The mysterious case of man who can read letters—but not numbers—exposes roots of consciousness

With a condition that’s “too strange for words,” patient can do mental math but cannot recognize numerals

A man looks at a computer with an 8 and attempts to draw what he sees
A man attempts to draw what he sees when looking at an 8. Numerals fail to register in his brain.SCHUBERT ET AL., PNAS, 117, 16055 (2020)

In the video, the man sounds creeped out. "This is too strange for words," he mutters. He's holding a plate-size, green foam 8. When upright, it looks to him like an incoherent jumble. But when he rotates it 90°, the shape snaps into focus; it looks like "a mask." He begins to rotate the numeral back and forth, watching it melt and cohere over and over. He finally hands it to a nearby scientist, saying, "You gotta take that away."

This man, known as RFS, is the subject of a new case study that would have made neurologist Oliver Sacks proud. RFS can read words and letters just fine. But as researchers report this month in the Proceedings of the National Academy of Sciences, he cannot see numerals at all—at least not consciously. His amazingly specific deficit could help neuroscientists understand how conscious awareness arises in the brain. "What it tells me," says Christof Koch, a neuroscientist at the Allen Institute who specializes in consciousness, "is that … you can get dissociation between cognition and consciousness."

RFS, an engineering geologist in his late 60s, began to experience headaches, amnesia, tremors, and difficulty walking in October 2010. Doctors couldn't determine the cause—they suspected a stroke, then discovered that he had a disease called corticobasal syndrome, which kills off brain cells.

Then, numbers began to look strange to RFS. The 4 on a clock might flip backward, for instance. 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.

Image of an 8 and a jumble of what a person who can't see numbers drew
SCHUBERT ET AL., PNAS, 117, 16055 (2020)

Yet he could still do mental arithmetic and perform other mathematical operations. And strangely, although the digits 2 through 9 were scrambled, 0 and 1 looked normal—perhaps because those digits resemble letters, which RFS could read, or perhaps because they have associations with deep concepts such as absence and unity, which might allow his brain to process them. 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.

In 2011, RFS was referred to a team of neuroscientists at Johns Hopkins University (JHU) led by Michael McCloskey and his then–graduate students Teresa Schubert and David Rothlein. One test the trio ran with RFS involved the green foam 8. Schubert, now at Harvard University, says the team hoped that touching the numeral would help RFS see it, but no. He could feel the figure's curves, but its image remained stubbornly scrambled—probably, Schubert says, because the brain prioritizes sight over other senses. And feeling one thing but seeing another left RFS disturbed, McCloskey says. "He understood that he could contribute to scientific knowledge, and he was willing to put up with the testing. But it was not pleasant."

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.

Perhaps more important, RFS's deficit could shed light on how conscious awareness arises. In another test, the JHU scientists showed him large numbers and letters with tiny drawings of faces embedded inside them. When viewing the letters with embedded faces, RFS reported seeing both. Moreover, an electroencephalogram (EEG) recorded a characteristic brain wave called N170 that's strongly associated with seeing faces.

In contrast, when shown numbers with embedded faces, the number's effect apparently swamped that of the face: RFS reported seeing neither; everything looked like spaghetti. Yet an EEG still showed the characteristic N170 spike for registering faces. Somehow, his brain was still processing and identifying a face—a fairly high-level skill—even though his conscious mind was oblivious. This deficit shows high-level cognitive processing and consciousness are distinct, Koch says. "You can get one without the other."

Sara Ajina, a neuroscientist at the University of Oxford who studies visual awareness deficits such as blindsight—a residual, unconscious "sight" in people with damage to the brain's visual system—says she's often skeptical about how much scientists can learn from single case studies. But she praised the work with RFS for moving beyond just his basic number deficit to exploring how it affected face recognition and other higher level abilities.

Still, Ajina wishes the JHU team had embedded a wider variety of stimuli in the numbers. For instance, could RFS have noticed movement within the numbers, as opposed to static drawings? Given the human brain's emphasis on sight, perhaps movement could break through the number tangles and penetrate his conscious awareness.

Sadly, such questions will likely be impossible to answer: RFS's health has deteriorated recently, limiting his ability to speak and move. But his unique deficits and willingness to endure unsettling tests have already taught researchers a lot. "It's a very nice demonstration of all that awareness requires," Schubert says—as well as a reminder of just how fragile that gift is.

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