Convergent and divergent connectivity patterns of the arcuate fasciculus in macaques and humans
This work addresses the evolutionary divergence in brain connectivity for language processing, offering insights for neuroscience and clinical applications, though it is incremental in building on comparative anatomy.
The study compared the connectivity of the arcuate fasciculus between macaques and humans, revealing that the human version has greater expansion into the middle temporal gyrus and stronger prefrontal and parietal operculum connectivity, quantified using Kullback-Leibler analysis. These differences suggest a neuroanatomical basis for human language specialization and provide a framework for understanding disorders like aphasia and dyslexia.
The organization and connectivity of the arcuate fasciculus (AF) in nonhuman primates remain contentious, especially concerning how its anatomy diverges from that of humans. Here, we combined cross-scale single-neuron tracing - using viral-based genetic labeling and fluorescence micro-optical sectioning tomography in macaques (n = 4; age 3 - 11 years) - with whole-brain tractography from 11.7T diffusion MRI. Complemented by spectral embedding analysis of 7.0T MRI in humans, we performed a comparative connectomic analysis of the AF across species. We demonstrate that the macaque AF originates in the temporal-parietal cortex, traverses the auditory cortex and parietal operculum, and projects into prefrontal regions. In contrast, the human AF exhibits greater expansion into the middle temporal gyrus and stronger prefrontal and parietal operculum connectivity - divergences quantified by Kullback-Leibler analysis that likely underpin the evolutionary specialization of human language networks. These interspecies differences - particularly the human AF's broader temporal integration and strengthened frontoparietal linkages - suggest a connectivity-based substrate for the emergence of advanced language processing unique to humans. Furthermore, our findings offer a neuroanatomical framework for understanding AF-related disorders such as aphasia and dyslexia, where aberrant connectivity disrupts language function.