bilateral spectrum

   Also known as bilateral scotoma. The term bilateral spectrum comes from the Latin words bi (two), latus (side), and spectrum (image, apparition). It is used to denote a relatively rare fortification spectrum (i.e. a * scintillating sco-toma), occurring in the context of a *migraine aura, whose development is exactly synchronized in the two hemifields. As a result, the bilateral spectrum may present in the form of a single central or pericentral spectrum. Various types of bilateral spectra (including the * rainbow spectrum) were described in 1904 by the British neurologist Sir William Richard Gowers (18451915). Bilateral negative scotomata (i.e. regions of loss of vision) occasionally develop into transient * blindness. In those rare cases where the tactile cortex is affected as well, a total loss of body-sense (i.e. *acenesthesia) may be experienced. As the involvement of both hemifields in fortification spectra is extremely rare, and cannot be explained with recourse to current hypotheses involving the mediation ofthese 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.