Abstract
The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex. It was computationally reconstructed for the first time to the level of all individual folia from multicontrast high-resolution postmortem MRI scans. Its total shrinkage-corrected surface area (1,590 cm2) was larger than expected or previously reported, equal to 78% of the total surface area of the human neocortex. The unfolded and flattened surface comprised a narrow strip 10 cm wide but almost 1 m long. By applying the same methods to the neocortex and cerebellum of the macaque monkey, we found that its cerebellum was relatively much smaller, approximately 33% of the total surface area of its neocortex. This suggests a prominent role for the cerebellum in the evolution of distinctively human behaviors and cognition.
Highlights
The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex
By performing the same procedure on a monkey brain, we found that the surface area of the human cerebellum has expanded even more than that of the human cerebral cortex, suggesting a role in characteristically human behaviors, such as toolmaking and language
By reconstructing the human cerebellar surface down to the level of individual folia, we found that its surface area was considerably larger than previously reported—amounting to 78% of the entire surface area of the human neocortex
Summary
The surface of the human cerebellar cortex is much more tightly folded than the cerebral cortex. Our interest in creating an accurate folia-level surface reconstruction was driven by the results of finely detailed (60+ sites/mm2) microelectrode mapping of the granule cell layer in somatosensory input regions in the cerebellum [9, 10], which revealed a unique “fractured somatotopy” in which tiny, internally somatotopic 2D patches of the body surface were arranged into a mosaic that routinely juxtaposes distant body parts to a much greater degree than is the case in primary somatosensory cortex These intricately “fractured” maps were detected in the Purkinje cell layer using localized naturalistic stimulation of the skin [11], likely the result of the higher synaptic density per Purkinje cell of the ascending limb of a parallel fiber compared with the more obvious mediolateral parallel fiber itself. Future studies of the spatial arrangement of these fractured functional maps on the cerebellar surface, and their implications for the computational role of the cerebellum, including possible analogs in the “cognitive cerebellum,” will first require a detailed quantitative representation of the complexly folded surface of the human cerebellum
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