Cerebral torque is human specific and unrelated to brain size

Abstract

The term “cerebral torque” refers to opposing right–left asymmetries of frontal and parieto-occipital regions. These are assumed to derive from a lateralized gradient of embryological development of the human brain. To establish the timing of its evolution, we computed and compared the torque, in terms of three principal features, namely petalia, shift, and bending of the inter-hemispheric fissure as well as the inter-hemispheric asymmetry of brain length, height and width for in vivo Magnetic Resonance Imaging (MRI) scans of 91 human and 78 chimpanzee brains. We found that the cerebral torque is specific to the human brain and that its magnitude is independent of brain size and that it comprises features that are inter-related. These findings are consistent with the concept that a “punctuational” genetic change of relatively large effect introduced lateralization in the hominid lineage. The existence of the cerebral torque remains an unsolved mystery and the present study provides further support for this most prominent structural brain asymmetry being specific to the human brain. Establishing the genetic origins of the torque may, therefore, have relevance for a better understanding on human evolution, the organisation of the human brain, and, perhaps, also aspects of the neural basis of language.

Automatic analysis of cross‐sectional cerebral asymmetry on 3D in vivo MRI scans of human and chimpanzee

“…In the chimpanzee brain, we did find a segment in part of the Broca's area showing significant rightward width asymmetry (but not perimeter asymmetry) and which may correspond to the Broca's cap that projects more laterally on the right compared to the left in both human and chimpanzee brains (Balzeau, Gilissen, Holloway, Prima, & Grimaud-Herve, 2014), profiles of both the cross-sectional width and perimeter appeared identical in both cerebral hemispheres, particularly in the posterior region. These finding do not support the reports of the presence of the torque in great ape brains (Hopkins & Marino, 2000;LeMay, Billig, & Geschwind, 1982) but is consistent with studies that concluded that the cerebral torque is a new feature in the hominin lineage (Holloway & De La Costelareymondie, 1982;Xiang et al, 2018Xiang et al, , 2019Zilles et al, 1996). It is also worth noting that we did find a segment in part of the Broca's area showing significant rightward width asymmetry, but not perimeter asymmetry asembling the human brain and this region may correspond to the Broca's cap which was reported to project more laterally on the right compared to the left in both human and chimpanzee brains.…”

“…Gratiolet and Leuret () reported there to be a greater gyrification of the left frontal and right occipital regions relative to the contralateral hemisphere and Best () later proposed the existence of three growth gradients that emerge early in the embryo. The growth gradients along the dorso‐ventral and antero‐posterior axes have been found to be specific in the human brain in our previous work (Xiang et al, ). The present study additionally suggests that torque asymmetry in the left‐right direction is also of late origin in the past 6 million years.…”

“…On the basis of absence of torque characteristics in our previous studies (e.g. Xiang et al, , Xiang, Crow, & Roberts, ), we hypothesize that asymmetry in the tissue distribution will only be present in the human brain.…”

Large-scale Phenomic and Genomic Analysis of Brain Asymmetrical Skew

“…An early study reported that the left hemisphere was shifted dorsally relative to the right [20], but recent work based on MRI analysis of 91 human brains found the opposite pattern, i.e. the left hemisphere shifted significantly downward relative to the right [9]. Again, this pattern was reported to be human specific, in comparison to chimpanzees [9].…”

“…Torque has been suggested to be a human-specific feature of the brain, through comparative work with chimpanzees [9,10]. There is also evidence for alterations of torque in cognitive and neuropsychiatric disorders, including developmental stuttering [11], dyslexia [12], schizophrenia ( [13][14][15]; but see [16]), attention-deficit/hyperactivity disorder [17], and depression [18,19].…”