Oro-facial and manual motor functions are differentially associated with short-fiber white matter connectivity within the chimpanzee “homunculus”

Using diffusion-weighted imaging, we quantified the microstructure of U-shaped fiber bundles connecting the primary motor and somatosensory cortices in chimpanzees. We tested for sex and age effects, lateralization, and associations with manual and orofacial motor functions. Manual skills were assessed with a tool-use task; orofacial communication was assessed by individual variation in attention-getting sound production. Chimpanzees showed population-level leftward asymmetries in fractional anisotropy in U-fibers connecting central and inferior cortices, especially in females. Age was inversely associated with radial, axial, and mean diffusivity in these bundles. Right-handed motor skill was linked to stronger leftward fractional anisotropy asymmetries in superior regions. In contrast, more frequent attention-getting sound production was associated with increased leftward asymmetries in inferior regions. These findings show that different motor functions in chimpanzees are linked to region-specific variation in U-fiber integrity along the dorsal-ventral axis, aligning with previous representations of the chimpanzee motor "homunculus." The observed association between orofacial skill and leftward asymmetry in inferior sensorimotor regions suggests a potential preadaptation for the lateralized speech functions found in modern humans.