Also known as fortification figure, fortification of Vauban, geometrical spectrum, herringbone, flittering scotoma, Norman arch, scintillating scotoma, telehopsia, and teichopsia. The introduction of the term fortification spectrum is commonly attributed to the British physician John Fothergill (1712-1780). It is used to denote a * geometric visual hallucination consisting of an extremely bright, sometimes coloured, zigzag line, or 'fortification wall', that may begin near the fovea in one hemifield, and then spread out towards the periphery of the same hemi-field without touching the vertical meridian. This bright zigzag line may have a pulsating, scintillating, flashing, or flickering quality, and it may be followed (or occasionally preceded) by a region of * blindness or negative * scotoma of the same form. The phenomenon has been known and described since ancient times. Some authors credit the German abbess and mystic * Hildegard of Bingen (1098-1180) with providing the first detailed description of the fortification spectrum avant la lettre. A historical description of the fortification spectrum by the British physician and migraineur Hubert Airy (1838-1903) runs as follows. "When it was at its height it seemed like a fortified town with bastions all around it, these bastions being coloured most gorgeously... All the interior of the fortification, so to speak, was boiling and rolling around in a most wonderful manner as if it was some thick liquid all alive." Etiologically, the fortification spectrum is associated primarily with paroxysmal neurological conditions such as migraine with aura, *migraine aura without headache, and epilepsy. Pathophysi-ologically, it is associated primarily with a spread of neural activation (sometimes followed by a neural depression) across a retinotopically organized part of the visual system, notably striate cortex (more specifically, the cortical areas V1 or V3a). The speed of this spreading depressionlike process is estimated as lying between 2 and 3mm/min. The fortification spectrum is commonly classified as a * geometric hallucination. As such, it is believed to come second in a classic series of geometric hallucinations associated with migraine (consisting of *phosphenes, fortification spectra, and intricate, rapidly changing geometrical patterns). When it moves across the visual hemifield, it can also be classified as a * moving scotoma. A relatively rare variant ofthe fortification spectrum is the *bilateral scotoma, which develops symmetrically in both hemifields, and may constitute a central or pericentral sco-toma called a * rainbow spectrum. As noted by the British neurologist Oliver Wolf Sacks (b. 1933), "The existence of such scotomata poses very difficult problems to those who postulate a local, unilateral process as the basis of migraine auras."
References
Fothergill, J. (1784). The duration of the fortification spectrum in migraine. Medical Observations & Inquiries, 6, 103-137.
Levene, J.R. (1975). Sir G. B. Airy, F.R.S. (18011892) and the symptomatology of migraine. Notes and Records ofthe Royal Society of London, 30, 15-23.
Sacks, O. (1992). Migraine. Revised and expanded. New York, NY: Vintage Books.
Wilkinson, F. (2004). Auras and other hallucinations: Windows on the visual brain. Progress in Brain Research, 144, 305-320.
Dictionary of Hallucinations. J.D. Blom. 2010.