Why big brains are more likely to develop mental health disorders than smaller ones

The study shows that the cortical network structure in mammals is governed by the so-called "exponential distance rule". The idea is that there is a strong relationship between distances and connection strength – the closer two brain areas are to each other, the more connections exist between them.

The study is based on the idea that the cortical network structure in this primate is governed by the so-called exponential distance rule. This rule suggests there is a robust association between distances and connection strength – the closer two areas are to each other, the more connections exist between them.

Published in PLOS Biology, the findings show that the global architecture of the cortical networks in primates and rodents is organised by common principles but that primates – who have much bigger brains – are more likely to have weaker brain connections, just as the exponential distance rule predicts.

Past studies have hinted that a weakness or disruption to neural connections can influence the development of a range of mental and neurodegenerative disorders, like schizophrenia.

Mice vs macaque

In their research, the authors have compared the features of the cortical networks in the macaque – a mammal with a large cortex – with those in the mouse, with a much smaller cortex. They used detailed tracing data to quantify connections between functional areas in both types of brains.

Despite the substantial differences in the cortex size between the species, they discovered that their brain connections followed the exponential distance rule. The data confirmed that neural connections weaken exponentially with distance. In bigger brains, the distance between neurons is greater, and weakened connections were observed by the scientists in macaques.

These findings could potentially apply to humans, the scientists say. Long-distance connections could be quite weak in the human cortex, as it is approximately five times larger than that of the macaque.

"One could speculate that the low weight of human long-range connections may contribute to an increased susceptibility to disconnection syndromes, such as have been proposed for Alzheimer disease and schizophrenia", the researchers conclude.