July 3, 2022

Neuroanatomy and Brain Evolution

 

Brain anatomy and its evolutionary history form important foundations for the work in our lab and underpins our functional investigations. Our work has adopted comparative approaches to ask questions about the evolution of the human cerebellum and its connections with the cerebral cortex, and has made the anatomy of the human cerebellum accessible to researchers with the publication of an MRI anatomical atlas.

Concerted Evolution in the Cortico-Cerebellar System

Comparative anatomy of the cerebral peduncle: Cortico-pontine fibres from the prefrontal cortex (green) occupy a larger fraction in humans compared with non-human primates

One of the goals of our work is to better understand the relationships between the prefrontal cortex and the cerebellum in the human brain. The connectional organisation of the cerebellar cortex shows that the prefrontal cortex and the motor cortex are each connected to separate areas of the cerebellar cortex in the primate brain. Both of these neocortical areas have been subjected to different selection pressures: there has been rapid evolutionary growth of the human prefrontal cortex compared with the primary motor cortex. If selection pressures act in a ‘concerted’ manner on whole functional systems, rather than on individual brain areas, then cerebellar targets of the prefrontal cortex, along with the pathways that connect them to the prefrontal cortex, should have undergone similar expansions in the same timeframe. We have taken  comparative anatomical approach to test this hypothesis with diffusion imaging to map fibre pathways and structural MRI to characterise anatomy in humans and non-human primates.We have contributed evidence that the prefrontal cortex and its connected areas in the cerebellar cortex have not evolved independently, but in concert with each other as a single functional unit that has been subjected to the same selection pressures. First, diffusion tensor imaging used in humans and macaque monkeys showed that descending fibres from the prefrontal cortex occupy a much larger fraction of the cerebral peduncle that those in macaque monkeys. We have also show using data from three primate species that cerebellar cortical lobules that are connected with the prefrontal cortex are significantly enlarged in the human brain compared with those connected to the motor cortex. Both of these studies suggest that in humans, the connections between the cerebellum and the prefrontal cortex probably play particularly important roles in behaviour.

A Probabilistic Atlas of the Human Cerebellum

Researchers who use neuroimaging methods increasingly rely on probabilistic atlases to guide their anatomical inferences. The anatomical organisation of the human cerebellum is complex, and varies considerably from case to case, suggesting the need for a probabilistic atlas of the human cerebellum in MNI standard stereotaxic space that can supplement anatomical inferences made on the basis of single-subject anatomical MRI scans. In collaboration with Joern Diedrichsen we have produced such an atlas, which is available for FSL, SPM,  and MRIcron. Further details can be found at Joerns website.

Selected papers:

  • Ramnani N (2011), “Frontal Lobe and Posterior Parietal Contributions to the Cortico-cerebellar System.”, Cerebellum (in press).
  • Balsters JH, Cussans E, Diedrichsen J, Phillips KA, Preuss TM, Rilling JK, Ramnani N (2010) Evolution of the cerebellar cortex: The selective expansion of prefrontal-projecting cerebellar lobules. Neuroimage, 43:388-98.
  • Jill X. O’Reilly, Christian F. Beckmann, Valentina Tomassini, Narender Ramnani and Heidi Johansen-Berg (2010). “Distinct and overlapping functional zones in the cerebellum defined by resting state functional connectivity”, Cerebral Cortex, 20:953-65.
  • Diedrichsen J, Balsters JH, Flavell J, Cussans E, & Ramnani N (2009). A probabilistic MR atlas of the human cerebellum. Neuroimage, 46(1), 39-46.
  • Ramnani N (2006) “The Primate Cortico-Cerebellar System”, Nature Reviews Neuroscience, 7(7):511-22
  • Ramnani N et al. (2006), “The evolution of prefrontal inputs to the cortico-pontine system: Diffusion imaging evidence from macaque monkeys and humans”, Cerebral Cortex, 16(6):811-8. (doi:10.1093/cercor/bhj024)
  • Ramnani N and Owen AM (2004), “The Anterior Prefrontal Cortex: What can functional imaging tell us about function?” Nature Reviews: Neuroscience 5, 184-194. [Download PDF]

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