to investigate the effect of transcranial direct current stimulation of the somatosensory area on upper limb motor performance in healthy adults

60 healthy subjects will be randomly allocated to one of three groups: (a) 15- min of High-Definition transcranial direct current stimulation (HD-tDCS) on primary somatosensory cortex (S1) with an intensity of 1 milliampere (mA) (HD-tDCS S1); (b) 15-min of HD-tDCS on primary motor cortex (M1) with an intensity of 1 mA (HD-tDCS M1); and (c) 15-min of sham HD-tDCS (HD-tDCS sham).

The stimulation will be administered noninvasively using an M x N 9-channel high definition transcranial electrical current stimulator from Soterix Medical (New York, NY). In the HD-tDCS S1 group, the anodal stimulation will target the right postcentral gyrus. In the HD-tDCS M1 group, the anodal stimulation will target the right primary motor cortex (brodmann area 4). The location of the electrodes will be placed based on HD-Targets brain modelling software (Soterix Medical, New York, NY).

Tests (motor and sensory tasks): The non-dominant left arm will be tested. Motor task: The subjects will perform a sequential point-to-point movement task on the graphics tablet. Initially, the participants will be required to perform 3 sequences without errors to familiarize themselves with the setup, the task and the sequence. Then, they will perform the pre-test which consists of two blocks of 6 sequences, i.e. 12 sequences, with a 30 s break between blocks. Two min after starting the appropriate stimulation, they will perform 2 blocks of 6 sequences (identical to the pre-test). After finishing the tDCS stimulation, the participants will perform a post-test, which is also identical to the pre-test. Three outcome measures will be used: movement time (s), reaction time (s) and straight line deviation (cm) of the reaching movements. Sensory tasks: discriminative task (two point orientation discrimination) and proprioception task will be included. The sensory tasks will be performed with eyes blindfolded. In the discriminative task the experimenter will deliver a stimuli by a two-pronged instrument at intervals of 2,3,4,5 mm to the distal phalanx of fingers 2 and 5 of the left hand. The stimulus will be oriented either across or down (total of 16 stimuli). Subjects will be asked to Identify the orientation of the stimulation. The outcome measure will be the percentage of correct answers. In the proprioceptive task, the subjects will perform a one direction point-to-point (target) movement task on the graphics tablet. The task will include 20 movements, 10 passive movements and 10 active movements. After every passive movement with eyes blindfolded, the subject will be asked to perform a similar active movement. Initially, the participants will be asked to perform four movements to familiarize themselves with the setup (the first two movements will be passive and active movements with eyes open, and the following two movements will be passive and active movements with eyes blindfolded). The outcome measure will be the end point error (cm) which is the distance between the target and the actual arrival location.

Inclusion Criteria:

• aged between 20 and 35
• right-hand dominant
• healthy according to self report

Exclusion Criteria:

• taking psychiatric medications
• a history of drug / alcohol abuse or dependence
• psychiatric or neurological disorder
• a history of seizures
• metal implants in their head
• musculoskeletal deficits interfering with task performance (proper reaching performance in sitting)

The datasets (Study Protocol, Statistical Analysis Plan, Informed Consent Form, Analytic Code generated during and/or analyzed during the current study) will be available from the corresponding author on reasonable request.