# Convergent and Divergent Connectivity Patterns of the Arcuate Fasciculus in Macaques and Humans

## TL;DR

This study compares the arcuate fasciculus in macaques and humans, revealing human-specific connectivity patterns linked to language evolution.

## Contribution

The study introduces a multimodal framework combining viral tracing and MRI to compare AF connectivity across species.

## Key findings

- Macaque AF connects temporal-parietal cortex to prefrontal regions via auditory cortex and parietal operculum.
- Human AF shows expanded connectivity in the middle temporal gyrus and stronger prefrontal-parietal links.
- Divergences in AF connectivity suggest evolutionary adaptations supporting human language networks.

## Abstract

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, viral‐based genetic labeling and fluorescence micro‐optical sectioning tomography (fMOST) are integrated for single‐neuron tracing with whole‐brain tractography derived from 11.7T diffusion MRI in four macaques. Augmented by human 7.0T MRI spectral embedding analyses, this multimodal framework enabled cross‐species comparative connectomics of the AF. Complemented by spectral embedding analysis of 7.0T MRI in humans, a comparative connectomic analysis of the AF across species is performed. It is demonstrated 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 differences underscore the critical role of AF expansion and differentiation in the evolution of human language capabilities.

This study employs viral‐based single‐neuron tracing and dMRI‐based whole‐brain tractography to investigate arcuate fasciculus (AF) trajectories in macaque monkeys, and compares with the human AF connectome using spectral embedding. Results demonstrate conserved AF topography spanning temporoparietal‐auditory‐frontal pathways across primates, with human‐specific expansions in the middle temporal gyrus and reinforced prefrontal‐parietal connectivity pointing to AF's evolutionary specialization for higher‐order language functions.