In 1950, researchers Hector Chevigny and Sydell Braverman set out to, as they put it, “demolish old fables about the emotional life of the blind” and demonstrate that the mental health issues of the blind are no different from those experienced by the sighted.
But they did discover one big — and very surprising — difference: There have been no reported cases of schizophrenia in people blind from birth (or who became blind very shortly after birth). At the time, there was limited patient data available, so it wasn’t clear if this astonishing finding would hold up. But in the almost 80 years since, more national databases of mental illness have been maintained, and still no cases have been found, according to a study in Frontiers in Psychology.
Is it just a coincidence? Or does never having been able to see somehow offer protection from schizophrenia? And if it does, how would that even work?
Read More: Can Animals Get Schizophrenia, or Is It Unique to Humans?
Schizophrenia and Departures from Consensus Reality
Schizophrenia is a neurodevelopmental disorder that interferes with the way people interpret reality, Philip Corlett, a neuroscientist at Yale University who studies psychosis and delusional thinking, told Discover. Schizophrenia can cause symptoms such as hallucinations, disorganized speech and thinking, and in some cases, a lack of motivation or engagement with the world.
However, the most familiar characteristic of the illness is what Corlett described as “departures from consensus reality,” or, put another way, believing things that most people in your culture don’t believe.
Though the causes and mechanisms of schizophrenia are not yet well understood, one increasingly accepted theory is that the illness results from errors in prediction. To understand that, we need to take a look at how the healthy brain processes information.
How the Brain Makes Predictions
We tend to think of our brains as something like extremely sophisticated video recorders. But over the past few decades, neuroscience has shown that’s not quite how our brains work. What really happens is that the brain predicts what it expects reality to be, based on what it knows from prior experience.
Then, in a series of iterations, it tweaks that prediction as sensory data becomes available. Sussex University neuroscientist Anil Seth calls this process “controlled hallucinations.” For people with schizophrenia, however, the hallucinations are not always controlled.
What goes awry with the perceptions of people with schizophrenia? One theory, developed by Corlett and others, is that the illness results from prediction errors. People with schizophrenia are likely to give too much weight to sensory input that the rest of us might discount or even ignore.
In one experiment, Corlett and colleagues conditioned study participants to hallucinate, something that he points out is not difficult to do, according to a study from Cambridge University Press. The experiment was simple enough. A visual stimulus, such as a flashing light, was paired with a tone. After several rounds, people began to perceive the tone when they saw the flash of light, even if the tone wasn’t present.
However, people who were already prone to hearing voices in their heads were 10 times more likely than other participants to hallucinate a tone in the lab. The fact that people who hear voices in their heads report hearing the tone much more frequently suggests that they have a bias toward expecting auditory stimulation, Corlett explained.
A Differently Wired Brain
So what does this have to do with blindness? The visual system is the area of the brain most extensively connected with other parts of the brain. It’s important not just for seeing but also for attention, learning, and even processing emotions. This prominence of the visual system is why it plays an important role even in auditory hallucinations.
The visual system is extremely important when it comes to psychosis tendencies because it’s important for creating expectations, Corlett said. We base our expectations on previous experience, and for sighted people, that experience is predominantly visual. However, when the visual cortex receives no input, as in people blind from birth, the brain reorganizes itself along different pathways for processing information. That may mean it has different ways of making predictions, ways that are less prone to delusion, according to a study in Biological Psychiatry.
“If you don't have the visual system, which is a dominant way of making predictions, then maybe you don't have this over-expectation of auditory stimuli, or maybe your expectations about auditory stimuli are different,” Corlett told Discover. “And that might protect you against this subsequent development of psychotic symptoms.”
Experts are still not sure why there have been no documented cases of schizophrenia in people blind from birth, but this strange but true fact does fit with what neuroscience is learning about how the mind constructs reality.
Read More: Hearing Voices and the Inner Monologue Can Get Mixed Up for Those with Schizophrenia
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Frontiers in Psychology: Cognitive and Neuroplasticity Mechanisms by Which Congenital or Early Blindness May Confer a Protective Effect Against Schizophrenia
- This article references information from a study published by Cambridge University Press: "Whatever next?" Predictive brains, situated agents, and the future of cognitive science
- This article references information from a study published by Biological Psychiatry: The Predictive Coding Account of Psychosis
