Cerebral asymmetry and the lateralization of language: core deficits in schizophrenia as pointers to the gene
- Timothy J. Crow
Purpose of review
Cerebral asymmetry (the torque from right frontal to left occipital) is the defining feature of the human brain, and as Broca proposed, the putative neural correlate of language. If as has been suggested schizophrenia is the price that Homo sapiens pays for language, the torque together with its functional correlates is of central significance. Recent evidence from anatomical, functional and genetic studies is reviewed.
Recent findings
Both post-mortem and anatomical imaging studies show evidence of a reduction or reversal of aspects of asymmetry particularly in the occipito-temporo-parietal association cortex. In some studies there is an interaction with sex. There is evidence that change in the left temporal lobe is sometimes progressive. Functional studies add substance to the concept that the lateralization of language is reduced and in some aspects reversed.
Summary
The dimension of asymmetry stands out as the variable that can make sense of observations across fields of investigation, and provides a key to the genetic basis of psychosis. Discordant monozygotic twin studies indicate strongly that the relevant variation is epigenetic; this is consistent with the possibility that the variation is related to recent structural changes (the Xq21.3/Yp duplicative transposition) on the sex chromosomes.
