Poster Slam Session D, Wednesday, August 21, 2019, 5:00 - 5:15 pm, Finlandia Hall, Emily Myers
Failure to replicate the effects of transcranial Direct Current Stimulation (tDCS) on articulation of tongue twisters: a pre-registered study
Charlotte Wiltshire1,2, Emily West1, Kate E. Watkins1,2;1University of Oxford, 2Wellcome Centre for Integrative Neuroscience, University of Oxford
Introduction: tDCS has been shown to modulate the cortex in a polarity-specific way. Local cortical excitability, measured by the size of transcranial magnetic stimulation (TMS)-induced Motor Evoked Potentials (MEPs) is upregulated by anodal stimulation and down regulated by cathodal (Nitsche and Paulus, 2000). When used in combination with a behavioural task, a single session of tDCS can also modulate performance (Stagg and Nitsche, 2011). Fiori et al., (2014) showed that a single session of 2-mA tDCS applied to the left inferior frontal gyrus (IFG) leads to modulated performance on a speech motor task (repetition of tongue twisters) during stimulation. Here, we aimed to replicate this finding and extend it by also measuring MEPs from the lip representation of the motor cortex. The study design and analysis plan were pre-registered on the Open Science Framework (https://osf.io/p84ys/). Method: Sixty right-handed, native-English speaking volunteers (age: M=22.3, SD=4.8) took part in a double-blind randomized sham-controlled study. Three gender-balanced groups received: 1) Anodal tDCS to the left hemisphere IFG/LipM1 and cathodal tDCS to the right hemisphere homologue; or 2) cathodal tDCS (the reverse montage) or 3) sham stimulation. In the active stimulation groups, 1-mA current was applied via 5 x 7cm saline-soaked electrodes for 13 minutes. In the sham group, the current was ramped up to 1-mA over the first 15 seconds and then turned off. The behavioural task was performed prior to, during and 10 minutes post, the stimulation. Participants heard and repeated 36 sentences with complex articulation (tongue twisters; TT) and 36 syllable- and word form-matched simple sentences (SS). The primary outcome of the study was the change in response duration (post minus pre-stimulation), which was measured from the offset of the recorded sentence to the end of the utterance. MEPs were elicited using TMS over the lip representation in left primary motor cortex before and immediately after the tDCS. Here, change (post- minus pre-stimulation) in peak-to-peak amplitude was the dependent measure. Results: There were no significant differences in response times for either TT or SS among the three stimulation groups (anodal vs. cathodal vs. sham). The magnitude of reduction in response time was significantly greater for TT than for SS (p=.003). For the change in MEP size, neither the anodal nor the cathodal group differed from sham (all p>.15), and none were different from zero (no change). Conclusion: We failed to replicate previous findings that tDCS modulates performance on a tongue twisters task (Fiori et al., 2014). There was greater learning for TT compared with the SS, as expected, but there was no difference in learning among the three stimulation groups. Furthermore, the tDCS applied concurrently with the behavioural task had no measurable effect on motor excitability measured with MEPs from the lip. The failure to replicate the behavioural findings of Fiori et al., (2014) could be due to changes in the stimulation protocol. Nevertheless, our large sample size gave us 80% power to detect a medium sized effect at p<.05 should one exist.
Themes: Speech Motor Control, Methods
Method: Neurostimulation
Poster D63