The role alpha (10Hz) oscillations in spatial updating: a transcranial alternating current stimulation study

We perceive the world as a stable reality, despite the ubiquitous changes in
visual input due to our own movements. When the body is brought in motion, like
when driving a car, vestibular feedback informs the brain about the motion, but
if and how these signals contribute to visual stability has not been revealed.
In a recent study, using a moving vestibular sled, we showed that internal
representations of remembered visual objects are stored as a spatial-selective
alpha power (10Hz) reduction in the posterior brain and remapped
trans-hemispherically due to the translation, when objects cross the midline
relative to the fixation point. The activity seen in occipito-parietal areas
may be an implementation of an *attentional pointer* to the remembered location
in gaze coordinates. Importantly, in posterior parietal cortex, the strength of
these remapping alpha power modulations correlated significantly with the
quality of the visuospatial updating. It may thus be that the posterior
parietal cortex integrates signals related to whole-body translation, and
applies this information to update spatial representations in parieto-occipital
areas. However, causal evidence for this hypothesis is still missing.

The primary objective of this study is to establish a causal role of the alpha
band oscillations in lateralized parieto-occipital areas during spatial
updating with whole-body translation using online transcranial alternating
current stimulation (tACS) in the alpha frequency range (10Hz). We expect that
updating of remembered targets is affected when information about self-motion
needs to be applied in order to remap the location of a remembered target,
during stimulation of occipito-parietal areas.

Placebo-controlled double-blind within-subjects design with healthy volunteers.

The experiment consists of two sessions: an experimental session and a control
(*sham*) stimulation session. During the experimental session 25 minutes of
tACS will be administered over parieto-occipital areas of the left or right
hemisphere during performance of a spatial updating task. In the control
stimulation session, the current will be ramped up and down at the beginning
and end of the session, without any stimulation during the experiment. Online
transcranial alternating current stimulation (tACS) will be delivered by a
battery-driven electric current stimulator (Eldith DC Stimulator (CE 0118),
Ilmenau) using a pair of electrodes over the occipital parietal area and over
the brain midline.

Participants will not directly benefit from their participation in the study,
except for a compensatory (financial) incentive. Transcranial current
stimulation (tCS) is a widely used non-invasive brain stimulation technique,
applying weak direct/alternating currents (tDCS/tACS) via conductive
rubber/sponge electrodes to the scalp. These weak currents can slightly shift
the neurons* membrane potential and thereby increase or decrease spontaneous
neuronal activity in the stimulated cortex, but (unlike TMS) they do not evoke
action potentials. During the stimulation, participants may transiently
experience light tingling, itching or burning sensations on the skin underlying
the electrodes, which can be unpleasant. The most common side effects are a
light transient headache and a feeling of fatigue. In the current study,
healthy participants will be stimulated with a protocol that is considered safe
with respect to the latest published safety guidelines. All subjects are
screened for their relevant medical history and other tCS safety aspects (e.g.
metal parts in the head, skin allergies). In summary, because the risk is
negligible and the burden associated with participation can be considered
minimal, we do not expect serious adverse events during the project.