The effects of transcranial current stimulation on visual oscillations

Neuronal oscillations in the visual cortex subserve different roles in
processing of relevant input and inhibition of irrelevant input. Normal human
subjects are able to endogenously modulate these oscillations; however subjects
with disorders of attention exhibit disrupted oscillatory dynamics which have
consequences for behaviour. Transcranial current stimulation (tCS) is a
technique that may eventually have applications for exogenously modulating
neuronal oscillatory responses in a manner similar to directed attention.
However until recently it has not been possible to concurrently stimulate the
brain using tCS and record oscillatory activity, meaning this relationship
could not be studied directly. Recent developments have enabled this via
concurrent tCS and MEG. Therefore it is now for the first time possible to
change these oscillatory dynamics by increasing or decreasing cortical
excitability, to attempt to subject these oscillations to external modulation
(*entrainment*), and to assess the relationship between these oscillatory
dynamics between the two hemispheres of visual cortex by selectively altering
the excitability of one hemisphere.

We aim to provide causal evidence for the following:

Study arm one: That the oscillatory brain response produced by a visual
stimulus can be modulated by tonically altering cortical excitability using
tDCS.

Study arm two: That functionally relevant brain oscillations can be entrained
using tACS, and that this oscillatory entrainment mimics the effect of
endogenously modulations (ie, that transcranial modulation of alpha
oscillations will produce phasic modulation of stimulus-induced gamma
oscillations).

Study arm three: That the human visual cortex exhibits *interhemispheric
inhibition*, and that increase of cortical excitability in one hemisphere using
tDCS results in both an increase in stimulus-induced gamma responses in that
hemisphere, plus a concomitant reduction of stimulus-induced gamma responses in
the opposite hemisphere.

Experimental within-subject (cross-over) design with healthy volunteers.

Short blocks of either tDCS or tACS (2mA; 2 min blocks; cumulative duration of
max. 32 min per session) will be applied to the visual cortex. In both cases,
commonly used, well-documented stimulation protocols will be applied (for
references, see section *study design*). Either tDCS or tACS will be applied
whilst participants perform a visual attentional task, concurrently with
recording of whole-brain activity using magnetoencephalography (MEG).

For the assessment of risks and burden associated with transcranial brain
stimulation in this study please refer to paragraph 5.2 of the approved
Standard Operating Procedure for Non-Invasive Brain Stimulation (v. 2.1., CMO
No. 2013/245) at the Donders Institute for Brain, Cognition and Behaviour.