Rate perception and the auditory 40-Hz steady-state fields evoked by two-tone sequences.

The rate perception of tone sequences reflects the physical repetition rate for identical sound elements. More complex sequences are perceived at the physical rate or at lower rates, depending on perceptual organization. Here, we used magnetoencephalography and psychophysical studies to evaluate the possible relationship between rate perception of such rapid, 40-Hz tone trains and the 40-Hz steady-state response (SSR) in human primary auditory cortex. In Experiment 1, the 40-Hz SSR evoked by monotone sequences of 1000 and 600 Hz were compared to the response evoked by alternating-tone sequences of the same frequencies. The results showed that the 40-Hz SSR for the alternating-tones was attenuated compared to the monotones. In Experiment 2, frequency differences across a range of 25-300 Hz were studied. Compared to a 1000-Hz monotone sequence, the 40-Hz SSR was reduced. Amplitude reduction was most prominent for frequency differences of 200 Hz and more, which were generally perceived with half-the-physical rate. We discuss possible physiological mechanisms of this finding and its relationship to perception.