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   The term number form was introduced in or shortly before 1880 by the British scientist Sir Francis Galton (1822-1911) to denote a mental map or configuration of numbers which may appear automatically and involuntarily whenever one thinks of numbers. As observed by Galton, some individuals are capable of conjuring up numerals with a greater or lesser degree of vividness: "Persons who are imaginative almost invariably think of numerals in some form of visual imagery. If the idea of six occurs to them, the word 'six' does not sound in their mental ear, but the figure 6 in a written or printed form rises before their mental eye... There are a few persons in whom the visualising faculty is so low that they can mentally see neither numerals nor anything else; and... there are a few in whom it is so high as to give rise to hallucinations. Those who are able to visualise a numeral with a distinctness comparable to reality, and not in some sort of dreamland, will define the direction in which it seems to lie, and the distance at which it appears to be." In some individuals, Galton found these numerals to be arranged in a peculiar configuration or 'number form'. As he wrote, "The peculiarity in question... consists in the sudden and automatic appearance of a vivid and invariable 'form' in the mental field of view, whenever a numeral is thought of in which each number has its own definite place. This Form may consist of a mere line of any shape, of a peculiarly arranged row or rows of figures, or of a shaded place... The Forms are sometimes variously coloured, occasionally very brilliantly. In all of these the definition and illumination vary much in different parts. Usually the Forms fade away into distinctness after 100; sometimes they come to a dead stop." Galton was under the impression that the capacity to conjure up number forms constitutes an inheritable trait, occurring in about 1 out of 30 adult males, and in 1 out of 15 females, most of whom developed this capacity during childhood. Today number forms are conceptualized as a special form of * synaesthesia, called * number-form synaesthesia. They are classified as a type of * synaesthetic configuration and should not be confused with * grapheme-colour synaesthesia. Pathophysiologically, the mediation of number forms is associated primarily with * cross-activation between regions of the parietal lobe that are involved in numerical cognition and spatial cognition. For other pathophysiological hypotheses, see the entry Synaesthesia.

   References

   Galton, F. (1880). Visualised numerals. Nature, 21, 252-256.

   Galton, F. (1883). Inquiries into human faculty and its development. London: J.M. Dent & Sons.

   Ramachandran, V.S., Hubbard, E.M. (2001). Synaesthesia - A window into perception, thought and language. Journal ofConscious-ness Studies, 8, 3-34.