales that were amplitude modulated at a rate of 41.5 Hz, generating an ongoing, stimulus-related oscillatory brain signal, the auditory steady-state response (aSSR). The aSSR generated energy at the amplitude modulation rate that was recorded using magnetoencephalography. As in previous work, the timing (phase) of this response varied with stimulus carrier frequency over the entire course of minute-long tone sequences ('phase tracking' of carrier frequency). The length of the time interval over which phase was calculated was systematically varied; significant phase tracking was regularly observed at analysis intervals of <50 ms in length. The right auditory cortex exhibited better phase tracking performance than the left at analysis intervals of 24-240 ms, and frequency dependent phase delays were consistently larger than those predicted by cochlear mechanics. Based on these empirical data, a model of the neural populations responsible for phase tracking suggests that it is produced by a subpopulation ( approximately 25%) of the cells generating the aSSR.