Connectogram: A Graphical Representation of Connectomics

A connectogram is a graphical representation of connectomics , the field of knowledge concerned with mapping all the white matter connections in the human brain. It is usually a circular graph obtained by applying graph theory to diffusion MRI to demonstrate white matter connections.

Structure

A connectogram showing the averaged connections and cortical properties of 110 healthy, right-handed men aged 25 to 36 years

A connectogram of a healthy control subject, including 5 additional nodal measures not included in the standard connectogram. From the outside in, the rings represent cortical area, gray matter volume, surface area, cortical thickness, curvature, degree of connectivity, node strength, betweenness centrality, eccentricity, node efficiency, and eigenvector centrality. An empty ring was added between the degree of connectivity and node strength as a placeholder.

Description

The connectogram as a graphical representation of brain connectomics was first proposed in 2012 . A connectogram is usually a circular graph in which the left half shows the connections of the left hemisphere and the right half those of the right. In turn, the halves are divided into the frontal lobe , the insular lobe, the limbic lobe, the temporal lobe, subcortical structures, and the cerebellum. At the very bottom, between the two hemispheres, the brainstem is also shown. Within these lobes, each cortical region is labeled with its own abbreviation and assigned its own color. The colors are used to find regions in other images of brain surfaces and to assess their relative positions. Inside the ring of cortical regions, nested rings describe such quantities as the gray matter volume of the region, the area of the region, the cortical thickness of the region, the surface curvature of the region, and the degree of connectivity of the region (the relative proportion of the connections of this region to the connections of the whole brain), in order of nesting.

Legend for the metadata presented in the various rings of the connectogram.

Background and description

The Connectogram , as a graphical representation of brain connectomics , was proposed in 2012

Circular representations of connections have been used in a number of disciplines; examples include representations of aspects of epidemics, geographic networks, musical rhythms, diversity of bird populations, and genomic data. Connectograms have also been mentioned as a source of inspiration for the style of Tony Stark's helmet heads-up display in " Iron Man 3" .

Brains colored according to the outer ring of the connectogram.

Connectograms are circular, with the left half depicting the left hemisphere and the right half depicting the right hemisphere. The hemispheres are further subdivided into the frontal lobe , the insular cortex , the limbic lobe , the temporal lobe , the parietal lobe , the occipital lobe , subcortical structures, and the cerebellum.. At the bottom, between the two hemispheres, the brainstem is also depicted. Within these lobes, each cortical region is labeled with an abbreviation and assigned its own color, which can be used to denote the same cortical regions in other figures, such as the parcellated brain surfaces in the adjacent image, so that the reader can find the corresponding regions of the cerebral cortex on a geometrically accurate surface, and see how different the connected regions can be. Inside the ring of the cortical surface, concentric circles represent different attributes of the corresponding cortical regions. These metric rings, in order from the outermost to the innermost, represent gray matter volume , surface area., cortical thickness, curvature, and degree of connectivity (the relative share of fibers beginning or ending in a given region, compared with the whole brain). Inside these circles, lines connect regions that were found to be structurally connected. The relative density (number of fibers) of these connections is reflected in the opacity of the lines, so that different connections and their structural significance can be easily compared. The fractional anisotropy of each connection is reflected in its color.

Uses

Brain mapping

Given the recent concerted effort to map the entire human brain and its connections , it is becoming increasingly important to find ways to graphically represent the large amounts of data involved in connectomics . Most other representations of the connectome use 3 dimensions and therefore require an interactive graphical user interface. A connectogram can display 83 cortical regions in each hemisphere and visually show which regions are structurally connected, all on a flat surface. It is therefore convenient to store in patient charts or to display in print. The graphs were originally created using a visualization tool called Circos ,. [10] [11]

Clinical use

A connectogram typical of clinical use, depicting the presumed connection damage in Phineas Gage , who in 1848 survived a large iron rod piercing his skull and brain. The connectogram shows only those connections that are presumed to have been damaged.

At the individual level, connectograms can be used to inform the treatment of patients with neuroanatomical abnormalities. Connectograms have been used to monitor the progress of neurological recovery in patients who have suffered traumatic brain injury (TBI). [12] They have also been applied to the famous patient Phineas Gage to assess the damage to his neural network (as well as damage at the cortical level, which was the main focus of earlier studies of Gage). [13]

Empirical research

Connectograms can represent average cortical measures (gray matter volume, surface area, cortical thickness, curvature, and degree of connectivity), as well as tractography data such as the average density and fractional anisotropy of connections, in populations of any size. . This allows visual and statistical comparison between groups, such as men and women [14], cohorts of different ages, or healthy control individuals and patients. Some versions have been used to analyze how networks are divided in patient populations [15], or to analyze the relative balance between inter- and intrahemispheric connections. [16]

Modified versions

There are many possibilities for which measures are included in the rings of the connectogram. Irimia and Van Horn (2012) published connectograms that explore correlational relationships between regions and use figures to compare graph theory and connectomics approaches. [17] Some have been published without the inner cortical metrics. [18] Others include additional measures related to neural networks , [19] which can be added as extra rings toward the inside to show graph theory metrics, as in the extended connectogram here:

Regions and their abbreviations

See also

• Connectome
• Connectomics
• Human Connectome Project
• Brain mapping
• Tractography
• Chord diagram