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Poster C3, Friday, August 17, 10:30 am – 12:15 pm, Room 2000AB

The Role of Primary Motor Cortex in Second Language Word Recognition

Beatriz Barragan1, Kazumasa Uehara1, Marissa Miller1, Yuto Tauchi2, Marco Santello1, Julie Liss1;1Arizona State University, 2Okayama University

The involvement of primary motor cortex (M1) activity is a common feature in speech perception tasks that involve difficult listening conditions (Adank, 2012; Devlin & Aydelott, 2009; Nuttall et al., 2016). Moreover, this M1 activity has shown to be linked to improved performance on those tasks. It has been therefore hypothesized that recruitment of M1 facilitates perception. Learning a second language (L2) can be thought of as a specific instantiation of listening in difficult conditions because it involves additional processing and cognitive-perceptual resources to distinguish among the new language sounds. If this is the case, M1 activity during L2 word recognition is expected. The purpose of this study was to investigate the role of M1 representation of the lip orbicularis oris (OO) muscle in processing acoustic inputs in the native language (L1) and L2. A repetitive Transcranial Magnetic Stimulation (rTMS) protocol was employed with a double-blinded, sham-controlled and crossover design, to selectively alter neural activity in M1 in twenty-four healthy English/Spanish bilingual participants. The performance on a bilingual listening word-to-picture matching task was measured before and after the rTMS and sham conditions. rTMS was applied for 15 minutes at 0.6Hz frequency, and 100% of active motor threshold intensity, whereas sham was applied without magnetic pulses as a control condition. Single-pulse TMS-induced motor evoked potential (MEP) from the OO muscle was used to assess the effect of rTMS on M1. This manipulation revealed high variability in the aftereffect of the rTMS protocol among participants; although inhibitory changes in M1 were expected (Cirillo et al., 2017, Möttönen et al., 2014), approximately 50% of participants exhibited inhibitory changes, whereas the other 50% had facilitatory changes. This observation suggests that rTMS led to complex influences on M1 excitability and relying on grand-average results can obscure important individual differences in rTMS physiological and functional outcomes. We found evidence of motor support to word recognition in L2. Participants who showed inhibitory changes in M1 by rTMS were significantly slower and less accurate in L2 word recognition after rTMS compared to the sham condition. This result suggests that the rTMS-induced disruption of M1 associated with speech articulators interfered with L2 speech recognition. On the other hand, the participants who exhibited facilitatory changes on M1 by rTMS were more accurate compared to the sham condition. Reaction time in pre- and post-rTMS performance was comparable, suggesting that a similar speed performance was associated with more accurate speech recognition for L2 after rTMS-induced facilitation. In both groups of participants (inhibitory and facilitatory changes on M1 by rTMS) no effect of rTMS was found on L1, where accuracy and speed were quite similar after sham- and real-rTMS. Together, our results provide substantial support for the role of M1 on L2 word recognition, and revealed a functional relation between M1 modulation and word recognition. These results suggest that M1 excitability contributes to L2 speech perception, and that sensorimotor integration is important for L2 neural information processing within the brain.

Topic Area: Speech Motor Control and Sensorimotor Integration