Modulatory Effects of Multichannel tDCS During Prolonged Experimental Pain

Corticomotor excitability is expressed as the peak-to-peak amplitude of motor evoked potentials to single-pulse transcranial magnetic stimulation. It is expected that network-tDCS will reduce corticomotor inhibition induced by tonic pain during 24 hours.

Conditioned pain modulation effect is an indicator of descending inhibitory control in humans. In this study, it is performed using pressure cuff algometry. The CPM protocol consisted of a constant cuff pressure stimulation (conditioning stimulus) at 70% of the PTT (recorded for pain sensitivity assessment on that day) applied to the left leg and simultaneously one ramped cuff stimulation (test stimulus) at 1 kPa/s applied to the right leg. It is hypothesized that network-tDCS might normalize the CPM effect reduced by tonic pain during 24 hours.

Warm detection thresholds will be assessed using a thermode applied on the volar forearm. The thermode will start increasing the temperature from 32 C (Celsius degrees) until the participant can detect a warm sensation and will immediately press a stop button. This task will be repeted a total of 4 times.

It is hypothesized that 2 daily treatments of active network-tDCS will not modulate this sensitivity measure during prolonged pain due to a small effect size.

Heat pain thresholds will be assessed using a thermode applied on the volar forearm. The thermode will start increasing the temperature from 32 C (Celsius degrees) until the participant can detect a temperature increment from just "a warm sensation" to an additional of either "burning" or "painful" and will immediately press a stop button. This task will be repeted a total of 3 times.

It is hypothesized that 2 daily treatments of active network-tDCS will not modulate this pain sensitivity measure during prolonged pain due to a small effect size.

Mechanical pain thresholds (MPT) were assessed using seven pinprick stimulators (MRC Systems GmbH, Germany) exerting forces ranging from 8 mN to 512 mN over the distal part of the volar forearm. Starting with the lightest, each stimulator is applied in an ascending order until the participant reported a perception of sharpness or pain. If the pain threshold is not reached, the value of 1024 mN was registered. The MPT will be expressed as the geometric mean of five series of ascending/descending series of stimuli.

It is hypothesized that 2 daily treatments of active network-tDCS will not modulate this pain sensitivity measure during prolonged pain due to a small effect size.

A computer-controlled cuff algometer combined with two 10-cm wide air-pressured cuffs and an electronic visual analogue scale (eVAS; anchored at 0 cm [no pain] and 10 cm [worst pain imaginable]) are used to assess cuff pressure pain detection thresholds (PDT) and pain tolerance thresholds (PTT).

It is hypothesized that 2 daily treatments of active network-tDCS will not modulate these pain sensitivity measures during prolonged pain due to a small effect size.

It is expected that the peak to peak amplitude of SEPs (in Cz position) will be modulated by tonic pain during 24 hours and by network-tDCS.

it is expected that the latency of SEPs will remain unchanged during the 24 hours period and will not be affected by neither prolonged pain nor by network-tDCS.