hat illusion

   Also referred to as hat-wearing illusion. Both terms are used to denote a "tactile hallucination or " illusion of a band exerting pressure around the head. It would seem that the term was introduced in 1962 by the American psychologists Harold L. Williams et al. to denote a peculiar phenomenon reported by some 20% of the participants in an experiment involving " sleep deprivation. As Williams et al. noted, "The subjects usually labelled this 'wearing a hat,' and often were observed making repeated efforts to remove the nonexistent hat." The notion of hat illusion should not be confused with the notion of "top hat illusion, which refers to a "geometric-optical illusion associated with the hat of the former US president Abraham Lincoln (1809-1865).

   References

   Williams, H.L., Morris, G.O., Lubin, A. (1962). Illusions, hallucinations and sleep loss. In: Hallucinations. Edited by West, L.J. New York, NY: Grune & Stratton.