Using Facial Electromyography to Assess Facial Muscle Reactions to Experienced and Observed Affective Touch in Humans

A protocol allows researchers to overcome potential issues, such as recall buys, or communication barriers, paving the way for novel lines of research that may not otherwise be possible. This method allows us to track facial muscle movements in real time. As a result, we can obtain an objective measure of affective response, in addition to self rapport, throughout touch stimulation.

It takes time to learn how to consistently apply the electrodes properly. However, assuring you're collecting quality data takes a lot of time afterwards, during data processing and analysis. Begin by informing the participant of the electrode sensor application process in which, the purpose is to measure muscle and sweat activity during the session.

Then, use water to clean the participant's skin where sensors will be applied. Use an exfoliant scrub to lightly abrade the same areas. Next, apply adhesive collars to the electrode pairs, consisting of two, four millimeter shielded, bipolar recording electrodes, plus one, monopolar reference electrode, so that they adhere to the skin.

After the collar is adhered to the outer rim of the electrodes, fill sensors with a conductive electrode gel, taking care to prevent the formation of air bubbles. Then, affix one electrode directly above the eyebrow along an imaginary vertical line, that traverses the inner corner of the eye. Place the second electrode one centimeter lateral and slightly superior to the first, along the border of the eyebrow.

For zygomatic reactivity, place the first sensor midway, along an imaginary line that connects the upper ear, and the corner of the mouth. Place the second electrode one centimeter medial, towards the mouth, and take care to avoid the masseter muscle. Next, use an eight millimeter, unshielded, monopolar recording electrode, as a reference electrode, and place it in the middle of the forehead, equidistant above the inner brows and below the hairline.

Ensure that the electrode wires are placed such that they do not impede vision. Finally, check that impedance levels are below 20 kiloohms for each electrode with an impedance monitor. Begin by seating the participant in front of the computer with the experimental arm extended laterally, and resting comfortably.

Then, block the participant's lateral view of the arm with a curtain separator. Lastly, administer touch using a 75 millimeter goat hair brush, applied to designated sections marked on the arm. Results indicated that experienced CT-optimal touch was rated as more pleasant than non-optimal touch, regardless of touch location.

Fast, non-optimal touch was always rated as more intense, regardless of whether the touch was experienced. Corrugator response significantly differed between CT-optimal and non-optimal touch, for touch to the arm, but only trend level effects were seen for touch to the palm. On the other hand, zygomatic activity was not significantly affected by experienced touch.

Further, for the second experiment, results indicated that slow, non-optimal touch was rated as less pleasant, and less intense than CT-optimal touch. Similar to fast, non-optimal touch, slow, non-optimal touch elicited robust corrugator activity that was attenuated by CT-optimal touch. Lastly, the EMG responses indicated that during the first 700 milliseconds, there was no difference in corrugator reactivity.

However, over the next 5.6 seconds, corrugator reactivity and response to CT-optimal touch decreased gradually, whereas it gradually increased in response to slow, non-optimal touch. In addition to EMG, you can also collect other psychophysiological measurements, such as skin conductance, or heart rate, at the same time. This would provide you with a more comprehensive psychophysiological profile.