Number form

A number form is a mental map of numbers, which automatically and involuntarily appears whenever someone who experiences number-forms thinks of numbers. Numbers are mapped into distinct spatial locations and the mapping may be different across individuals. Number forms were first documented and named by Sir Francis Galton in his The Visions of Sane Persons (Galton 1881a). Later research has identified them as a type of synesthesia (Seron, Pesenti & Noël 1992; Sagiv et al. 2006).

Neural mechanisms

It has been suggested that number-forms are a result of "cross-activation" between regions of the parietal lobe that are involved in numerical cognition and angular gyrus for spatial cognition (Ramachandran & Hubbard 2001; Hubbard et al. 2005). Since the areas that process numerical and spatial representations are close to each other, this may contribute to the increased cross-activation. Compared to non-synesthetes, synesthetes display larger P3b amplitudes for month cues, but similar N1 and P3b responses for arrow (<- or ->) and word (left or right) cues. (Teuscher et al. 2010).