Frontal White Matter Volume Is Associated with Brain Enlargement and Higher Structural Connectivity in Anthropoid Primates

Previous research has indicated the importance of the frontal lobe and its ‘executive’ connections to other brain structures as crucial in explaining primate neocortical adaptations. However, a representative sample of volumetric measurements of frontal connective tissue (white matter) has not been available. In this study, we present new volumetric measurements of white and grey matter in the frontal and non-frontal neocortical lobes from 18 anthropoid species. We analyze this data in the context of existing theories of neocortex, frontal lobe and white versus grey matter hyperscaling. Results indicate that the ‘universal scaling law’ of neocortical white to grey matter applies separately for frontal and non-frontal lobes; that hyperscaling of both neocortex and frontal lobe to rest of brain is mainly due to frontal white matter; and that changes in frontal (but not non-frontal) white matter volume are associated with changes in rest of brain and basal ganglia, a group of subcortical nuclei functionally linked to ‘executive control’. Results suggest a central role for frontal white matter in explaining neocortex and frontal lobe hyperscaling, brain size variation and higher neural structural connectivity in anthropoids

Correlations between neocortical substructures and basal ganglia and rest of brain.

<p>For abbreviations see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009123#pone-0009123-t001" target="_blank">tables 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009123#pone-0009123-t002" target="_blank">2</a>. Values indicate partial R² values of a PGLS regression between basal ganglia and rest of brain (‘Brain minus N’) with frontal and non-frontal lobe white and grey matter. Partial correlations were computed by using the residuals of each variable to the control variable (‘control’).</p

Volumetric measurements of neopallium and frontal lobe white and grey matter.

<p>Species #: identification number of the individual used in the collection; N: neopallium; w: white matter; g: grey matter; FR: frontal neopallial lobe; Brain: total brain volume; BG: basal ganglia. The ratio of frontal lobe relative to neopallium are indicated for the current study and for the analyses done on different individuals by Semendeferi et al (1997) and Semendeferi et al. (2002). Basal ganglia is measured as the sum of its two largest constituent parts (striatum and pallidum) that are also responsible for (respectively) the primary input from neopallium and primary output to other brain structures. Volumetric data for substantia nigra and subthalamic nucleus (the two other, but significantly smaller, constituent parts of the basal ganglia) are not available for the individuals used in the current analysis. Data from Stephan (unpublished) comprises the individual-specific data underlying the data presented in Stephan et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009123#pone.0009123-Stephan1" target="_blank">[37]</a> and was made available by H. Frahm. Data on brain size from <i>Pan paniscus</i> and <i>Pongo pygmaeus</i>, however, comes from Macleod et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009123#pone.0009123-MacLeod1" target="_blank">[61]</a>.</p