Speech comprehension is correlated with temporal response patterns recorded from auditory cortex

Speech comprehension depends on the integrity of both the spectral content and temporal envelope of the speech signal. Although neural processing unde rlying spectral analysis has been intensively studied, less is known about the processing of temporal information. Most of speech information conveyed by the temporal envelope is confined to frequencies below 16 Hz, frequenci es that roughly match spontaneous and evoked modulation rates of primary au ditory cortex neurons. To test the importance of cortical modulation rates for speech processing, we manipulated the frequency of the temporal envelop e of speech sentences and tested the effect on both speech comprehension an d cortical activity. Magnetoencephalographic signals from the auditory cort ices of human subjects were recorded while they were performing a speech co mprehension task. The test sentences used in this task were compressed in t ime. Speech comprehension was degraded when sentence stimuli were presented in more rapid (more compressed) forms. We found that the average comprehen sion level, at each compression, correlated with (i) the similarity between the frequencies of the temporal envelopes of the stimulus and the subject' s cortical activity ("stimulus-cortex frequency-matching") and (it) the pha se-locking (PL) between the two temporal envelopes ("stimulus-cortex PL"). Of these two correlates, PL was significantly more indicative for single-tr ial success. Our results suggest that the match between the speech rate and the a priori. modulation capacities of the auditory cortex is a prerequisi te for comprehension. However, this is not sufficient: stimulus-cortex PL s hould be achieved during actual sentence presentation.