Blindness and Schizophrenia: The Exception Proves the Rule

Congenital cortical—but not peripheral—blindness protects against schizophrenia.

Posted Nov 21, 2014

In a previous post, I showed how the diametric model of mental illness could explain why congenital blindness appears to prevent schizophrenia. Since then, a parallel implication of the diametric model of mental illness has found unexpected confirmation in the finding that, in a population of almost 5 million Danes, smaller birth size increases risk of a psychotic spectrum disorder, while reducing that of an autism spectrum one, and vice versa for larger birth size. My original suggestion was similar: because congenital blindness is known to increase risk of autism, it must also by the logic of the diametric model, reduce—or perhaps even eliminate—the risk of psychosis

But now a new study by Evelina Leivada and Cedric Boeckx published in Frontiers in Human Neuroscience finesses the finding and suggests that there is an exception which seems to prove the diametric rule. Crucially, these authors make a distinction between congenital cortical blindness (CCB), where the lesion causing blindness is in the brain, and congenital peripheral blindness (CPB), where it is in the eye. Although the authors identified some cases of congenital blindness and schizophrenia, they were all of the peripheral type and their “research did not identify a single case of CCB and schizophrenia.” 

Leivada, E. and Boeckx, C. (2014).
Source: Leivada, E. and Boeckx, C. (2014).

How is this to be explained? The authors offer two suggestions. First, they follow previous studies in pointing out that genes implicated in CPB show multiple connections to genes implicated in schizophrenia, for example OTX2. (Diagrammed above extreme left, where different lines between nodes represent different types of evidence for the reported association between genes: black is for co-expression, dark blue for co-occurrence, light blue for databases, dark green for neighborhood, light green for text mining, pink for experiments, red for gene fusion, purple for homology. Protein nodes are colored and sized randomly). Another critical gene is Disrupted-in-Schizophrenia 1 (DISC1, diagrammed extreme right): “one of the most frequently discussed susceptibility loci for schizophrenia.” The authors point out that DISC1 plays a role in  Bardet-Biedl syndrome, the genetic basis of which “suggests that this is a syndrome that may manifest comorbidity between schizophrenia and retinitis pigmentosa” (a major cause of CPB).

The authors' second suggestion is that CCB—and to a lesser extent CPB—results in secondary cortical compensations protective against schizophrenia linked to the role of the thalamus (other brain areas involved are the occipital cortex, the lateral geniculate nucleus and the pulvinar).  One  finding that might illustrate this effect is that, as these authors comment, “patients with CPB are significantly better than sighted individuals in ultra-fast speech perception.” But, I would add, not so autistics, who typically are sometimes thought to be deaf thanks to their evident deficits in perceiving and reacting to speech, despite evidence of heightened sensory sensitivity overall. 

The authors point out that “many thought-disordered patients with schizophrenia have a tendency toward over-abstraction and over-inclusion in their thinking, as well as overly elaborated (and more easily primed) semantic networks.” This is what I would term hyper-mentalism, and according to the diametric model of mental illness, it is the central pathology in all psychotic spectrum disorders, notably paranoid schizophrenia. They add that congenitally blind “children have the opposite tendency in being characterized by a lack of over-generalization of concepts and categories, and a reduced number of word inventions.” This, by contrast, is symptomatic of what I would call hypo-mentalism, and according to the diametric model is the core pathology of autism spectrum disorders.

The implication of these findings for the imprinted brain theory and its distinctive diametric model of mental illness is that the reason that CCB protects against schizophrenia is that it is associated with autistic, hypo-mentalistic brain function, as I argued in my original post. CPB, however, proves the rule by sometimes producing its opposite: hyper-mentalism, via genetic comorbidity linking peripheral blindness and schizophrenia. In this case, a few small exceptions fit the diametric model just as neatly as do oppositely-oriented trend lines in huge data sets like the Danish one

(With thanks and acknowledgement to Evelina Leivada.)