Yesterday (May 16) the Public Library of Science (PLoS) published a fascinating article titled “Mapping Connectivity Damage in the Case of Phineas Gage“. It analyzes the brain damage which the famous trauma victim sustained after an accident drove a steel rod through his skull. Railroad worker Phineas Gage survived the accident and continued to live for another 12 years, albeit with significant behavioral changes and anomalies. Those changes were severe enough for him to have to discontinue his work and also get estranged from his friends who stated he was “no longer Gage”. This has become a much studied case about the impact of brain damage on behavior anomalies. Since the accident happened more than 150 years ago there are no autopsy data or brain scans from Phineas Gage’s brain. So how did the scientists reconstruct the likely damage?
Since a few years there has been interest in the human connectome. Just like the genome is a map of human genes, the connectome is a map of the connectivity in the human brain. The human brain is enormously complex. Most estimates put the number of neurons in the hundreds of billions and the synaptic interconnections in the hundreds of trillions! Using diffusion weighted (DWI) and magnetic resonance imaging (MRI) one can identify detailed neuron connectivity. This is such a challenging endeavor that it drives the development of many new technologies, including the data visualization. The image resolution and post-processing power of modern instruments is now large enough to create detailed connectomes that show major pathways of neuronal fibers within the human brain.
The authors of the Laboratory of Neuro Imaging (LONI) in the Neurology Department at UCLA have studied the connectomes of a population of N=110 healthy young males (similar in age and dexterity to Phineas Gage at the time of his accident). From this they constructed a typical healthy connectome and visualized it as follows:
Details of the graphic are explained in the PLoS article. The outermost ring shows the various brain regions by lobe (fr – frontal, ins – insula etc.). The left (right) half of the connectogram figure represents the left (right) hemisphere of the brain and the brain stem is at the bottom, 6 o’clock position of the graph.
Connectograms are circular representations introduced by LONI researchers in their NeuroImage article “Circular representation of human cortical networks for subject and population-level connectomic visualization“:
This article introduces an innovative framework for the depiction of human connectomics by employing a circular visualization method which is highly suitable to the exploration of central nervous system architecture. This type of representation, which we name a ‘connectogram’, has the capability of classifying neuroconnectivity relationships intuitively and elegantly.
Back to Phineas Gage: His skull has been preserved and is on display at a museum. Through sophisticated spatial and neurobiological reasoning the researchers reconstructed the pathway of the steel rod and thus the damaging effects on white matter structure.
Based upon this geospatial model of the damaged brain overlaid against the typical brain connectogram from the healthy population they created another connectogram indicating the connections between brain regions lost or damaged in the accident.
From the article:
The lines in this connectogram graphic represent the connections between brain regions that were lost or damaged by the passage of the tamping iron. Fiber pathway damage extended beyond the left frontal cortex to regions of the left temporal, partial, and occipital cortices as well as to basal ganglia, brain stem, and cerebellum. Inter-hemispheric connections of the frontal and limbic lobes as well as basal ganglia were also affected. Connections in grayscale indicate those pathways that were completely lost in the presence of the tamping iron, while those in shades of tan indicate those partially severed. Pathway transparency indicates the relative density of the affected pathway. In contrast to the morphometric measurements depicted in Fig. 2, the inner four rings of the connectogram here indicate (from the outside inward) the regional network metrics of betweenness centrality, regional eccentricity, local efficiency, clustering coefficient, and the percent of GM loss, respectively, in the presence of the tamping iron, in each instance averaged over the N = 110 subjects.
The point of the above quote is not to be precise in terms of neuroscience. Experts can interpret these images and advance our understanding of how the brain works – I’m certainly not an expert in this field, not even close. The point is to show how advances in imaging and data visualization technologies enable inter-disciplinary research which just a decade ago would have been impossible to conduct. There is also a somewhat artistic quality to these images, which reinforces the notion of data visualization being both art and science.
The tool used for these visualizations is called Circos. It was originally developed for genome and cancer research by Martin Krzywinski at the Genome Sciences Center in Vancouver, CA. Circos can be used for circular visualizations of any tabular data, and the above connectome visualization is a great application. Martin’s website is very interesting in terms of both visualization tools as well as projects. I have already started using Circos – which is available both for download and in an online tableviewer version – for some visualization experiments which I may blog about in the future.