Connectivity-driven white matter scaling and folding in primate cerebral cortex

Area of Science:

• Neuroscience
• Comparative Anatomy
• Evolutionary Biology

Background:

• Larger brains exhibit more folded cerebral cortices, with white matter scaling faster than gray matter.
• Understanding the cellular composition of subcortical white matter is crucial for explaining brain size evolution.

Purpose of the Study:

• To analyze the cellular composition of subcortical white matter across primate species.
• To investigate the scaling relationships between white matter, neuronal number, and cortical folding.

Main Methods:

• Cellular composition analysis of subcortical white matter in 11 primate species and one Scandentia.
• Examination of scaling laws for white matter mass, non-neuronal cell counts, and axonal properties.
• Modeling the relationship between connectivity, cortical surface area, and neuronal number.

Main Results:

• White matter mass scales linearly with non-neuronal cell number, indicating constant average axonal cross-sectional area.
• White matter surface area scales with neuronal number to the power of 0.87.
• Connectivity, defined as the fraction of cortical neurons sending myelinated axons into white matter, decreases with increasing neuronal number (N^-0.16).

Conclusions:

• Connectivity decreases in larger cerebral cortices, potentially slowing the increase in global conduction delay.
• A direct relationship exists between connectivity and cortical folding.
• White matter tension, driven by connectivity, may be a mechanism for increased cortical folding in larger brains.