In the past several years, I have had occasional episodes of a false sense-perception that tiny insects or insect-like creatures are crawling on me. The technical term for such experience is
formication, loosely adapted from
formicate, which means crawl like an ant (from the Latin
formica, meaning ant), though in my case the sense-perception is that of being crawled upon by something such as fleas or mites.
I use the term
sense-perception because, in my case, an underlying sensation is real; the falsity is in how that sensation is unconsciously interpretted. The sensation is actually produced by an allergic reaction. My skin is allergic to many things in my present environment. I don't know what all of these allergens are, but normally I can keep my discomfort and other symptoms at a low level by avoiding detergents and scented cleaning products. However, sometimes a threshold is exceeded, perhaps by something I recognize, perhaps not, and one consequence might be sensations so much like those of having tiny bugs crawling on me that my unconscious mind signals that just that is happening. None-the-less, when I can see some of the areas in question, no creatures (other than myself) are visible. Further, the inferred creatures don't make any progress; it is as if they are crawling-in-place. And, finally, wounds are not later discoverable on these sites. (As it happens, an episode can be triggered by an actual insect bite, but then further wounds are not found.)
Being rational in how I consciously interpret the sense-perception doesn't seem to cause it to change. That's probably because, for most of my life, when I felt as if creatures were crawling on me, it was because I had creatures crawling on me. Years of neural networking would have to be revised for my brain not first to think that bugs were crawling on me. And a practical adaptation would account for the possibility, when the sensation were felt, that bugs were crawling on me and warranted a swift response.
If I were less rational or didn't have other awareness of my allergies, it would be natural for me to conclude that, while I could not see tiny creatures, they must never-the-less be crawling on my skin or perhaps just below its surface. Let's add to such inclinations that most of those who would deny that these creatures were crawling on-or-under my skin would themselves get matters fundamentally wrong. In particular, failing to distinguish the raw sensation from the sense-perception, most people would deny the reality of the former, not only in their protestations but in their attempts to locate the dysfunction. It wouldn't be all in my head.