The Final Ph.D. Thesis Defense of Steven Hauser, candidate for the Doctor of Philosophy Degree will be held on August 26 at 9:00 a.m. in Olsson Hall 104. The examining committee consists of Gregory Gerling (ESE-Systems, Advisor), Jason Kerrigan (Mechanical Engineering), Håkan Olausson (Linköping University, Sweden), Nicola Bezzo (ESE-Systems), and Cody Fleming (ESE-Systems)
Skin contact interactions and neural encoding mechanisms underlying social and affective touch
Our sense of touch provides an intuitive means of expressing social and emotional sentiment. For example, one might shake the hand of a coworker to show gratitude, or stroke the arm of a romantic partner to express love. When a “toucher” expresses a sentiment by using their hands to physically contact the forearm of a touch “receiver,” populations of thousands of sensory neural afferents in the skin of the receiver respond to this contact in a way that encodes the emotion. However, several steps in this pathway are not yet well understood. In prior efforts, the contact interactions that underlie social touch—i.e., how quickly someone moves their hands, or how they stretch the skin of the touch recipient—have been studied in only a qualitative fashion. Moreover, although certain sensory afferent types are thought to be involved in social touch—such as C-tactile afferents which respond to light, stroking touch—it is not yet clear how these afferents work alongside other mechanosensitive and muscle spindle afferents in encoding emotional percepts. In this work, we employed methods such as motion tracking, psychophysics, and microneurography in order to decipher the contact interactions which readily evoke an emotional response for six common social touch expressions. First, we ran human-subjects experiments to determine how people naturally perform the touch expressions, by employing external motion tracking systems to measure skin-to-skin contact “primitives” such as contact area and velocity of stroking across the arm. Next, we identified “expert” touch patterns for each expression that can be intuitively understood by strangers. Finally, we measured how a set of A-beta and C-tactile afferents responded to this measured physical contact via microneurography and motion tracking experiments performed with collaborators at Linköping University. We found that the six touch expressions could be differentiated by the firing patterns of a single afferent, namely a hair follicle afferent (HFA) or slowly-adapting type II afferent (SA-II), even amidst significant variability in how the touches were performed. Although the responses of the C-tactile were not good at differentiating the expressions, we found them to correlate well with the perceived pleasantness of the touches. A better understanding of the social and emotional touch pathway could allow us to help those with social deficits to perform proper touch gestures, or create augmented means of communicating or recognizing them.