The Neural Dynamics of Amplitude Modulation Processing in the Human Auditory System

Abstract

The neural, auditory amplitude modulation transfer function (MTF) is estimated from 3 - 50 Hz using magnetoencephalography (MEG). All acoustic stimuli are amplitude modulated (AM). Two different dynamical stimulus types are used: exponential sweeps with the AM rate changing from 2 up to 60 Hz, and 89 down to 3 Hz. Several carriers are also employed, including 3 pure-tone carriers (250 Hz, 707 Hz and 2 kHz) and 3 bandlimited pink-noise carriers (1/3, 2 and 5 octaves centered at 707 Hz). Neural response magnitudes, phases, group delays and impulse responses are all estimated. Our results show that the shape of modulation transfer function is flat but with a slightly low pass shape below 10 Hz. The phase of the response is approximately linear in many frequencies. The group delay is around 50 ms at 40 Hz for increasing-frequency sweeps and closer to 100 ms for decreasing-frequency sweeps.