Left motor cortex contributes to auditory phonological discrimination.

Evidence suggests that the articulatory motor system contributes to speech perception in a context-dependent manner. This study tested 2 hypotheses using magnetoencephalography: (i) the motor cortex is involved in phonological processing, and (ii) it aids in compensating for speech-in-noise challenges. A total of 32 young adults performed a phonological discrimination task under 3 noise conditions while their brain activity was recorded using magnetoencephalography. We observed simultaneous activation in the left ventral primary motor cortex and bilateral posterior-superior temporal gyrus when participants correctly identified pairs of syllables. This activation was significantly more pronounced for phonologically different than identical syllable pairs. Notably, phonological differences were resolved more quickly in the left ventral primary motor cortex than in the left posterior-superior temporal gyrus. Conversely, the noise level did not modulate the activity in frontal motor regions and the involvement of the left ventral primary motor cortex in phonological discrimination was comparable across all noise conditions. Our results show that the ventral primary motor cortex is crucial for phonological processing but not for compensation in challenging listening conditions. Simultaneous activation of left ventral primary motor cortex and bilateral posterior-superior temporal gyrus supports an interactive model of speech perception, where auditory and motor regions shape perception. The ventral primary motor cortex may be involved in a predictive coding mechanism that influences auditory-phonetic processing.