Multi-Modal Signals for Analyzing Pain Responses to Thermal and Electrical Stimuli

The data acquired with the presented method may shed light on how to recognize pain intensities, independently of the underlying pain model. It may also help to assess the pain duration. The main advantage of this technique is its multimodality in terms of signal recording and pain stimulation.

It significantly broadens the scope of pain research. This technique can be applied to monitor for pain presence in pathology treatment for more efficient pain management. The execution of he protocol requires many complex manual actions, such as electrode application, which are faster and easier to learn through visual demonstration.

Demonstrating the procedure will be Sandra Gebhardt, a student from my laboratory who is trained as an experimenter for this study. To prepare for the procedure, welcome the arriving subject and lead her to the calibration room. Inform her in detail about the calibration procedure and the following pain stimulation.

Next, conduct the pain stimulation in a camera-monitored, temperature-controlled and low-noise experimental room next to the calibration monitoring room. Use appropriate recording computers, software, and recording devices for capturing physiological data, audio, videos, and thermal and electrical stimulator outputs during the experiment. Enter the values of the phasic electrical and heat pain intensities and the tonic electrical and heat intensities calculated based on the performed calibration in the thermal and electrical stimulator.

Set the baseline temperature to 32 degrees Celsius and the temperature rate to 8 degrees Celsius per second, then save all settings. Subsequently, prepare the pain elicitation script, as shown here. Set the number of each phasic stimulus intensity to 30 and the number of each tonic stimulus intensity to one.

Then, set the duration of each phasic stimulus to five seconds and the duration of each tonic stimulus to 60 seconds. Randomize the order of all stimuli and the pauses between the phasic stimuli to eight to 12 seconds. After that, set the pauses after the tonic stimuli to 300 seconds.

To begin pain stimulation, ask the subject to lie down comfortably on the examination couch. Clean all skin areas where the electrodes will be attached with alcohol solution. Remove any dead skin cells on the surface of the left cheek, behind the left ear, above the left eyebrow, and on the forehead with abrasive gel.

Then, re-clean these areas with an alcohol solution. For the measurement of the skin conductance level, attach two pre-gelled nonpolorizable silver, silver chloride electrodes to the underside of the distal phalanx of the right index and middle finger with Velcro straps. To record ECG, use three pre-gelled adhesive silver, silver chloride snap electrodes with circular contact areas.

Place the cathode electrode on the chest, approximately six centimeters below the right collarbone and the anode electrode on the left 9th and 10th rib. Attach the reference electrode to the right side waist next to the pelvic bone. To record EMG of of musculus trapezius, also use three pre-gelled adhesive silver, silver chloride snap electrodes with circular contact areas.

Place the cathode and anode electrodes side by side on the trapezius muscle at the left side of the neck. Then, place the reference electrode below, on the left collarbone. Use six reusable shielded silver, silver chloride electrodes to measure the EMGs of musculus corrugator supercilii and musculus zygomaticus major.

Attach one side of the double-sided adhesive collars to the electrodes. Subsequently, fill the cavities of the electrodes with electrolyte gel. Remove the paper backings on the other side of the double-sided adhesive collars.

For musculus corrugator supercilii, place the anode electrode directly above the left eyebrow, next to the glabella line. Then, place the cathode electrode one centimeter lateral to the anode electrode. Attach the reference electrode to the middle of the frontal bone, just below the hairline.

For musculus zygomaticus major, draw an imaginary line from the left oral commissure to the left ear lobe. Place the anode electrode slightly below the middle of the line and the cathode electrode one centimeter medial next to it. Attach the reference electrode to the left mastoid.

Then, connect all the electrodes to the corresponding inputs of the biosignal recording device. Perform a visual check, using the biosignal recording software to make sure all physiological signals are of good, excellent quality. Next, place the silver, silver chloride anode electrode on the upper side of the intermediate phalanx of the left index finger and the cathode electrode on the upper side of the proximal phalanx of the left middle finger.

Connect the electrodes to the electrical stimulator. Apply the thermode to the upper side of the subject's left forearm, about 30 millimeters proximal to the wrist, with a Velcro strap. Start all the cameras.

Ensure that the participant is perfectly visible in the camera. Then, start all the recording devices, complying with the requirements for data synchronization. Run the pain elicitation script.

Carefully monitor the subject and the progress of the pain stimulation part. After the ending of the pain elicitation script, stop all the recording devices. Check if the participant is all right and detach all the electrodes and the thermode.

Next, wrap a hygienic, non-woven towel around a cold gel pack. Ask the participant to apply it on the skin area where the thermode was placed for at least five minutes. Then apply ointment to the skin area where the thermode was placed.

Afterward, thank the participant and say goodbye. Dispose of all the disposable electrodes. Clean all the reusable electrodes, and the examination couch.

Shown here are the images of the subject before, during, and after an intense pain stimulus. All the recorded signals are non-filtered and synchronized in time. For clarity, only representative screenshots of the video signals are shown.

While attempting this procedure, it's important to ensure all signals are optimized before starting the pain stimulation. Also ensure that all the recording devices are on before running the pain elicitation script. Following this procedure, other methods, like implementing a near infrared night vision camera, or recording for biosignals can be performed in order to recognize pain more efficiently in a nightly environment.

This technique allows for recognizing pain objectively without a need to communicate with the patient. It may be helpful to distinguish different pain properties and provide insight to the underlying source of pain. Pain stimulators can cause serious injuries if not operated by trained personnel.

Always follow the ethical and safety restrictions regarding stimulus intensity cutoffs to ensure the participants'safety.