Healthy Clinical Trial
Official title:
Effects of the Transcranial Alternative Electric Stimulation on the Consciousness Access in Healthy Subjects
The aim of this study is to investigate the effect of transcranial Alternating Current Stimulation (tACS) at theta frequency and the the effect of transcranial Direct Current Stimulation (tDCS) on the conscious access to visual stimuli. tACS and tDCS are non-invasive stimulation techniques that are used to induce brain oscillations at certain frequency or to increase the brain activity in applied region. Healthy participants will perform a behavioral task measuring conscious access (visual backward metacontrast masking task) before, during and after fronto-parietal tACS or tDCS stimulation which will increase neural activity in the two sites. This will allow us to examine online and remaining effects of the stimulation and the causal role of fronto-parietal activity on conscious access.
Behavioral task
The behavioral task consists in presentation of a para-foveal numerical target on a screen
for 16 ms either right or left of a central fixation point, followed by a visual mask, made
of letters surrounding the target, presented for 250 ms. The delay between the target and the
mask (Stimulus Onset Asynchrony - SOA) will be variable (16 ms, 33 ms, 50 ms, 66 ms or 83
ms). Participants will then be asked to perform two tasks. First (objective task),
participants will be asked to indicate as quickly as possible whether the target was bigger
or smaller than the digit "5". Second (subjective task), they will be requested to judge the
visibility of the presented target by indicating whether they had seen the target or not.
tACS
Study design
The study design is composed of three experimental sessions. In these sessions, participants
will first complete the behavioral paradigm as presented above while having their brain
activity recorded with EEG. Afterwards, they will re-perform the task during and after tACS
stimulation. tACS stimulation type will differ in each session : in-phase, anti-phase or sham
(see below for further details). Stimulation sessions will occur in a double-blind randomized
crossover design (neither the participant nor the experimenter knows which session includes
which stimulation type), with at least 2 days of interval between them.
Stimulation
During tACS sessions, 6 Hz stimulation (1000 μA) will be applied simultaneously over the left
prefrontal dorso-lateral cortex (F3 of the 10-20 international scalp EEG system) and the left
parietal cortex (P3) using an 8-channels stimulator (Starstim NE, Neuroelectrics, Barcelona,
Spain) with small round sponge electrodes (25 cm2 surface, maximal current density of 0.06
μA/cm2), controlled via Bluetooth. In the in-phase condition, the phase difference between
the two stimulation sites will be 0° which will entrain synchronization between sites. In the
anti-phase condition on the other hand, the phase difference will be 180°, which should
desynchronize the activity in the two cortices. In the sham condition, the stimulation (also
anti-phase) will start with a current intensity of 2000 μA lasting for 30 seconds.
Afterwards, the intensity will progressively decrease over 20 seconds until cessation. Each
stimulation session will take 20 minutes and the positions of stimulation electrodes and the
duration of the stimulation will be kept identical for all conditions.
tDCS
Study design
The study is composed of two experimental sessions: one tDCS session and one sham session. In
the tDCS session, participants will do the behavioral task (with an EEG recording) during and
after 20 minutes of 1000 μA tDCS stimulation. As a control, participants will do the
behavioral task in a second session (sham session) during which they will have a sham
stimulation instead of tDCS stimulation. The two sessions will occur in a double-blind
randomized crossover design (neither the participant nor the experimenter knows which session
includes which stimulation type), with at least 2 days of interval between them.
Stimulation
During tDCS session, 1000 μA stimulation will be applied simultaneously over the left
prefrontal dorso-lateral cortex (F3 of the 10-20 international scalp EEG system) and the left
parietal cortex (P3) using an 8-channels stimulator (Starstim NE, Neuroelectrics, Barcelona,
Spain) with small round sponge electrodes (25 cm2 surface, maximal current density of 0.06
μA/cm2), controlled via Bluetooth. In the sham condition, the stimulation will start with a
current intensity of 1000 μA lasting for 30 seconds. Afterwards, the intensity will
progressively decrease over 20 seconds until cessation. The reason for this subtle
manipulation is to make sure that participants will be unable to notice the difference
between active tDCS and sham tDCS stimulation. Each stimulation session will take 20 minutes
and the positions of stimulation electrodes and the duration of the stimulation will be kept
identical for all conditions.
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