Extracting functional, phylogenetic and structural data from the subcortical brain: an exploratory geometric morphometric survey

Abstract

The neocortical brain is notoriously difficult to research among extant and extinct primates because its complexity can negate assumptions of spatial and functional homology. Despite this, the neocortical brain remains the centre of attention whilst the subcortical brain is often dismissed as conserved and lacking in any significant phylogenetic or functional signal. However, it is possible that while the information held in the subcortical brain is limited it may also be more amenable to investigation and shed new light on the evolution of the primate brain. Here we use shape analysis to survey the changes of the subcortical brain across extant apes. Twenty-eight specimens covering nine primate species were examined through midsagittal magnetic resonance images (MRI). Coordinates for 10 2D-landmarks representing the subcortical morphology were collected from midsagittal scans and analysed with geometric morphometrics. The main axis of variation was size-related, and associated in most species with flexion of the encephalic trunk, anterior rotation of the corpus callosum, and geometric compression of the thalamic region. Gibbons and orangutans showed only minor shape changes, despite the differences in size. This similarity suggests a possible phylogenetic component that should be further investigated. Both hypotheses (allometric vs. phylogenetic variation) must be considered when taking into account the interaction between the brain cortical changes and the cranial base dynamics.