Neuro-evolutionary evidence for a universal fractal primate brain shape

Abstract

The primate cerebral cortex can take on a bewildering diversity of shapes and sizes within and across species, whilst maintaining archetypal qualities that make it instantly recognisable as a "brain". Here we present a new way of expressing the shape of a cortex explicitly as the hierarchical composition of structures across spatial scales. In computational simulations, as one successively removes sulci and gyri smaller than a specified scale, the cortices of 11 primate species are gradually coarse-grained into less folded brains until lyssencephaly (no folding). We show that this process, in all cases, occurs along a common scale-free morphometric trajectory overlapping with other mammalian species, indicating that these cortices are not only approximately fractal in shape, but also, strikingly, are approximations of the same archetypal fractal shape. These results imply the existence of a single universal gyrification mechanism that operates in a scalefree manner on cortical folds of all sizes, and that there are surprisingly few effective degrees of freedom through which cortical shapes can be selected for by evolution. Finally, we demonstrate that this new understanding can be of practical use: biological processes can now be interrogated in a highly scale-dependent way for increased sensitivity and precision. To our knowledge, this is the most parsimonious universal description of the brain's shape that is at the same time mechanistically insightful, practically useful, and in full agreement with empirical data across species and individuals.

The formation of brain shape in human newborns

“…2–3). Interestingly, recent evidence from statistical physics suggests that the cortical topologies observed across a variety of primate species may be an expression of the same archetypal fractal shape ( 39 ). Given the highly canonical shape developments observed here, this raises the intriguing possibility that the early-life formation of cortical complexity is not only a key process in human brain development but may rather be the result of a more general, evolutionarily conserved mechanism of cortical expansion.…”

“…We analyze structural MRI recordings from the third dHCP release (11). This dataset includes 782 human neonates and covers a wide range of infant maturity -from very preterm to well post-term at the time of scanning (27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45) weeks post-menstrual age). Figure 1A illustrates the profound differences in brain shape over these varying degrees of maturity, where the latter are defined by the age criteria of the World Health Organization (WHO) (31) and the American College of Obstetricians and Gynecologists (ACOG) (32).…”