Abstract
The tracts between cortical areas are conceived as playing a central role in cortical information processing, but their actual numbers have never been determined in humans. Here, we estimate the absolute number of axons linking cortical areas from a whole-cortex diffusion MRI (dMRI) connectome, calibrated using the histologically measured callosal fiber density. Median connectivity is estimated as approximately 6,200 axons between cortical areas within hemisphere and approximately 1,300 axons interhemispherically, with axons connecting functionally related areas surprisingly sparse. For example, we estimate that <5% of the axons in the trunk of the arcuate and superior longitudinal fasciculi connect Wernicke's and Broca's areas. These results suggest that detailed information is transmitted between cortical areas either via linkage of the dense local connections or via rare, extraordinarily privileged long-range connections.
Highlights
The major tracts connecting cortical areas have long been central to models of information processing in the human brain [1]
We find 8.4 × 108 total axons and 1.1 × 108 callosal axons per participant on average, with medians of approximately 1,100 and approximately 130 axons for pairwise intra- and interhemispheric connections between cortical areas (S1 Fig), i.e., even more sparse than those calculated from our tractography data
We estimated the absolute number of axons interconnecting cortical areas by calibrating diffusion MRI (dMRI)-based tractography using the histologically ascertained cross-sectional fiber density of the corpus callosum and found that long-range corticocortical connections are quite sparse
Summary
The major tracts connecting cortical areas have long been central to models of information processing in the human brain [1]. Such models have been refined and applied to development and disease with the advent of diffusion MRI (dMRI) [2]. We describe and apply a novel method for translating from dMRI-derived streamlines to axon counts. The ratio between these 2 measures is obtained by comparing the number of streamlines pass through the corpus callosum to the number of axons, as measured with histology.
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