Neural systems underlying perceptual adjustment to non-standard speech tokens

Evidence suggests that listeners use top-down information to guide perception of speech sounds. A phenomenon termed 'perceptual learning for speech' shows that listeners also use top-down information to adjust perceptual boundaries in subsequent processing of speech from the same talker. The neural mechanisms that underlie this process are not well understood. Of interest is whether boundary shifts arise because of a retuning of phonetic sensitivities early in the neural processing stream or whether they result from decision-related or attentional mechanisms further downstream. In the current study, activation was measured using fMRI as participants underwent a behavioral replication of this paradigm. Sensitivity to boundary shifts emerged in right frontal and middle temporal regions, implicating adjustments of perceptual criteria downstream from primary auditory cortex. Later in the session, this same sensitivity emerged in left superior temporal areas, implicating a slowly-adapting system in regions of the brain related to phonetic processing.