Human Cortical Responses to the Speech Envelope

Abstract

Objective: 

To evaluate the response of the human auditory cortex to the temporal amplitude-envelope of speech. Responses to the speech envelope could be useful for validating the neural encoding of intelligible speech, particularly during hearing aid fittings—because hearing aid gain and compression characteristics for ongoing speech should more closely resemble real world performance than for isolated brief syllables.

Design: 

The speech envelope comprises energy changes corresponding to phonemic and syllabic transitions. Envelope frequencies between 2 and 20 Hz are important for speech intelligibility. Human event-related potentials were recorded to six different sentences and the sources of these potentials in the auditory cortex were determined. To improve the signal to noise ratio over ongoing electroencephalographic recordings, we averaged the responses over multiple presentations, and derived source waveforms from multichannel scalp recordings. Source analysis led to bilateral, symmetrical, vertical, and horizontal dipoles in the posterior auditory cortices. The source waveforms were then cross-correlated with the low frequency log-envelopes of the sentences. The significance and latency of the maximum correlation for each sentence demonstrated the presence and latency of the brain’s response. The source waveforms were also cross-correlated with a simple model based on a series of overlapping transient responses to stimulus change (the derivative of the log-envelope).

Results: 

Correlations between the log-envelope and vertical dipole source waveforms were significant for all sentences and for all but one of the participants (mean r = 0.35), at an average delay of 175 (left) to 180 (right) msec. Correlations between the transient response model (P1 at 68 msec, N1 at 124 msec, and P2 at 208 msec) and the vertical dipole source waveforms were detected for all sentences and all participants (mean r = 0.30), at an average delay of 6 (right) to 10 (left) msec.

Conclusions: 

These results show that the human auditory cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The delay between the envelope and the response is approximately 180 msec.

In Brief

The objective of this study was to evaluate the response of the auditory cortex to the temporal amplitude envelope of speech. Event-related potentials were recorded to sentences in a group of young, normal-hearing listeners. Source waveforms in the posterior auditory cortices were cross-correlated with the low frequency log-envelope of each sentence. Significant correlations were detected for all sentences and for all but one of the participants, with the cortical response occurring approximately 180 ms after the stimulus. The results show that the human auditory cortex either directly follows the speech envelope or consistently reacts to changes in this envelope.