Dirk-Bart den Ouden, Karim Johari, Keiko Bridwell, Caroline Hayden & Roozbeh Behroozmand Department of Communication Sciences and Disorders, University of South Carolina

Modulation of the Speech Motor Control Network through High-Definition Transcranial Direct Current Stimulation

Introduction • Speech motor control: Coordination of respiratory,

laryngeal, articulatory and facial muscles through feedforward motor and sensory feedback

• Speech motor network can be modulated • Increased contribution of feedforward commands

after musical training (e.g. Behroozmand et al., 2014)

• Reduced contribution of feedforward commands in Parkinson’s Disease (Chen et al., 2013)

Prediction: Neurostimulation of vMC modulates 1. Laryngeal motor commands for pitch control 2. Underlying feedforward mechanisms that provide

internal predictions about intended pitch output

• High-Definition transcranial Direct Current Stimulation (HD-tDCS) to modulate response to auditory feedback perturbation

• Electroencephalography (EEG) to investigate modulation of neural responses during vocal pitch motor control, before and after HD-tDCS

Results EEG • Downward Pitch Perturbation: N1: GroupXStim: F(2,27)=4.77, p<.05; frontal electrodes P2: GroupXStim: F(2,27)=6.68, p<.01; temporoparietal GroupXStim: F(2,27)=4.22, p<.05; parietal electrodes

• Upward Pitch Perturbation: N1: GroupXStim: F(2,27)=4.64, p<.05; temporoparietal P2: GroupXStim: F(2,27)=6.27, p<.01; temporoparietal

• Source Localization: P2 to downward pitch shift

Methods Pitch Perturbation cents), randomized. Onset randomized 800-1200ms post-vocalization • Pitch frequency extracted in Praat (Boersma and Weenink, 1996); exported to MATLAB • Segmented into epochs, from −100-500ms after the onset of pitch-shift.

EEG

• filter with cut-off freq. set to 1 and 30 Hz (−24 dB/oct); Automatic artifact rejection method (EEGLAB); Segmentation: −100-500ms rel. to pitch-shift

• Baseline correction, relative to window −100-0 ms. • Investigation of N1 and P2 components: sensitive to unpredictable stimuli • Source localization with standardized low-resolution brain electromagnetic

tomography (sLORETA; Pascual-Marqui, 2002); voxel-wise randomization tests with 5000 permutations, corrected for multiple comparisons (α=.05)

HD-tDCS • M×N HD-tDCS Stimulator (Soterix Medical Inc.); 10-20 cap (Easy-Cap GmbH) • Target coordinates vMC: MNI [-57, 3, 14] (Behroozmand et al., 2015b)

• 20 minutes stimulation, online task performance (starting after 10 minutes) • A- HD-tDCS (anodal), C-HD-tDCS (cathodal) and sham stim between subjects • Active sham condition; sensitivity monitored: no group FX

Acknowledgements This research was supported by a Magellan scholarship award to K.B. and C.H., from the University of South Carolina (Grant # 11560-15-38104).

Methods • 30 subjects (10 M; 20 F; age range: 18-25, mean 21) • HD-tDCS: 3 groups: age and gender matched • Pitch Perturbation Task: pre-post HD-tDCS (+ online) • EEG: pre-post stimulation, during Pitch Perturbation

Session 1 Session 2

Sham HD-tDCS

Anodal HD-tDCS

Cathodal HD-tDCS

EEG EEG 4-14 days

• Voice pitch motor control is supported by ventral (laryngeal) motor cortex - vMC (e.g. Behroozmand et

al., 2015)

Guenther, 2016

• Subjects maintain steady vocalizations of /a/ for 2-3 seconds, with 3-4 second breaks in between

• Brief (200 ms) pitch-shift in auditory feedback; either upward (+100 cents) or downward (-100

Anodal Cathodal Sham

Location Current Location Current Location Current

FC5 +2.0 mA FC5 -2.0 mA POz +1.0 mA

F7 -0.41 mA F7 +0.41 mA Oz -1.0 mA

C1 -0.43 mA C1 +0.43 mA

TP7 -0.55 mA TP7 +0.55 mA

AF7 -0.61 mA AF7 +0.61 mA

• Time-locked to pitch shift shift and vocalization onset • Active electrode system (Brain Products GmbH,

Germany); 10-20 montage; common reference. • EEG signals recorded at 1 kHz; low-pass anti-aliasing

filter with 200 Hz cut-off frequency. Offline band-pass

Conclusions • Both anodal and cathodal HD-tDCS reduce

compensation after pitch perturbation • Increased weight to feedforward commands

• N1 and P2 ERP amplitudes to pitch perturbations are enhanced following anodal and cathodal HD-tDCS • Intact detection (sensory feedback) of pitch shift

• Stimulation of left vMC results in modulation of sensorimotor mechanisms of speech motor control

• IPL provides a neural interface for sensorimotor integration during speech motor control

Results Pitch Perturbation

•Stim Type: F(1,27)=29.02, p<.001, η2p=0.62

•GroupXStim: F(2,27)=4.45, p=.022, η2p=0.48

•Vocal responses to downward pitch-shift stimuli reduced after anodal (p=.003) and cathodal (p=.009) stimulation

HD-tDCS Pitch Shift Brain area BA t-value x y z

Anodal Up Left Inferior Parietal Lobule 40 +4.96 -54 -38 40

Down Left Inferior Parietal Lobule 40 +5.67 -50 -40 50

Cathodal Up Left Inferior Parietal Lobule 40 +4.89 -46 -42 53

Right Inferior Parietal Lobule 40 +5.23 46 -60 48

Down Left Inferior Parietal Lobule 40 +5.19 -47 -41 45

Right Inferior Parietal Lobule 40 +5.41 47 -40 44

Current-density maxima, thresholded at p<0.05 (corr. for mult. comp). Positive t-values: increased neural response for post-stimulation vs. pre-stimulation.

References Behroozmand et al. (2014) Brain & Cognition, 84, 97–108. Behroozmand et al. (2015) Neuroimage 1, 418–28. Chen et al. (2013) Brain Research 1527, 99–107. Guenther, F. H. (2016). Neural Control of Speech. Cambridge, MA: MIT Press.