Poster Slam Session E, Thursday, August 22, 2019, 3:30 - 3:45 pm, Finlandia Hall, Brenda Rapp
Disruption of Speech Adaptation with Repetitive Transcranial Magnetic Stimulation of the Articulatory Representation in Primary Motor Cortex
Ding-Lan Tang1, Alexander McDaniel1, Kate. E Watkins1;1University of Oxford
Sensorimotor learning has gained growing interest in the domain of speech motor control due to its importance in the development and maintenance of fluent speech production. The primary motor cortex (M1), which was considered as executor of motor commands and merely responsible for producing or controlling the movement, has recently been shown to be involved in motor learning. However, whether M1 causally contributes to speech motor learning remains unknown. Here, we aimed to determine whether temporary disruption of the articulatory representation in left M1 by repetitive transcranial magnetic stimulation (rTMS) impairs speech motor learning. Forty right-handed native English speakers between the ages of 18 and 36 years read words containing the /ɛ/ vowel (as in “head” “bed” and “dead”). To induce sensorimotor learning, the first formant (F1) of these productions was shifted up and played back to participants without a noticeable delay using a Matlab Mex-based program Audapter. Typically, participants compensate for the increase in F1 by altering their speech production to reduce F1 and increase the frequency of the second formant (F2), which results in production closer to the vowel /i/ (as in “hid” “bid” and “did”). The changes to F1 and F2 provide a measure of sensorimotor learning. Two groups of 20 participants (10 male, 10 female) received low-frequency (0.6 Hz, subthreshold, 12 min) rTMS to either the hand or the tongue representation of primary motor cortex between the baseline phase with normal feedback and the learning phase when feedback was shifted. RTMS successfully inhibited motor cortex as demonstrated by a significant reduction in the amplitude of motor-evoked potentials (MEPs) elicited by single pulses of TMS over the representation of the target muscle. Participants who received rTMS over the hand representation showed the expected compensatory response for the upwards shift in F1 by significantly reducing F1 and increasing F2 frequencies in their productions. In contrast, such compensatory changes in both F1 and F2 were abolished by rTMS applied over the tongue representation. This was confirmed by statistical tests indicating a significant difference between hand and tongue groups during the learning phase for the changes in F1 (t(38) = 2.65, p =.012) as well as for the changes in the opposite direction for F2 (t(38) = -3.35, p = .002). Critically, rTMS (subthreshold) over the tongue representation did not affect vowel production, which was unchanged from baseline. As predicted, inhibitory TMS over the tongue representation, but not hand representation significantly impaired compensatory changes in both the shifted formant (F1) and the unaltered formant (F2). These results provide direct evidence that the articulatory representation in primary motor cortex causally contributes to sensorimotor learning in speech. Furthermore, these results also suggest that M1 is critical to the network supporting a more global adaptation.
Themes: Speech Motor Control, Multisensory or Sensorimotor Integration
Method: Neurostimulation
Poster E56