The term rainbow spectrum comes from the Latin noun spectrum, which means image, representation, shadow. It is used to denote a relatively rare * fortification spectrum (i.e. a * scintillating scotoma) which may occur in the context of a * migraine aura. The rainbow spectrum appears as an arc that is placed centrally and bilaterally in the visual field. It is classified as a * bilateral spectrum. Along with various other bilateral spectra, it was first described in 1904 by the British neurologist Sir William Richard Gowers (1845-1915). However, it has been speculated that the classical physician Aretaeus of Cappadocia (c. AD 150) refers to the same phenomenon in one of his medical writings. As the involvement of both hemifields in fortification spectra is extremely rare, and cannot be explained with recourse to current hypotheses involving the mediation of these phenomena in a single cerebral hemisphere, the British neurologist Oliver Wolf Sacks (b. 1933) notes that "the existence of such scotomata poses very difficult problems to those who postulate a local, unilateral process as the basis of migraine auras."
References
Gowers, W.R. (1904). Subjective sensations of sight and sound: Abiotrophy, and other lectures. Philadelphia, PA: P. Blakiston's Son & Co.
Sacks, O. (1992). Migraine. Revised and expanded. New York, NY: Vintage Books.
Dictionary of Hallucinations. J.D. Blom. 2010.