Effects of the Transcranial Alternative Electric Stimulation on the Consciousness Access in Healthy Subjects

Brief Summary

Detailed Description

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.

Primary Outcomes

Secondary Outcomes

• Change in visual evoked potentials between before and after stimulation.(Measured at each stimulation sessions (three sessions in a randomized cross-over order) for each participant. The three sessions will span in an average of two weeks.)
• Change in quantitative EEG measures between before and after stimulation.(Measured at each stimulation sessions (three sessions in a randomized cross-over order) for each participant. The three sessions will span in an average of two weeks.)
• Change in objective performance on the behavioral task (% of correct response on the number comparison task for masked target) between before and after stimulation.(Measured at each stimulation sessions (three sessions in a randomized cross-over order) for each participant. The three sessions will span in an average of two weeks.)