Eventually, numerals deteriorated into messy, unrecognizable “spaghetti” blobs—a disaster for someone who did math all the time. And it wasn’t just his work that suffered. He couldn’t read price tags or speed limit signs. At hotels, he had to mark the doorframe of his room with a magic marker.

Yet he could still do mental arithmetic and perform other mathematical operations. [...] He eventually mastered an entirely new digit system (where ⌊ stood for 2, ⌈ for 8, etc.); determined to keep working, he had his computer rigged to present the new numerals onscreen.

[...]

The tests also revealed that RFS’s deficit is not a simple visual malfunction. After all, he could see the foam 8’s shape clearly in certain orientations. Rather, the deficit depended on his interpretation: As soon as his unconscious brain circuits registered a number, everything went haywire. That he could still interpret letters, Schubert says, lends support to the idea that the brain has a specialized module for processing numbers. And his ability to recognize and manipulate new figures as representations of numbers suggests his higher level math abilities remained intact.

From the article:

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