Previous exposure to intact speech increases intelligibility of its digitally degraded counterpart as a function of stimulus complexity

Recent studies have shown that acoustically distorted sentences can be perceived as either unintelligible or intelligible depending on whether one has previously been exposed to the undistorted, intelligible versions of the sentences. This allows studying processes specifically related to speech intelligibility since any change between the responses to the distorted stimuli before and after the presentation of their undistorted counterparts cannot be attributed to acoustic variability but, rather, to the successful mapping of sensory information onto memory representations. To estimate how the complexity of the message is reflected in speech comprehension, we applied this rapid change in perception to behavioral and magnetoencephalography (MEG) experiments using vowels, words and sentences. In the experiments, stimuli were initially presented to the subject in a distorted form, after which undistorted versions of the stimuli were presented. Finally, the original distorted stimuli were presented once more. The resulting increase in intelligibility observed for the second presentation of the distorted stimuli depended on the complexity of the stimulus: vowels remained unintelligible (behaviorally measured intelligibility 27%) whereas the intelligibility of the words increased from 19% to 45% and that of the sentences from 31% to 65%. This increase in the intelligibility of the degraded stimuli was reflected as an enhancement of activity in the auditory cortex and surrounding areas at early latencies of 130-160 ms. In the same regions, increasing stimulus complexity attenuated mean currents at latencies of 130-160 ms whereas at latencies of 200-270 ms the mean currents increased. These modulations in cortical activity may reflect feedback from top-down mechanisms enhancing the extraction of information from speech. The behavioral results suggest that memory-driven expectancies can have a significant effect on speech comprehension, especially in acoustically adverse conditions where the bottom-up information is decreased.