Volume of Frontal Lobe White Matter

The frontal lobe is the area of the brain at the front of the skull, with its posterior border at the central sulcus. It has a wide range of functions that primarily consist of so-called higher mental functions, such as decision-making and suppressing inappropriate behaviors. As is true for all cortical areas, it comprises a surface layer of gray matter, rich in neuronal cell bodies, with underlying white matter, containing primarily the projections to and from those cells.

Since humans have much more complex higher cognitive capacity than other apes, it might be expected that human-specific changes have occurred in the frontal lobe. Indeed, early studies suggested that humans have a relatively large frontal lobe (as fraction of total brain volume) when compared to other primates. However, more recent work has called that into question. The use of magnetic resonance imaging (MRI) has allowed more comprehensive study of living primates, including nonhuman hominids. In 2002, work by Semendeferi et al. showed that humans are not in fact unusual among great apes when considering the size of both gray and underlying white matter of the frontal cortex. However, the overall size of frontal cortex is increased in great apes compared to other primates.

A later paper by Schoenemann et al. suggested that humans have an increase in brain volume specifically in prefrontal cortical white matter, but not gray matter. This may suggest that the connectivity of prefrontal cortex has changed substantially in human evolution. The methods used to calculate this result have been challenged, and the difference may in fact be quite small. Interestingly, however, a paper from Schenker et al. found that humans have increased gyral white matter, i.e. the amount of white matter directly underlying a gyrus, or bulge in the brain, in prefrontal cortex. This may indicate an increase in local connectivity within this area or to nearby regions. Others have more recently suggested that the human increase in white matter volume may specifically be lateralized to the left prefrontal cortex, perhaps indicating a specialization of this region in human evolution.

The possible human-specific increase in frontal lobe white matter is still under investigation, and further work will be needed to determine the details and significance of this potential difference.