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Poster A10, Thursday, August 16, 10:15 am – 12:00 pm, Room 2000AB

Effects of HD-tDCS Current Intensity on Vocal Pitch Motor Control

Dirk B. Den Ouden1, Danielle Fahey1, Taylor McDonald1, Janelle Rocktashel1, Roozbeh Behroozmand1;1University of South Carolina

Introduction: High-Definition transcranial Direct-Current Stimulation (HD-tDCS) is a relatively recent innovation to the tDCS technique, by which the focality of cortical stimulation is improved through the use of multiple electrodes. This may increase the potential for tDCS to enhance treatment outcome for neurogenic disorders. However, with a method that allows greater control over stimulation parameters comes an increase in associated configurational choices. To a large extent, the different settings currently used in experimental studies are based on what is known from traditional tDCS, even though modeling studies show that the regional direction and intensity of electric field potentials in HD-tDCS are very different. Traditional tDCS studies have typically applied intensities of 1 and 1.5mA, but accepted safety practices allow HD-tDCS current intensities up to 2mA. It is unclear to what extent this should lead to increased effects, or rather whether there may be an upper effective intensity threshold, or even the possibility to ‘overstimulate’, yielding decreased behavioral effects beyond a certain threshold. In a previous study by our group, HD-tDCS of the left ventral motor cortex modulated the magnitude of compensatory vocal responses to pitch-shift stimuli in speech auditory feedback, particularly with cathodal stimulation for downward pitch shifts. This finding suggested that HD-tDCS can modulate the underlying neural mechanisms of vocal pitch motor control, and that the magnitude of behavioral compensation provides a sensitive outcome measure. Here we have used this outcome measure to test the effects of current intensity in a partial replication of our previous experiment. Methods: Twenty right-handed participants (19 females; mean age 23, range 18-28) received either 1mA or 2mA of cathodal HD-tDCS on left ventral motor cortex. Before, during and after stimulation, participants maintained steady vowel vocalizations and received randomized upward (+100 cents) and downward (-100 cents) pitch shift stimuli in their auditory feedback. We recorded participants’ vocalizations and measured the magnitude of pitch shifts in response to feedback pitch alterations. Results: A main effect of time point on the amount of pitch shift in response to altered auditory feedback was present for downward (F(1.371, 24.7)=4.941, p<.05), but not for upward pitch-shift stimuli (F(1, 18)=.821, ns). This main effect was driven by a significant reduction in the magnitude of vocal compensation between the pre-stimulation and the post-stimulation time points (pairwise t-test: p<.05). For neither of the two pitch-shift directions was there a main effect of intensity, nor an interaction between time point and intensity.   Conclusion: Results replicate our earlier findings, in that cathodal HD-tDCS reduced the magnitude of compensatory responses to downward pitch shifts in the auditory feedback. No effect of stimulation current intensity was present. At this point, therefore, we conclude that 2mA of HD-tDCS does not increase (nor reduce) behavioral effect sizes of HD-tDCS. This is in line with the idea that there is an upper threshold of stimulation intensity beyond which no added benefits may be generated. Practically, for our purposes, it also means we may increase the lifetime of HD-tDCS electrodes, by limiting their use to 1mA.

Topic Area: Speech Motor Control and Sensorimotor Integration