Simmonds, A. 1 , Wise, R. 1, 2 , Dhanjal, N. 2 & Leech, R. 2
1 Medical Research Council Clinical Sciences Centre, Imperial College London
2 Department of Experimental Medicine, Imperial College London
Speaking is a highly automatic skill. In our native language we do not have to concentrate on how we move our articulators. In contrast, adults who acquire a second language never rid themselves of an accent that clearly marks them as a non-native speaker. However, the motor-sensory neural consequences of speaking in a second language have only very rarely been studied; previous studies of bilingualism have concentrated on changes in linguistic or cognitive neural processes as subjects switch between languages. Unlike previous studies that have treated bilingualism as a linguistic problem, we have approached speaking a second language from a sensorimotor perspective. The persistent accent in late learners of L2 is likely to be the result of a failure to achieve the same proficiency in integrating the motor feedforward and sensory feedback control of articulation that we achieve when speaking in our native tongue.
Here, we present a functional MRI study contrasting native and non-native propositional speech production that for the first time has revealed the changes in motor-sensory control when switching from native speech production to accented speech in a second language. The specific hypothesis of the study related to the temporo-parietal cortex, where we predicted that production of L2 would place increased demands on sensory control. This was apparent within the somatosensory cortex in the parietal operculum, the increased activity expressing a mismatch between the efference copy of motor commands to the articulators, the intended auditory target (L2 without an accent), and actual somatosensory and auditory feedback. Sensory gating prevented signal change for the familiar L1. We have demonstrated a central role for sensory feedback within parietal opercular cortex when speaking a second language with an accent, establishing that bilingualism depends as much on integration of sensory with motor discharges as on linguistic and cognitive processing.