In February 2019, an intriguing study made the rounds. Researchers from the University of Western Australia found conclusive evidence that congenital blindness is protective against schizophrenia.
The unusual discovery offered new insights into the inner workings of schizophrenia, a condition which still has many unknowns, despite decades of research.
"This leads us to think there is a link that must be explored," said Professor Vera Morgan from the UWA Neuropsychiatric Epidemiology Research Unit, lead author of the study.
Now, a new study sheds new light on that phenomenon and could help us better understand how schizophrenia works -- as well as how we build a mental model of the world around us.
The mind's eyes
The relationship between vision and psychosis is complex. While congenital visual loss appears to be protective against psychotic disorders, gradual visual loss can give rise to hallucinations or even psychotic episodes.
The age at which vision loss happens appears to be a crucial factor. The absence of vision at birth or very early in life appears to have a protective effect, whereas later-life visual loss appears to predispose one to the development of psychotic symptoms. This is particularly unusual as there are very few medical disorders that may be protective against psychosis (the already-notorious example is rheumatoid arthritis).
Researchers suspect that it's not just the loss of sight that is at work here -- it's also the way the other senses and the brain are rearranged after vision loss. For instance, the brains of blind people adapt to sharpen the other senses, which they are also better at processing.
When something interferes with a person's vision, it can also send all kinds of confusing signals to the brain. If a person with functioning eyesight is blinded, the auditory and tactile information that he or she receives will be chaotic, confusing, and potentially overwhelming. For a blind person, that just doesn't happen, as the brain is used to processing this sort of information. Simply stated, their internal model of the world is simpler and more resilient to malfunctions.
"In simple terms, we argue that when people cannot see from birth, they rely more heavily on the context they extract from the other senses," the researchers write. They also add that this can also explain the
relationship between later visual loss and other psychotic
symptoms, as well as the effects of visual deprivation and
hallucinogenic drugs on psychosis and schizophrenia.
While this is still somewhat speculative, this hypothesis would explain the effect. If this is true, then it would mean that congenitally blind people have overall lower psychosis susceptibility than the sighted population. There are also ways to test this, researchers say.
Congenitally blind individuals will show fMRI prediction error responses during causal learning, unlike people with psychosis whose prediction error is aberrant. In addition, they will experience reduced psychoactive effects from drugs such as ketamine
The study has been published in Schizophrenia Bulletin.