Characterizing the application of transcranial direct current stimulation in human pharyngeal motor cortex

Samantha Jefferson, Satish Mistry, Salil Singh, John Rothwell, Shaheen Hamdy, Samantha Jefferson, Satish Mistry, Salil Singh, John Rothwell, Shaheen Hamdy

Abstract

Transcranial direct current stimulation (tDCS) is a novel intervention that can modulate brain excitability in health and disease; however, little is known about its effects on bilaterally innervated systems such as pharyngeal motor cortex. Here, we assess the effects of differing doses of tDCS on the physiology of healthy human pharyngeal motor cortex as a prelude to designing a therapeutic intervention in dysphagic patients. Healthy subjects (n = 17) underwent seven regimens of tDCS (anodal 10 min 1 mA, cathodal 10 min 1 mA, anodal 10 min 1.5 mA, cathodal 10 min 1.5 mA, anodal 20 min 1 mA, cathodal 20 min 1 mA, Sham) on separate days, in a double blind randomized order. Bihemispheric motor evoked potential (MEP) responses to single-pulse transcranial magnetic stimulation (TMS) as well as intracortical facilitation (ICF) and inhibition (ICI) were recorded using a swallowed pharyngeal catheter before and up to 60 min following the tDCS. Compared with sham, both 10 min 1.5 mA and 20 min 1 mA anodal stimulation induced increases in cortical excitability in the stimulated hemisphere (+44 +/- 17% and +59 +/- 16%, respectively; P < 0.005) whereas only 10 min 1.5 mA cathodal stimulation induced inhibition (-26 +/- 4%, P = 0.02). There were neither contralateral hemisphere changes nor any evidence for ICI or ICF in driving the ipsilateral effects. In conclusion, anodal tDCS can alter pharyngeal motor cortex excitability in an intensity-dependent manner, with little evidence for transcallosal spread. Anodal stimulation may therefore provide a useful means of stimulating pharyngeal cortex and promoting recovery in dysphagic patients.

Figures

Fig. 1.
Fig. 1.
Schematic demonstrating the principles of transcranial direct current stimulation (tDCS) including the placement of electrodes for anodal tDCS (A) and cathodal tDCS (B) of left hemisphere motor cortex.
Fig. 2.
Fig. 2.
Percentage change (means ± SE) in pharyngeal motor evoked potential (MEP) amplitudes in response to single-pulse transcranial magnetic stimulation (TMS) over the stimulated hemisphere (A) and the unstimulated hemisphere (B), following either anodal (triangles), cathodal (squares), or sham (diamonds) tDCS given for 10 min at 1 mA tDCS. There were no significant changes from baseline.
Fig. 3.
Fig. 3.
Overlaid traces of pharyngeal MEPs from the stimulated hemisphere of 1 subject, showing typical responses to 3 different tDCS paradigms [sham (C), anodal (A), and cathodal (B)]. Note the changes in amplitude following anodal and cathodal stimulation, which are not seen with sham stimulation alone.
Fig. 4.
Fig. 4.
Percentage (means ± SE) change from baseline of stimulated hemisphere pharyngeal motor cortex excitability following anodal (A) or cathodal (B) tDCS for 20 min at 1 mA (squares), 10 min at 1.5 mA (triangles), or sham stimulation (cross). Significant changes in excitability are seen following 10 min 1.5 mA, 20 min 1 mA anodal, and 10 min 1.5 mA cathodal stimulation with repeated-measures ANOVA; *t-test P < 0.05 compared with baseline, **t-tests P < 0.05 compared with both baseline and sham.

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