On the role of attention in modulating the alpha oscillation phase-dependency of cortical excitability and visual perception

Over the occipital region, shifts in covert visuospatial attention are
associated with a lateralization of posterior alpha band power (8-14 Hz). Alpha
power is higher in the hemisphere ipsilateral to attention (Jensen and
Mazaheri, 2010). Recently, Transcranial Magnetic Stimulation (TMS) has been
used to causally link ongoing oscillations, cortical excitability and visual
perception. Thus, we know that over the occipital region, alpha power is
negatively related to cortical excitability (Romei et al., 2008) but also that
alpha phase mediates the causal relation between brain excitation and visual
perception (Dugué et al., 2011). Using attention to modulate ongoing alpha
amplitude, we aim to determine how the phase effect behaves depending on alpha
power (i.e., depending on attention). In particular, a stronger phase effect is
expected for the unattended condition.

We aim to test how attention exerts its facilitative effects on perception. We
expect a validation of the *pulsed inhibition* theory, i.e., that - for an
unattended location - perceptual probability is normal during the excitatory
phase but decreased during the inhibitory phase of the alpha oscillation.

This study will combine TMS with EEG. Two TMS coils will be positioned over the
left and right primary visual cortex (V1) to induce phosphenes (illusory
flashes) by single pulses. For both target areas, TMS intensity will be set to
phosphene threshold (i.e., evoking phosphenes in 50% of the cases). Subject
will perform two tasks, intermingled on a trial-by-trial level: a Gabor
orientation task (determining the orientation of a tilted Gabor patch in the
cued hemifield) and a phosphene localization task (perceiving and localizing a
TMS-induced phosphenes in the left or right hemifield). The Gabor spatial
attention task is used to warrant that subjects covertly orient their attention
to the cued hemifield and allows us to test phosphene probability depending on
alpha power as modulated by visuospatial attention.

Participants will receive single pulses of TMS over left or right V1 at the
individual subject*s and target site*s phosphene threshold.

TMS of the visual cortex is not painful, even at maximum stimulator output.
From the literature we know that, in rare cases, participants could report a
(light) headache. Paracetamol is sufficient to treat these symptoms. On the
basis of incidental epileptic seizures triggered by TMS in early 90*s,
safety-guidelines were established (Wassermann, 1998) and recently updated
(Rossi et al., 2009). All stimulation parameters in this study will follow
these safety guidelines. Furthermore, all participants will be pre-screened for
relevant medical history, epilepsy, drug abuse, head trauma, neurological or
psychiatric illness, pregnancy, heart disease, cardiac pacemakers, medication
pumps, tricyclic antidepressants, neuroleptics and a family history of
neurological illness, psychiatric illness or epilepsy. Because the risk
associated with participation can be considered negligible and the burden can
be considered minimal, we do not expect adverse events during the project.