Stearns (1972) discusses the physiology of laughter and tickling; a few of his points are summarized here. Laughter is physiologically spasmodic, rhythmic, vocalized, expiratory, and (when due to tickling) involuntary. Stearns (1972, chapter I) discusses in some detail the neural pathways of the tickle-laughter reflex arc. Regarding the structure of tickling, Stearns (1972:4) defers to Houssay (1951:p849), who ``contends that [tickling] is due to `simultaneous excitation of both touch and pain receptors'... because tickling cannot be produced after section of the spinothalamic tract (which does not interfere with touch sensation); also, tickling cannot be provoked when the circulation of an extremity is arrested, which first eliminates the sensation of touch, and with it the tickling sensation. The pain sensation is eliminated later.'' In short, tickling involves simultaneous sensation of touch and of pain. This is perfectly isomorphic to the elements N and V of the present theory of humor.
Pain, after all, is a violation of physical integrity and comfort; these are principles which we certainly care about quite viscerally. Pain is essentially a sensory representation of a violation of one's body's natural order. It represents V, a violation of moral principle, reduced to the level of a physiological response to a physical stimulus. Touch sensations, on the other hand, provide an internal representation of the external, touched stimulus for the organism to process. This representation of the stimulus is painless by itself; it is a representation of a normal contact with a stimulus, N.
The fact that tickling requires a sensation of pain as well as a ``normal'' touch sensation, is a remarkable piece of evidence that appears to support the present theory of humor. The physiology of tickling is actually a restatement at the physiological level of the present theory of humor. Indeed, this suggests that physical tickling and more cerebral and cognitive forms of humor may have the same basic representation in the human nervous system, and that biological implementations of the two may at least be evolutionarily related.
It should be pointed out in making this comparison that the tickle response is not a purely physiological reflex response. While tickling of the type, ``research scientist applies feather to plantar surface of foot'', may be thought to be purely physiological, there are kinds of tickling which clearly involve other mechanisms. Some people, for example, may be tickled without actually being touched. Such cases appear to involve a perceived attack in combination with a perceived lack of a real threat. Also, some people are simply ``not ticklish''. Finally, one of the most robust and mysterious facts about ticklishness is that people usually cannot tickle themselves, but rather can only be tickled by some other agent. It would seem that the tickle response is not an innate physiological reflex, but involves something else that is possibly learnable, presumably cognitive. I suggest that this something is the judgement that one is being physically attacked in some way: a perceived fake attack. A perceived attack is obviously a violation of physical integrity and corresponds to a V interpretation. The falsity of the attack allows for a predominating N interpretation at the same time. The findings above follow from this suggestion: First, people for whom no sense of violation, invasion, or attack is evoked by light stimulation on footsoles, armpits, etc., will not be ticklish; conversely, ticklish people, on this account, are not of this character. Second, people who perceive an attack ``in fun'' may have a tickle response without actual touch, simply because the requisite judgment - a perceived fake attack - is present. And third, people ought not to be able to tickle themselves, either, since and to the extent that it is impossible to convince oneself that one is attacking oneself. You can't fake an attack on yourself; you see through it every time. So both the physiological facts regarding the tickle response, and the more psychological findings are fully consistent with the present theory of humor.