Investigating inhibitory processes within the motor cortex during conflict resolution using transcranial magnetic stimulation

In daily life, we often need to deal with situations in which a number of
actions are potentially possible and are in conflict with each other,
situations of so-called *response conflict*. Our brain needs to select the best
of these competing responses and suppress unwanted actions. There is a
substantial literature regarding which higher-level areas of the human brain
assist in this process of response selection. However, it remains unknown how
and by which physiological mechanism these areas affect the *output station* of
the brain, the primary motor cortex. A potential mechanism is the involvement
of inhibitory interneurons, which can be directly tested using transcranial
magnetic stimulation (TMS).

We aim to test how inhibitory processes within the primary motor cortex change
over time during the resolution of response conflict.

In order to probe inhibitory processes within the primary motor cortex, this
study will employ single- and paired-pulse TMS. A TMS coil will be positioned
over the primary motor cortex of the left hemisphere. The dependent measure in
the task is the change across single- and paired-pulse trials in the amplitude
of the motor-evoked potential (MEP). This change in amplitude is termed short
intra-cortical inhibition* or sICI. The MEP is elicited by the TMS in the
muscle of one of the response hands. The TMS will be applied while participants
are performing a simple choice reaction time task. The stimuli are arranged
such that they elicit only one response (no conflict) or two competing
responses of which only one is correct (conflict). Four different time points
will be used for stimulation with one time point per trial. In this way the
individual time specific physiological pattern of inhibitory processes can be
identified for either conflict or no conflict situation

Participants will receive a total of 864 trials. On half of the trials a single
pulse of TMS will be applied, on the other half of trials two TMS pulses will
be applied in quick succession.

Participants will not directly benefit from their participation in the study.
Transcranial magnetic stimulation (TMS) is a widely used non-invasive brain
stimulation technique, based on the principle of electromagnetic induction.
During stimulation the participant will likely hear the clicks of the TMS
pulses and experience a slight stimulation of nerves and muscles of the head.
The most common side effect is a light transient headache (2-4% occurrence). A
severe headache is uncommon (0.3-0.5% occurrence). In TMS studies using certain
patient populations (e.g. epilepsy) or that exceeded the standard protocols
(e.g. in intensity or frequency) epileptic seizures have been reported in rare
cases. In the current study healthy participants will be stimulated with a
protocol that falls within the safety guidelines. All subjects are screened for
their relevant medical history and other TMS safety aspects (e.g. presence of
metal parts in the head). In summary, because the risk and burden associated
with participation can be considered negligible-to-minimal, we do not expect
serious adverse events during the project.