Although it is well accepted that the speech motor system (SMS) is activated during speech perception, the functional role of this activation remains unclear. Here we test the hypothesis that the redundant motor activation contributes to categorical speech perception under adverse listening conditions. In this functional magnetic resonance imaging study, participants identified one of four phoneme tokens (/ba/, /ma/, /da/, or /ta/) under one of six signal-to-noise ratio (SNR) levels (-12, -9, -6, -2, 8 dB, and no noise). Univariate and multivariate pattern analyses were used to determine the role of the SMS during perception of noise-impoverished phonemes. Results revealed a negative correlation between neural activity and perceptual accuracy in the left ventral premotor cortex and Broca's area. More importantly, multivoxel patterns of activity in the left ventral premotor cortex and Broca's area exhibited effective phoneme categorization when SNR ≥ -6 dB. This is in sharp contrast with phoneme discriminability in bilateral auditory cortices and sensorimotor interface areas (e.g., left posterior superior temporal gyrus), which was reliable only when the noise was extremely weak (SNR > 8 dB). Our findings provide strong neuroimaging evidence for a greater robustness of the SMS than auditory regions for categorical speech perception in noise. Under adverse listening conditions, better discriminative activity in the SMS may compensate for loss of specificity in the auditory system via sensorimotor integration.
Keywords: fMRI; forward sensorimotor mapping; multivariate pattern analysis; speech categorization.