Over the past 60 some years, scientists around the world have been writing about this mystery. They've analyzed past studies, combed the wards of psychiatric hospitals, and looked through agencies that treat blind people, trying to find a case.

It was something Tom Pollak had heard whispers about—an odd factoid, referred to now and again, usually with bewilderment: No person who was born blind has ever been diagnosed with schizophrenia.

More strangely, vision loss at other periods of life is associated with higher risks of schizophrenia and psychotic symptoms. Even in healthy people, blocking vision for just a few days can bring about hallucinations. And the connections between vision abnormalities and schizophrenia have become more deeply established in recent years—visual abnormalities are being found before a person has any psychotic symptoms, sometimes predicting who will develop schizophrenia.

These findings suggest that something about congenital blindness may protect a person from schizophrenia. This is especially surprising, since congenital blindness often results from infections, brain trauma, or genetic mutation—all factors that are independently associated with greater risk of psychotic disorders.

As time goes on, larger data sets have emerged: In 2018, a study led by a researcher named Vera Morgan at the University of Western Australia looked at nearly half a million children born between 1980 and 2001 and strengthened this negative association. Pollak, a psychiatrist and researcher at King's College London, remembered checking in the mental health facility where he works after learning about it; he too was unable to find a single patient with congenital blindness who had schizophrenia.

In 2004, 13 healthy people were blindfolded for 96 hours, and 10 of them reported having visual hallucinations between their first and second day in the dark.

But the whispered-about fact persists: Being born blind, and perhaps specific types of congenital blindness, shield from the very disorders vision loss can encourage later in life. A myriad of theories exist as to why—from the blind brain's neuroplasticity to how vision plays an important role in building our model of the world (and what happens when that process goes wrong). Select researchers believe that the ties between vision and psychotic symptoms indicate there's something new to learn here. Could it be that within this narrowly-defined phenomenon there are clues for what causes schizophrenia, how to predict who will develop it, and potentially how to treat it?

One subject, a 29-year-old woman, saw a green face with big eyes when she was standing in front of where she knew there was a mirror—though she couldn’t see it. Another 24-year-old man, by the end of the second day, was having difficulty walking because of all the hallucinations that appeared to be in his way. He reported seeing "mounds of pebbles, or small stones...and between them was running a small stream of water." By the end of the study, he reported seeing "ornate buildings of white-green marble" and "cartoon-like figures."

We've known for a long time about the link between vision loss and hallucination. Charles Bonnet syndrome, first described in 1760, is a disorder in which people lose their vision and then start to experience hallucinations. These kinds of mental conjurings don't necessarily come with mental illness, though people with schizophrenia have been regularly shown to also have issues with their sight.

Having unusual eye movements, problems with the retinas, unusual blink rates, and other visual aberrations makes it more likely for a person to be diagnosed with schizophrenia. One study found that those vision problems start before a person has their first psychotic episodes, not after.

Yet this association falls away among people born without sight. Pollak and Phil Corlett, an associate professor of psychiatry and psychology at Yale University, have a theory about why—which they published late last year in the journal Schizophrenia Bulletin. It's rooted in the hypothesis that one of our brain's most important jobs is to make predictions about the world.

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This view of the brain argues that rather than perceiving the world around us in real time, our brains create a model of what’s out there, predict and simulate what we experience, and then compare our predictions to what’s actually happening—using any errors to update or change the model in our minds. The accuracy of your past predictions are crucial for the accuracy of your overall model—it's what you're comparing new inputs to, and how you're making any adjustments.