Dichotic training modulates phonetic feature processing in late English learners: Evidence from categorical perception and the mismatch negativity

In English, voiceless stop consonants (i.e., /p-t-k/) have long voice onset times (VOT), and voiced stop consonants (i.e., /b-d-g/) have short VOTs. In French, voiceless stop consonants have short VOTs while voiced stop consonants have pre-lag VOTs, which can make late second language (L2) learning of English difficult. The Asymmetric Sampling in Time (AST) model proposes that slow and fast cerebral rhythms dominate in the auditory cortex of the right hemisphere and left hemisphere (LH), respectively, and preferentially support processing of long and short speech events. Our study extends the application of this model to sub-phonemic levels, where short VOTs were presented to the right ear (stimulating the LH), and long VOTs were presented to the left ear (dichotic stimulation). Pre-/post-tests assessed categorical perception performance in adults trained dichotically, binaurally, or without training. Consonant perception improved in dichotic and binaural groups, with generalization of training only in the dichotic group (experiment 1). Following dichotic training, the Mismatch Negativity amplitude increased for voiced English deviants, and hemispheric dominance shifted appropriately for voiceless deviants (experiment 2). These findings demonstrate that perceptual training grounded in an AST framework can induce phonological perception changes in late English learners, suggesting promising directions for L2 perceptual training.