The overall goal of this procedure, is to obtain a robust estimate of individual pain thresholds, using a well-established psychophysical estimation method. This method can help answer key questions in the field of pain research, such as identifying oscillatory neural activity that is connected to the subjective perception of pain, and of constant stimulation. The main advantage of this technique is that it yields very robust estimates of the stimulation intensity necessary to elicit a certain subjective sensation.
Begin by selecting the appropriate cap size for the participant. And prepare the electroencephalography, or EEG electrodes based on the system's instruction manual. Then, set the sample rate and high-low cut off, as well as the impedance limits of the recording equipment.
Run the EEG system on a battery, to ensure the stimulation device and the EEG are not electrically coupled. Then, make sure that any link between the EEG system and the computer controlling the electrical stimulation device, is potential-free. Next, set the stimulator to a single mono-phasic stimulation pulse, with one millisecond duration and 400 volt maximum voltage.
Make sure that the electrical stimulator is switched on, but the output to the electrode is switched off. Then ask the participant to lay their hand on a flat surface, with all digits stretched out, and opposed. Identify the stimulation site by bisecting the distance between the first knuckles of the thumb, and the index finger.
Clean the skin by applying electrode preparation gel. Then attach the stimulation electrode, and fasten it in place with textile tape. Next, ask the participant to place their hand comfortably, so that it will be still for the duration of the experiment.
Place a soft tissue under the hand, to absorb any humidity. Finally, flip the output switch to the upward position. Begin by instructing the participant on how to operate the rating scale on the screen, using the mouse.
Inform them, the left half represents the non-painful sensations, and the right half corresponds to the standard pain VAS scale. Point out to the participant that the absolute center point of the scale cannot be selected, and provide standardized instructions about the anchor points. Next, give the participant the opportunity to get comfortable with the rating process by applying stimuli of varying intensity, and recording the responses.
Pick the intensities randomly, exploring the most extreme ends, and repeating some to evoke responses around the scale center. Ask the participant to provide verbal feedback on the intensities. Use the information gathered during this phase to get an estimate for two intensities that consistently evoke strong but non-painful sensations, a low point, and moderately painful sensations, a high point.
After the low and high point starting intensities have been obtained, inform the participant that the first part of the experiment is about to start, and that he or she could keep on rating as practice, while random stimulus intensities are presented. To determine the threshold estimate, use the prior high and low starting intensities as a baseline. Then, as the participant is presented with stimulation, update the distribution of the threshold estimate, depending on the likelihood of participant's response.
Finally, compute a new estimate by combining the prior likelihood distributions and use the resulting estimate as the new prior baseline estimate in the new iteration of the procedure. Begin by informing the participant that for the remaining part of the experiment, more blocks with random stimulation will be following, and they should keep rating as they did before. Start the EEG recording.
Set the electrical stimulator to the previously determined mean threshold estimate, and keep the setting constant throughout the rest of the experiment. Next, start a rating block and observe the data quality of the EEG recording. Depending on the EEG data quality, run four to five rating blocks, and allow the participant to take short breaks in between the blocks.
After the experiment is complete, stop the EEG recording, switch the stimulator output to off, and remove the stimulation electrode. Finally, remove the EEG cap and debrief the participant. Results indicate that taking the mean of the high and low intensity starting point estimates yields the final threshold estimate.
Furthermore, the subjective stimulation intensity evoked by repeated stimulation at the estimate threshold is stable across multiple blocks of 30 trials, each within one experimental session. Here, you can see the sum of the lateralized differences between painful and non-painful stimulation in the theta-band EEG frequency range of four to seven Hertz. Once mastered, this procedure can be completed within 40 minutes, if it it done properly.
While attempting this procedure, it's important to remember to explore a wide range of stimulus intensities, so the participants will give reliable pain ratings. Using this procedure, it's also possible to conduct experiments with other subjective pain intensities, such as levels well above the pain threshold. This method and its derivatives will enable pain researchers to not just control the intensity of the presented stimulus, but to also take into account the subjective sensation that is perceived by the participant.
After watching this video, you should have a good understanding of how to perform psychophysical estimation of individual pain threshold, and how to use this technique in your own experiments. Don't forget that when working with painful stimulation, care has to be taken to ensure the safety of the participant at all times, and also the experiment has to be approved by the responsible ethics committee.
