Cortical areas tracking the speech amplitude envelope revealed by electrocorticographic recordings
Research on auditory features that contribute to speech comprehension has pointed out the importance of temporal cues and the slow amplitude envelope. Specifically, manipulating the speech amplitude envelope degrades recognition of speech sounds and comprehensibility of sentences and, further, it has been shown that speech comprehension is relatively preserved even when the frequency content is disrupted if the speech amplitude envelope is preserved. While previous studies have shown that electrical potentials generated by neural activity during listening to speech correlate with the speech envelope, and that the degree of this correlation predicts speech comprehensibility, it has not been determined precisely which cortical regions track the speech envelope.
In their recent study, Kubanek et al. (2013) recorded electrical activity intracranially with electrode grids placed on left-hemisphere cortical surface in five epileptic patients undergoing pre-surgical mapping of seizure foci. During recording, the subjects listened to four short stories narrated by a male voice. The stimulus amplitude envelope was computed as the sound power in consecutive 50-ms time windows, as was power of high-frequency gamma activity (75-115 Hz) for each of the recording electrodes. The results show that the speech amplitude envelope is most faithfully tracked by human non-primary auditory cortex that surrounds the primary auditory cortex located within the confines of the Heschl’s gyrus, and that the gamma-band signal in these recordings correlates best with the speech envelope. Using non-speech control stimuli, the authors further demonstrated that the non-primary auditory cortical areas, while also tracking amplitude envelope of melody, do track more specifically the speech amplitude envelope. Higher-order structures (superior temporal gyrus and inferior frontal cortex), in contrast, tracked the speech amplitude envelope more weakly but at the same time even more specifically than the non-primary auditory cortical areas.
These findings provide important further knowledge on the cortical mechanisms underlying processing of speech amplitude envelope. The results reveal cortical areas that track the speech amplitude envelope, and further suggest that there is a processing hierarchy with the non-primary auditory cortical areas tracking the envelope of speech more robustly but less speech-specifically than superior temporal gyrus and posterior inferior frontal gyrus (i.e., Broca's area) that are considered higher-order language areas.
Reference: Kubanek J, Brunner P, Gunduz A, Poeppel D, Schalk G. The tracking of speech envelope in the human cortex. PLoS ONE (2013) 8: e53398. http://dx.doi.org/10.1371/journal.pone.0053398