Silvan S. Tomkins (1911-1991) and scholars advanced the understanding of facial expressions and their connection to emotions in affect theory in confluence with the etho- and ethnological Darwinian traditions. While Tomkins at first attributed the origins of feelings to the mimical muscles, he soon realized that the overlying skin, as moved by the muscles, was the actual, not just facial, agent of feelings. Variants of the contested hypotheses on Emotional Facial Feedback (EFF) struggled since, while basic mostly clinical research on the sensory trigeminal (TGS) and the facial motor system (CN7S) couldn't offer a (patho-)physiology of affect concerning the lay experience of emotions to be felt around (activated) mimical muscles in a variant of soft touch - in a way similar to how "feelings" are elicited by emotional mental content alone. Here a broad psycho-physiological review concludes on three explanations: 1). the many "anastomotic" tracts with close adjacency of motor and sensing branches point to a simulating "shortcut" from the CN7S to the TGS thus just faking "feeling" without ensuing contraction – since branches of CN7S can just be alerted by readiness to move. This could be due to an orthogonal gaseotransmission by H2S and NO, also regulating its axonally transported enzymes (nNOS e.g.). 2). The very circumscribed sensorial areas in the face creating specific EFF gradients could function more precisely and adapting as a "somatotopic grid" by recruiting the second "onion-shaped" dermatomes of sensorial defluence providing "pain by rate", but also localization. 3). Merely intended movements of the jaw (via preparatory potentials) possibly provide psychological localizations of emotional and other semantic meanings within the Cartesian abstract mental space - limited to 4=3+1 (time) dimensions sustaining the role of movement in future AI. Self-constituting and empathic automatic mimicry and touch interactively point to their core clinical disturbances in "borderline personality" amenable to trigeminal inflammations e.g. at the cavernous sinus.
Gottfried R.S. Treviranus (Wed,) studied this question.