Slide Slam F14
The influence of 2kHz Transcranial Alternating Current Stimulation (tACS) over the articulatory motor cortex on cortical excitability and categorical perception of speech sounds
Jessica Jacobs1, Alexis Hervais-Adelman2, René Müri3, Dario Cazzoli4,5,6, Basil Preisig2; 1Department of Psychology, University of Fribourg, Fribourg, Switzerland, 2Neurolinguistics, Department of Psychology, University of Zürich, Zürich, Switzerland, 3Department of Neurology, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland, 4Gerontechnology and Rehabilitation Group, ARTORG Center for Biomedical Engineering Research, University of Bern, Bern, Switzerland, 5Perception and Eye Movement Laboratory, Department of Neurology, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland, 6Neurocenter, Luzerner Kantonsspital, Lucerne, Switzerland
Although the ventral motor cortex (vMC), the area in which the neural representations of the articulators are, is activated during speech perception, the functional role of this activation remains unclear. Previous studies reported that the inhibition of this region with repetitive transcranial magnetic stimulation (rTMS) impairs categorical speech sound perception. Moreover, listening to speech sounds has been shown to facilitate cortical excitability in this region. Based on this evidence, we hypothesized that enhancing cortical excitability in this area would increase categorical speech sound perception. For this purpose, high frequency (2kHz) transcranial alternating current stimulation (tACS) was applied over the left ventral motor cortex to enhance cortical excitability. The cortical excitability of the left vMC was assessed before and after tACS, by measuring the amplitude of motor evoked potentials (MEPs) elicited by monophasic single pulse TMS. The MEPs were recorded on the muscle orbicularis oris (OO) of the lips. Categorical perception of speech sound was assessed by a syllable identification and a syllable discrimination task. The presented syllables were part of artificial acoustic continua that either involve lip articulators (/ba/-/da/) or not (/ga/-/ka/). Such continua consist of eight steps that gradually vary and are typically perceived as one or the other. Thus, the continua can be divided into two discrete regions depending on the subjective perceived boundary. The identification task allows to analyse the individual category boundary by computing the individual psychometric curves. In the discrimination task, all subjects were presented two stimuli at a time, separated by two steps of the continua (1-3, 2-4…). Depending on the individual category boundary, the pairs were classified as across-category or within-category pairs. If the categorical perception is improved, a steeper slope and therefore clearer category boundary perception is expected in the identification task, whereas in the discrimination task a higher proportion of “different” responses in the across-category pairs (pairs containing the individual boundary) is expected. The analysis of the neurophysiological data revealed no significant effect of high frequency tACS on lip motor excitability, contrary to expectations based on the literature. Further, tACS did not significantly modulate syllable identification and discrimination performance. Thus, no significant change in categorical speech sound perception could be observed. Overall, our results indicate that the 2 kHz stimulation does not reliably modulate cortical activity. In a next step, we will thus collect a new dataset applying random noise stimulation (tRNS) to the left vMC. Recent studies suggest that tRNS is more effective in enhancing motor cortical activity than kHz tACS.