Cerebral torque has been described as a right-left asymmetry of the frontal and parieto-occipital regions of the brain. It is associated with an increase in inter-hemispheric connectivity and the subsequent development of language. T.J. Crow has postulated links between cerebral torque and functional psychoses; with a focus on schizophrenia. Connections between psychotic disorders and language have been made where disordered speech, auditory hallucinations, and functional or structural deficits in language areas of the brain are seen in diagnosed individuals.
T.J. Crow hypothesises that cerebral torque arose after a genetic speciation event occurring 5-6 MYA – around the same time Homo sapiens diverged from the Pan lineage. This is thought to have triggered a chain of events mitigating the formation of the Xq21.2/Yp11.2 homology block, where a paracentric inversion occurred on the Y chromosome 160 KYA and cerebral torque is hypothesised to have formed. It is this timeline of genetic events that initially implies that torque is specific to Homo sapiens, as they only occur after the Homo-Pan split. However, contrary to Crow’s theory, asymmetries of the brain have been documented in nonhuman primates since the 1970’s as new technologies such as functional magnetic resonance imaging (fMRI) and computed tomography (CT) came into use for imaging the brain. Neurological asymmetries have been found in ape species Gorilla, Pan and Pongo, as well as monkey species Macaca mulatta; Macaca fascicularis; Saguinus oedipus, and Callithrix jacchus.
If theorised correlations between hemispheric asymmetries and psychosis are to be believed, evidence for such asymmetry in nonhuman primates could point towards their possession of an anatomical foundation for developing psychotic-type behaviours.
This research examines the genetic and anatomical precursors of psychosis through a meta-analysis of existing literature, genome comparison studies and examination of CT scans of human and primate brains. It further investigates the hypothesis that African great apes (Gorilla spp., Pan troglodytes, Pan paniscus) possess the neuroanatomical prerequisites for developing psychosis, and explores the question as to whether or not ancient human species (Neanderthals and Denisovans) also possessed them.