For Daniel Tammet, every number between 0 and 10 000 has a unique color, shape and texture. Complicated numbers take the form of landscapes." "he sees letters and numbers in different colors, shapes, textures and sizes. Words and numbers are associated with feelings and experiences.

https://spraktidningen.se/artiklar/sa-funkar-ett-sprakgeni/

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Edit: The web page is in Swedish and I thought it originally was available to read in full, but now it's asking for a login. And I cannot find an archived full page.