How the Brain Integrates Our External and Internal Worlds
Post by Lani Cupo
The takeaway
Humans are capable not only of intelligible communication with others, but also private internal thought, both of which are represented separately in the brain. The interaction between the two is facilitated by convergence of these systems in two hub regions of the brain.
What's the science?
As humans, we perceive the external world through senses, and act upon it with various muscle groups, allowing communication with others. We are also capable of internal thought, allowing us to consider abstract concepts and plan for the future. The brain regions underlying these processes have been fairly well characterized as two segregated systems. There is the dorsal system, which is dedicated to analyzing the external world in terms of space and time, allowing us to perceive and act upon it, while the ventral system is dedicated to meaning, allowing our internal world to comprehend more than the immediate environment. It was previously unknown if and how these systems interact, however this week in NeuroImage, Weiller and colleagues identified hub regions in the brain where the dorsal and ventral systems interact, connecting the “dual-loop” of the dorsal and ventral systems and regulating syntax across cognitive domains, both internal and external.
How did they do it?
The authors used diffusion magnetic resonance imaging (dMRI) data, a technique which allows researchers to estimate reconstructions of white matter tracts in the brain by analyzing the characteristics of water molecules’ diffusion. First the authors modelled the fiber tracts with information about their directional trajectories, known as “streamlines”, as well as where each streamline terminates (where the white matter tract ends). They specifically examined regions of interest corresponding with the dorsal and ventral language systems of the brain. Next, the authors calculated which cortical regions, or hubs, intersected both dorsal and ventral tracts. Finally, they examined hemispherical differences between the hubs, assessing whether the same brain areas were identified on the left and right hemisphere. Having identified the regions, the authors examine the involved brain structures through the context of evolutionary development, with phylogenetic trees and lifetime development with the process of myelination.
What did they find?
First the authors found two major streamline systems that cover the majority of the brain with dorsal streamlines that includes many primary sensory and motor areas, and a ventral streamline system that includes further regions, such as the frontal, temporal, and occipital poles. The authors then identified two cortical hubs at the intersection of the dorsal and ventral tracts: one in the frontal cortex (largely the lateral prefrontal cortex and inferior frontal gyrus [IFG]), and one posterior hub in the middle temporal gyrus (MTG). The hubs close the loop between the dorsal and ventral tracts, which can allow recursive thought and internalization even without external feedback. While the hubs were identified in both the left and right hemispheres, there were inter-hemispheric differences in the exact regions involved, such as increased involvement of the temporal gyrus in the left hemisphere and more regions in the IFG in the right hemisphere. The hub regions are high-level multimodal areas, meaning they receive information from many lower-level processing regions (such as primary sensory regions), and are well connected throughout the brain. In particular, the MTG is responsible for processes related to language comprehension and conceptual knowledge, while the IFG has previously been implicated as an intersection between ventral and dorsal systems. These regions are among some of the last to emerge through evolution, changing significantly between macaque monkeys and humans, and they are also among the last to be myelinated and finish developing.
What's the impact?
This study suggests that the identified regions integrate the ventral and dorsal systems, providing syntax for both external communication and internal thought. Of note, the authors do not suggest that syntax takes place wholly within these regions, but instead these regions are necessary to integrate the parallel computations of the two separate systems. The findings of the authors discuss traits that are uniquely human lending insight into what sets us apart from other animals.