Number form

Number forms were first documented and named by Sir Francis Galton in his The Visions of Sane Persons.

[4][5] Since the areas that process numerical and spatial representations are close to each other, this may contribute to the increased cross-activation.

Synesthetes display larger P3b amplitudes for month cues compared to non-synesthetes, but similar N1 and P3b responses for arrow (← or →) and word (left or right) cues.

The reaction times for valid cues are smaller than invalid cues (words and arrows), but in synesthetes the response time differences for months are larger than those of non-synesthetes.

Although this form of synesthesia has not been as intensively studied as grapheme–color synesthesia, Hubbard and colleagues have argued that similar neural mechanisms might be involved, but acting in different brain regions.

A number form from one of Francis Galton's (1888) subjects. Note the convolutions, and how the first 12 digits correspond to a clock face. Number forms are idiosyncratic to the person experiencing them.