HD-tDCS: Effects on the Somatosensory System

A hand-held pressure algometer (Somedic, Hörby, Sweden) with a 1-cm2 probe will be used to record the pressure pain threshold. The pressure is increased gradually at a rate of 30 kPa/s. The measurement is repeated three times on on flexor carpi radialis.

Tactile threshold will be measured using an anaesthesiometer consisting of a set of Von Frey filaments. The filaments are made of nylon fibre of various diameters so as to provide a range of forces of up to 300 grams. The minimum force that the subject can detect will be identified. This will be assessed on flexor carpi radialis on the right hand arm.

Mechanical pain threshold (MPT) will be measured using a set of weighted pinprick stimulators with a flat contact area of 0.25 mm diameter that exert forces between 8 and 512 mN. Threshold procedures will use a method of limits with up to five series of ascending and descending stimulus intensities. This will be assessed on flexor carpi radialis on the right hand arm.

A 3×3 cm (9 cm2) contact thermode (Medoc Advanced Medical Systems, Israel) will be used to apply thermal stimulation. Each stimulus will be started at 32°C and thresholds of heat and cold sensory detection will be assessed by ascending or descending ramps in temperature to the cutoffs of 0-50 °C. This will be assessed on flexor carpi radialis on the right hand arm.

A 3×3 cm (9 cm2) contact thermode (Medoc Advanced Medical Systems, Israel) will be used to apply thermal stimulation. Each stimulus will be started at 32°C and thresholds of heat and cold pain detection will be assessed by ascending or descending ramps in temperature to the cutoffs of 0-50 °C. This will be assessed on flexor carpi radialis on the right hand arm.

A computer-controlled cuff pressure algometer (Nocitech, Denmark) with an air-filled tourniquet cuff (VBM, Germany) will be used to record cuff pressure-induced pain thresholds over the muscle bulk on the calfs. The pressure is increased gradually at a rate of 1 kPa/s to assess pain tolerance on a 0-10 VAS scale, where 0 is no pain at all and 10 is the worst pain imaginable.

During CPM testing, the pain tolerance is first assessed on one leg marking the baseline. The pressure pain tolerance is then assessed on the same leg again, while the contralateral calf is applied a simultaneous painful pressure stimulus. The outcome of the CPM testing is the difference in pressure pain tolerance measured in kPA between the first and the second assessment. The larger the positive difference is between the baseline and the second assessment, the stronger CPM effect the subject has.

Vibration detection threshold (VDT) will be determined with a Rydel-Seiffer tuning fork (64 Hz, 8/8 scale), which will be placed three times over a bony prominence of the distal part of the ulna on the right arm. Subjects are to indicate the disappearance of vibratory sensations.

TSP will be applied using cuff pressure algometry. A total of 10 repeated mechanical pressure stimuli will be delivered at 0.5 Hz (1-s stimuli duration and 1-s interval between stimuli) to the test area. During the 10 repeated stimuli, subjects will continuously rate the pain intensity on a 10-cm continuous VAS. TSP is assessed on the calf of the right leg.