Mapping Connections in the Human Brain

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The first high-resolution structural connection map of the human cerebral cortex was published earlier this month in the journal PLoS Biology. The study reveals regions that are highly connected and central, forming a structural core network [1]. Intriguingly, this core network consists of many areas that are more active when we’re at rest than when we’re engaged in a task that requires concentration.

In the human brain, a complex network of fiber pathways link all regions of the cerebral cortex. This “brain wiring” is responsible for shaping neural activation patterns. To better understand the structural basis of functional connectivity patterns in the human brain, researchers at Indiana University and Harvard Medical School in the U.S., and at University Hospital Center and the University of Lausanne in Lausanne, Switzerland, used a brain imagining technique called diffusion spectrum imaging to map out axonal pathways in cortex white matter.

The outermost layer of the cerebrum is termed gray matter. Below the grey matter of the cortex, white matter, consisting of myelinated axons, interconnects different regions of the central nervous system. Axons or nerve fibers are long filaments that project from a nerve cell (called a neuron) and carry electrical impulses away from the cell body to other neurons. Myelinated axons are axons surrounded by an electrically-insulated phospholipid layer (meaning a fat-soluble molecule, think insulated brain wiring). The end of the axon comes close to but does not contact the next nerve cell; this gap between neurons is referred to as a synapse or synaptic junction.

Diffusion spectrum imaging is a type of MRI that identifies water molecules and monitors how they move. In nerve fibers, water molecules can be used to discern brain structural connectivity since they move along the length of fiber. The noninvasive technique offers researchers the ability to compare neural connection variability between participants and to relate it to differences in individual functional connectivity and behavior.

The scientists used network theory, usually applied in social network analysis, to measure network properties such as degree and strength (i.e. the extent to which the node is connected to the rest of the network), centrality and efficiency (i.e. how many short paths between other parts of the network pass through the node), and betweenness. Using measures from five healthy people (participants A — E), they calculated brain regions that were highly connected and contained numerous connector hubs.

The researchers found evidence for the existence of a structural core composed of densely connected regions at the top and back of the brain, straddling both hemispheres (figure). An earlier imaging study measured blood flow to different parts of the brain [2]. It positively correlated with the level of white brain matter axonal connectivity in individual participants.

Interestingly, regions that are in the most connected brain areas have been shown in various studies to have high levels of energy utilization and activation at rest [2], and significant deactivation during goal-directed tasks [2-4]. Termed the brain’s “default network”, it has been suggested to be involved with stimulus-independent thought (daydreaming) [5] and other internally focused thoughts and cognitions, such as remembering the past, envisioning future events and considering the thoughts and perspectives of other people [6]. These tasks all activate multiple regions within the default network.

According Dr. Olaf Sporns, co-author of the study and neuroscientist at Indiana University [7]:

This is one of the first steps necessary for building large-scale computational models of the human brain to help us understand processes that are difficult to observe, such as disease states and recovery processes to injuries.

Indeed, the default network is disrupted in several human disorders, including autism, schizophrenia and Alzheimer’s disease [6]. Using this technique to map out axonal pathways in the cortical network of patients with such diseases should help scientists identify default network regions that are disrupted. Researchers hope that they can better understand these disorders based on changes in the brain’s connectivity map.

For additional information on the human cortex and neurons, YouTube has a great video excerpt from the Discovery Channel on Neurons and How They Work.

References

1. Hagmann et al. Mapping the Structural Core of Human Cerebral Cortex. PLoS Biol. 2008 Jul 1;6(7):e159. [Epub ahead of print] DOI: 10.1371/journal.pbio.0060159
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2. Raichle et al. A default mode of brain function. Proc Natl Acad Sci U S A. 2001 Jan 16;98(2):676-82.
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3. Gordon et al. Common Blood Flow Changes across Visual Tasks: II. Decreases in Cerebral Cortex. J Cogn Neurosci. 1997;9(5):648-663.
4. Fox et al. The human brain is intrinsically organized into dynamic, anticorrelated functional networks. Proc Natl Acad Sci U S A. 2005 Jul 5;102(27):9673-8. Epub 2005 Jun 23.
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5. Mason et al. Wandering minds: the default network and stimulus-independent thought. Science. 2007 Jan 19;315(5810):393-5.
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6. Buckner et al. The brain’s default network: anatomy, function, and relevance to disease. Ann N Y Acad Sci. 2008 Mar;1124:1-38.
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7. New map IDs the core of the human brain. Indiana University Press Release. 2008 July 1.