Transcranial Direct Current Stimulation and Modulation of Affect and Pain

This study aims to examine the effects of M1 tDCS on induced experimental pain, pain-related decision making, and independently manipulate expectations based on the pain and monetary offered by others. This study involves 4 sessions, one thermal calibration session and three full study sessions where subjects will experience one of three tDCS conditions: anodal, cathodal, or sham. Participants receive only one condition of tDCS for each full session, and the order of which tDCS condition they receive is randomized. Participants are also asked to complete a decision-making task during each full study session, a Single-trial heat stimulation task based on the decision-making task, and finally a Social Decision Making and Placebo Responses task. Participants will complete all tasks following tDCS inside the MR scanner.

The initial pain calibration session is performed as a part of the screening process for each participant to ensure that all temperatures delivered during any subsequent procedures are within participants' tolerable range, and to obtain consent to participate in this research. The calibration procedure can improve the accuracy of the measurement because it adjusts the temperature based on the participants' reported sensation and allows for the determination of an individual's pain and tolerance level. Four runs (6 trials in each run and 24 trials totally) of noxious heat are administered to six forearm sites using iterative adaptive logistic regression. Each stimulus lasts 13 seconds, with 1.5 seconds for ramping. Sensation is rated on a circular scale from "no sensation" to "strongest imaginable". Participants report pain and tolerance with "yes" or "no" responses to "Painful?" and "Tolerate?" questions. The first calibration run familiarizes participants with 47°C heat on all sites. The second run includes 45°C, 47°C, and 48.9°C stimuli twice, randomly distributed. Runs three and four include low and high temperatures based on the pain and tolerance responses of previous trials. A logistic regression based on a 90% response to pain and tolerance of previous trials is used to determine the low and high threshold and is updated after each trial. After the final trial, low (pain) and high (tolerance) temperatures are determined using logistic regression across all trials, and three sites with the least mean absolute error values in rate-temperature linear regression are chosen for pain stimulation.

Each full session includes 3 run pains runs before tDCS, 20 min tDCS, 3 run pains tests after tDCS, Decision making task, and 3 runs of social decision making and placebo responses task. Each full session begins with three pain runs on three different regions of the right forearm (out of the 6 possible regions) used during the initial calibration session, and ends with three more pain runs on these same regions after tDCS. Each pain run consists of 12 trials using each participants' unique temperature range (pain and tolerance) obtained from the calibration session, such that there are 36 trials before tDCS, and 36 trials after tDCS.

The tDCS device is applied at 2 mA for 20 minutes using the StarStim system (Neuroelectrics, Spain). Electrodes are placed on the C3 and FP2 regions. There is a 30-second ramp-up and 30-second ramp-down of stimulation for each tDCS condition to minimize any discomfort during stimulation. During stimulation, a random fractal image is displayed on the participant's computer screen. Originally, the main purpose of presenting a random image during each tDCS stimulation was to facilitate a 1-month follow-up session for a memory test planned for future study. The assignment of pictures to task conditions is randomized for each participant and therefore picture identity is not confounded with task variables. After tDCS, participants complete a questionnaire related to sensations induced by tDCS.

Decision-making task During the initial phase of the decision-making task, participants are presented with 128 trials where they are given the option to either accept or reject offers for pain and monetary reward levels. The pain levels for the offers are randomized from 8 possible levels of each participant's own unique pain tolerance threshold (or 20%-90% of their tolerance), and the monetary rewards are also randomized from $1 to $8. The presentation order of the pain levels and monetary values for each offer are also randomized (e.g., $5 for 60%, or, 20% for $3, etc.). Participants are instructed that each monetary value represents 10 heat stimulation at that percentage of tolerance. The 128 offers are divided into two sets of 64 trials each, where the first set involves decisions about offers for oneself, while the second set involves decisions about offers for others. No heat stimulation is involved while participants complete the first phase of the task, and the trials are presented in blocks of 8 for each type. Upon completing the initial phase of the task, a random selection of trials decided for others are chosen, and the average reward value associated with the accepted trials among these trials is paid to the participants as compensation for their involvement in this task. In addition, one random trial from the decisions participants made for himself/herself is selected, and participants receive either 10 heat stimulation at that percent of tolerance and additional monetary value or no pain/monetary.