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

Abstract

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 thesentences. This allows studying processes specifically related to speech intelligibility since any change betweenthe responses to the distorted stimuli before and after the presentation of their undistorted counterparts cannotbe attributed to acoustic variability but, rather, to the successful mapping of sensory information onto memoryrepresentations. To estimate how the complexity of the message is reflected in speech comprehension, we appliedthis rapid change in perception to behavioral and magnetoencephalography (MEG) experiments usingvowels, words and sentences. In the experiments, stimuli were initially presented to the subject in a distortedform, after which undistorted versions of the stimuli were presented. Finally, the original distorted stimuliwere presented once more. The resulting increase in intelligibility observed for the second presentation of thedistorted stimuli depended on the complexity of the stimulus: vowels remained unintelligible (behaviorallymeasured intelligibility 27%) whereas the intelligibility of the words increased from 19% to 45% and that of thesentences from31% to 65%. This increase in the intelligibility of the degraded stimuliwas reflected as an enhancementof 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 atlatencies of 200–270 ms the mean currents increased. These modulations in cortical activity may reflect feedbackfromtop-down mechanismsenhancing the extraction of information fromspeech. The behavioral results suggestthat memory-driven expectancies can have a significant effect on speech comprehension, especially in acousticallyadverse conditions where the bottom-up information is decreased.