Congenital vision loss may protect against psychosis because of the high stability of an “internal world” characterized by other sensory modalities, according to an article published in Schizophrenia Bulletin.

Dr Thomas A Pollack and Dr Philip R Corlett, researchers at King’s College London in the United Kingdom and Yale University in New Haven, Connecticut, respectively, used a Bayesian prediction error minimization model to illuminate the role of vision loss in psychosis, particularly regarding positive symptoms.

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The Bayesian approach to psychosis in schizophrenia conceptualizes the brain as a “hierarchical…inference machine”: the brain interprets the probability of present events using information gleaned from past stimuli, or priors. The more precise a prior, the more strongly it influences decision making and beliefs at higher levels in the hierarchy. In schizophrenia, this hierarchy is disturbed, and irrelevant stimuli may become “abnormally salient….[to] beliefs higher in the hierarchy.”

Congenital blindness may protect against these “computational deficits” that underlie schizophrenia. Investigators hypothesize that blind individuals experience greater stability of high-level priors, possibly driven by increases in N-methyl-D-aspartate receptor (NMDAR)-mediated signaling. NMDAR-mediated signaling is thought to be “top-down and modulatory” in nature. Visual deprivation has been shown to cause higher-level, multisensory neurons to shift from sensory-driven responses to a more “modulatory influence,” a phenomenon that is likely NMDAR dependent.

According to the researchers, other published data suggest that visual deprivation causes increases in NMDAR-dependent cortical excitability. As such, increased top-down modulatory signaling — associated with stable higher-level priors — is more prevalent in the visually impaired brain. This computational rationale may explain the protective effect congenital blindness offers against psychosis. With more stable priors, blind individuals are less susceptible to the perception abnormalities that characterize schizophrenia.  

Notably, later-life vision loss offers no such protective effect against psychosis. The top-down model cortical hierarchy asserts that the brain is equipped with “hyperpriors,” or internal expectations concerning perceived features of the world. In psychosis, a hyperprior may generate false attributions of the causes of sensory input. According to this same model, hyperpriors also predispose the developmentally typical brain to have visual hallucinations following visual deprivation because the brain expects a cause for sensory data. In the absence of visual information, the brain creates false external attributions, which manifest as hallucinations.

According to this model, investigators outlined several experimentally testable hypotheses, including the theory that congenitally blind individuals will show lessened “psychosis-proneness” compared with their sighted counterparts and lower psychotomimetic response to ketamine. The Bayesian error minimization model may be useful in further efforts to explore the nature of psychosis, vision loss, and visual hallucinations.

Reference

Pollack TA, Corlett PR. Blindness, psychosis, and the visual construction of the world [published online October 11, 2019]. Schizophr Bull. doi: 10.1093/schbul/sbz098