Tracking cortical entrainment in neural activity: auditory processes in human temporal cortex

Abstract

A primary objective for cognitive neuroscience is to identify how features of the sensory environment are encoded in neural activity. Current auditory models of loudness perception can be used to make detailed predictions about the neural activity of the cortex as an individual listens to speech. We used two such models (loudness-sones and loudness-phons), varying in their psychophysiological realism, to predict the instantaneous loudness contours produced by 480 isolated words. These two sets of 480 contours were used to search for electrophysiological evidence of loudness processing in whole-brain recordings of electro- and magneto-encephalographic (EMEG) activity, recorded while subjects listened to the words. The technique identified a bilateral sequence of loudness processes, predicted by the more realistic loudness-sones model, that begin in auditory cortex at ~80 ms and subsequently reappear, tracking progressively down the superior temporal sulcus (STS) at lags from 230 to 330 ms. The technique was then extended to search for regions sensitive to the fundamental frequency (F0) of the voiced parts of the speech. It identified a bilateral F0 process in auditory cortex at a lag of ~90 ms, which was not followed by activity in STS. The results suggest that loudness information is being used to guide the analysis of the speech stream as it proceeds beyond auditory cortex down STS toward the temporal pole.

Relating dynamic brain states to dynamic machine states: human and machine solutions to the speech recognition problem

“…Evidence for the timing of such a lag has become available from recent research relating speech input to higher-order cortical interpretation. In a study using EMEG word data similar to the speech materials used here, Thwaites et al [30] found that the cortical computation of the fundamental frequency function F 0 was most strongly expressed in EMEG brain responses in bilateral STC at a lag of 90 ms. Related evidence from ECoG studies looking at phonetic discriminability in neural responses to speech suggest similar delays. Mesgarani et al ([19], Fig S4A) show a lag of around 125 ms for neural discrimination of phonetic categories relative to the acoustic information in the speech signal, while Chang et al ([27], Fig 2a) find a similar peak at around 110 ms.…”

Cortical Response to the Natural Speech Envelope Correlates with Neuroimaging Evidence of Cognition in Severe Brain Injury

“…Such tasks are cognitively demanding [1]; accordingly, recent studies support that fMRI command following in braininjured patients associates with preserved cerebral metabolism and preserved sleep-wake EEG [5,6]. We investigated the use of an EEG response that tracks the natural speech envelope (NSE) of spoken language [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] in healthy controls and brain-injured patients (vegetative state to emergence from minimally conscious state). As audition is typically preserved after brain injury, auditory paradigms may be preferred in searching for covert cognitive function [23][24][25].…”

Tonotopic representation of loudness in the human cortex

“…This procedure is repeated at 5-ms intervals (Fig. 2B) across a range of time lags (−200 <  l  < 800 ms), covering the range of plausible latencies (0–800 ms) and a short, pre-stimulation range (−200 to 0 ms) during which we would expect to see no significant match (The 0–800 ms range was chosen because the study of Thwaites et al. (2015) showed little significant expression for instantaneous loudness after a latency of 500 ms; the 5-ms interval step was chosen because it is the smallest value that can be used given current computing constraints).…”

“…We used an exact formula for the familywise false-alarm rate to generate a ‘corrected’ α, α* of approximately 3 × 10 −13 (see Thwaites et al., 2015, for the full reasoning); p-values greater than this are deemed to be not significant.…”