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.
Highlights
What neural structure has allowed humans to exceed other animals in “cognitive ability”? Two hypotheses have been proposed
Sex effects were present in both humans [F(3,87) = 24.22, p < 0.0005] and chimpanzees [F(3,74) = 7.16, p < 0.0005], the normalized difference between the sexes is comparatively smaller in the chimpanzee than in the human brain
Two main findings distinguish the human from the chimpanzee brain: (1) larger brain size with preferential expansion in the antero-posterior and dorso-ventral axes relative to the left–right axis and (2) the torque whereby the right-frontal and left-occipital poles are more prominent than the contralateral poles; the left-occipital pole is shifted downward compared to the right and bends towards the right side and dimensional asymmetries whereby the left hemisphere is elongated and with reduced height compared to the right
Summary
What neural structure has allowed humans to exceed other animals in “cognitive ability”? Two hypotheses have been proposed. The shape asymmetry is proposed as a potential substrate for hemispheric specialization of function, including left hemisphere dominance for language. Some authors suggested that chimpanzees share the patterns of asymmetry with humans, though to a lesser degree (Gomez-Robles et al 2013). This classical Darwinian concept of gradual transitions over long periods of time cannot explain the gap in functional abilities, relating to language and handedness between humans and chimpanzees. These issues require further investigation using state-of-the-art image analysis techniques
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