The journal PLOS Biology published a study that could explain why large brains are more vulnerable to brain disorders such as schizophrenia and Alzheimer’s disease. An extensive range of brain sizes is exhibited in mammals, reflecting their adaptation to the varied environments in which they live. The mammals’ cerebral cortex is a thin layer of gray matter, which is neural tissue composed of nerve cell bodies and nerve fibers that cover the brain.
The processing of information in the brain, thoughts, high-level brain functions as retrieving memories, calculating, language and consciousness is the responsibility of the gray matter. Data on the features of the brain that are preserved through evolutions and what processing structures are unique to a particular species has been provided by comparing the cortical networks in the brains of mammals of differing sizes.
Tracing studies in macaques with network theory were conducted by Zoltan Toroczkai from the University of Notre-Dame, IN, along with other colleagues. These results showed macaques’ cortical network is regulated by the exponential distance rule (EDR). EDR describes a consistent relationship between distances and connection strength and predicts there are fewer long-range axons than short axons.
The researchers found that by using area-tracing data from a macaque, a mammal with a large cortex, and a mouse, which has a significantly smaller cortex, they could introduce a standard model framework that enabled them to make comparisons. A general organization principle based on an EDR and cortical geometry was used to carry out the comparisons within the same model framework. It was noted that all the statistical features of all cortical networks followed EDR.
Even though the cortical networks in primates and rodents are similar, the long-distance connections in the primates’ brains were considerably weaker, which suggests that EDR plays a key role across the mammalian order to optimize the layout of the interareal cortical network allowing larger-brained animals to maintain communication efficiencies combined with increased neuron numbers. Neuronal connections weaken exponentially with distance, suggesting the human cortex (5 times the size of the macaque) could have moderately weak long-distance connections.
The weak long-range cortical connections in humans may contribute to the risk of disconnection syndromes such as Alzheimer’s disease and schizophrenia.
-Dr Fredda Branyon