Abstract
Transcranial direct current stimulation (tDCS) is a non-invasive method to modulate cortical excitability in humans. Here, we examined the effects of anodal tDCS on suprahyoid motor evoked potentials (MEP) when applied over the hemisphere with stronger and weaker suprahyoid/submental projections, respectively, while study participants performed a swallowing task. Thirty healthy volunteers were invited to two experimental sessions and randomly assigned to one of two different groups. While in the first group stimulation was targeted over the hemisphere with stronger suprahyoid projections, the second group received stimulation over the weaker suprahyoid projections. tDCS was applied either as anodal or sham stimulation in a random cross-over design. Suprahyoid MEPs were assessed immediately before intervention, as well as 5, 30, 60, and 90 min after discontinuation of stimulation from both the stimulated and non-stimulated contralateral hemisphere. We found that anodal tDCS (a-tDCS) had long-lasting effects on suprahyoid MEPs on the stimulated side in both groups (tDCS targeting the stronger projections: F(1,14) = 96.2, p < 0.001; tDCS targeting the weaker projections: F(1,14) = 37.45, p < 0.001). While MEPs did not increase when elicited from the non-targeted hemisphere after stimulation of the stronger projections (F(1,14) = 0.69, p = 0.42), we found increased MEPs elicited from the non-targeted hemisphere after stimulating the weaker projections (at time points 30–90 min) (F(1,14) = 18.26, p = 0.001). We conclude that anodal tDCS has differential effects on suprahyoid MEPs elicited from the targeted and non-targeted hemisphere depending on the site of stimulation. This finding may be important for the application of a-tDCS in patients with dysphagia, for example after stroke.
Highlights
Transcranial direct current stimulation is a non-invasive method of neuromodulation that provides insights into functions of specific brain areas and associated motor activity (Filmer et al, 2014; Flöel, 2014)
Volunteers and Transcranial direct current stimulation (tDCS) Impedance A total of 30 healthy volunteers completed the trials with good tolerance for transcranial magnetic stimulation (TMS) and tDCS; the only exception was one subject who complained of a headache after TMS and quit
Upon comparing the bilateral mean amplitudes of motor evoked potentials (MEP) traces induced by singlepulse TMS, 18 of 30 participants were observed to have a stronger suprahyoid projection in the left hemisphere, while 12 participants had a stronger right cortical representation
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive method of neuromodulation that provides insights into functions of specific brain areas and associated motor activity (Filmer et al, 2014; Flöel, 2014). This technique involves delivering constant, weak electrical current to a local region of the cerebral cortex to modulate neuronal excitability and plasticity. Substantial evidence from neuroimaging studies has demonstrated that both the cerebral cortex and subcortical regions play an important role in controlling the swallowing process These regions include the primary sensorimotor cortex, insula, anterior cingulate cortex, frontal operculum, and supplementary motor areas, basal ganglia, thalamus, and cerebellum (Sörös et al, 2009; Leopold and Daniels, 2010). Previous studies have established a distributed but functionally connected map of the neural structures involved in swallowing, which has contributed to the development of techniques that can be applied in clinical practice (Michou and Hamdy, 2009; Lowell et al, 2012; Babaei et al, 2013)
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