This week, our paper “Comparative diffusion tractography of cortico-striatal motor pathways reveals differences between humans and macaques” was published online ahead of print in the Journal of Neurophysiology.
This paper compared pathways connecting two cortical motor areas – the frontal eye fields, an area supporting eye movements, and the primary motor cortex, an area supporting limb movements – with the striatum, a structure in the center of the brain, between macaque monkeys and humans. These pathways are thought to be fundamental for the control over eye and limb movements, especially in complex environments and demanding tasks.
The rationale for this species comparison was that our understanding of these cortico-striatal pathways is largely based on studies performed in macaque monkeys. However, the model arising from these studies is also used to understand the structure and function of the human brain in health and disease, despite obvious differences between brains of humans and macaques.
The brains of nine macaque monkeys and nine humans were scanned non-invasively using an MRI scanner, using a technique called diffusion tensor imaging. Then, the pathways between cortical motor areas and the striatum were visualized and analyzed with a method called probabilistic diffusion tractography.
The present study shows that the pathways from cortical motor regions end in different sites of the striatum for macaque monkeys and humans. In humans as compared to monkeys, the pathways from both the frontal eye fields and the primary motor cortex to the striatum terminated more posteriorly (i.e. towards the back of the brain) and more in the putamen than in the caudate nucleus, two large regions that form the striatum together with a third regions called the nucleus accumbens. Also, in humans as compared to monkeys, the sites of the striatum where pathways from the primary motor cortex and frontal eye field terminated showed greater overlap. We speculate that all these differences in the topography of cortico-striatal pathways between macaque monkeys and humans relate to the expansion of frontal lobe in primate evolution and potentially the more intricate coordination of eye and hand movements in humans.
These findings point to differences in the organization of cortico-striatal pathways supporting eye and limb movements of macaque monkeys and humans and indicate that caution is required when translating findings from animal models to human studies.
‘Comparative diffusion tractography of cortico-striatal motor pathways reveals differences between humans and macaques’
Sebastiaan F.W. Neggers, Bram B. Zandbelt, Michelle S Schall, Jeffrey D Schall
Journal of Neurophysiology (in press)