Abstract
BackgroundThe robustness of speech perception in the face of acoustic variation is founded on the ability of the auditory system to integrate the acoustic features of speech and to segregate them from background noise. This auditory scene analysis process is facilitated by top-down mechanisms, such as recognition memory for speech content. However, the cortical processes underlying these facilitatory mechanisms remain unclear. The present magnetoencephalography (MEG) study examined how the activity of auditory cortical areas is modulated by acoustic degradation and intelligibility of connected speech. The experimental design allowed for the comparison of cortical activity patterns elicited by acoustically identical stimuli which were perceived as either intelligible or unintelligible.ResultsIn the experiment, a set of sentences was presented to the subject in distorted, undistorted, and again in distorted form. The intervening exposure to undistorted versions of sentences rendered the initially unintelligible, distorted sentences intelligible, as evidenced by an increase from 30% to 80% in the proportion of sentences reported as intelligible. These perceptual changes were reflected in the activity of the auditory cortex, with the auditory N1m response (~100 ms) being more prominent for the distorted stimuli than for the intact ones. In the time range of auditory P2m response (>200 ms), auditory cortex as well as regions anterior and posterior to this area generated a stronger response to sentences which were intelligible than unintelligible. During the sustained field (>300 ms), stronger activity was elicited by degraded stimuli in auditory cortex and by intelligible sentences in areas posterior to auditory cortex.ConclusionsThe current findings suggest that the auditory system comprises bottom-up and top-down processes which are reflected in transient and sustained brain activity. It appears that analysis of acoustic features occurs during the first 100 ms, and sensitivity to speech intelligibility emerges in auditory cortex and surrounding areas from 200 ms onwards. The two processes are intertwined, with the activity of auditory cortical areas being modulated by top-down processes related to memory traces of speech and supporting speech intelligibility.
Highlights
The robustness of speech perception in the face of acoustic variation is founded on the ability of the auditory system to integrate the acoustic features of speech and to segregate them from background noise
In the anterior inferior temporal (AIT), only the undistorted sentences (7 pA/m) resulted in significantly stronger activation than the first presentation of degraded stimuli (5 pA/m; p
The amplitude and latency effects of the P2m were substantially more pronounced in the right hemisphere than in the left. These findings indicate that transient activity of the auditory cortex is sensitive to the acoustic properties of sound during the early processing stages of connected speech, and that the right hemisphere is more sensitive to acoustic variability than the left
Summary
The robustness of speech perception in the face of acoustic variation is founded on the ability of the auditory system to integrate the acoustic features of speech and to segregate them from background noise. In electro- and magnetoencephalography (EEG & MEG, respectively) recordings of the human brain, the presentation of short-duration (300 ms), the transient responses are followed by a sustained response These responses are suited for revealing both the spatial and temporal evolution of cortical activity. The N1m has been found to be a sensitive measure of the extraction process with which the human brain segregates speech signals from various types of noise contributions [17,18,19] While these EEG/MEG studies, utilizing short (~200 ms) isolated vowel sounds in no-task (passive) recording conditions, have revealed the link between transient activation and the bottom-up extraction mode in acoustic feature processing, the role of top-down influences on the processing of meaningful speech has remained largely unaddressed. The above studies should be complemented by investigations focusing on the sustained activity elicited by connected speech, potentially revealing how activity spreads to multiple cortical brain areas performing speech processing
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