Temporal Envelope of Time-Compressed Speech Represented in the Human Auditory Cortex

Abstract

Speech comprehension relies on temporal cues contained in the speech envelope, and the auditory cortex has been implicated as playing a critical role in encoding this temporal information. We investigated auditory cortical responses to speech stimuli in subjects undergoing invasive electrophysiological monitoring for pharmacologically refractory epilepsy. Recordings were made from multicontact electrodes implanted in Heschl's gyrus (HG). Speech sentences, time compressed from 0.75 to 0.20 of natural speaking rate, elicited average evoked potentials (AEPs) and increases in event-related band power (ERBP) of cortical high-frequency (70 -250 Hz) activity. Cortex of posteromedial HG, the presumed core of human auditory cortex, represented the envelope of speech stimuli in the AEP and ERBP. Envelope following in ERBP, but not in AEP, was evident in both language-dominant and -nondominant hemispheres for relatively high degrees of compression where speech was not comprehensible. Compared to posteromedial HG, responses from anterolateral HG-an auditory belt field-exhibited longer latencies, lower amplitudes, and little or no time locking to the speech envelope. The ability of the core auditory cortex to follow the temporal speech envelope over a wide range of speaking rates leads us to conclude that such capacity in itself is not a limiting factor for speech comprehension.

Direct Recordings from the Auditory Cortex in a Cochlear Implant User

“…The main finding of the present study is the similarity of response properties that characterized noncore auditory cortex on the lateral STG with those previously described in normal-hearing individuals Nourski et al 2008Nourski et al , 2013a. Thus, while similarities in cortical responses between CI and normal-hearing subjects have been previously reported in extracranial studies, we were able to localize this to a specific auditory cortical area.…”

“…Representation of the temporal stimulus envelope in the cortical activity was quantified in the time domain using cross-correlation analysis (Ahissar et al 2001;Abrams et al 2008;Nourski et al 2009). Envelopes of the speech stimuli were obtained by calculating the magnitude of the Hilbert transform of the speech signal waveform and low-pass filtering at 50 Hz using a fourth-order Butterworth filter.…”

“…Phase locking of the cortical response to the stimulus envelope at different compression ratios was quantified using cross-correlation analysis (Ahissar et al 2001;Abrams et al 2008;Nourski et al 2009). Envelope following by the AEP and ERBP was measured as peaks of cross-correlograms between speech envelopes and AEPs and high-frequency ERBP envelope (70-150 Hz; see "Methods"), respectively.…”

Sound representation in higher language areas during language generation

“…This is true whether the encoded linguistic items will then be uttered or not (as when we think). Our findings concerning the correlation of cortical activity in high-level language areas to the sound envelope of the encoded linguistic items are symmetrical to those emerging by studying cortical activity during speech perception (3,5,6). Even if we assume that subvocalization with subthreshold activation of the phonatory apparatus is always active when we think (20), the correlation between ECoG and the envelope of the sound of the linguistic items involved is not limited to the late phonological and phonetic processing and the encoding of the articulatory motor commands that immediately precede sound production.…”

“…The correlation between the power envelope of the speech and the neural activity is maximal at low frequencies (2-7 Hz, theta range) corresponding to syllable rates, and becomes less precise at higher frequencies Hz, gamma range) corresponding to phoneme rates (1). Entrainment of neural activity to the speech envelope in auditory regions has allowed recognition of the phonetic features heard during speech perception (2)(3)(4)(5), and even the reconstruction of simple words (6). This evidence indicates that during listening, speech representation in the auditory cortex and adjacent areas of the superior temporal lobe reflects acoustic features directly related to linguistically defined phonological entities such as phonemes and syllables.…”