Calculate an asymmetry index, a ratio of intra-hemispheric edges in the left to right hemisphere of a graph for brain MRI data.
edge_asymmetry(g, level = c("hemi", "vertex"))An igraph graph object
Character string indicating whether to calculate asymmetry for
each region, or the hemisphere as a whole (default: 'hemi')
A data table with edge counts for both hemispheres and the asymmetry
index; if level is vertex, the data table will have
vcount(g) rows.
The equation is: $$A = \frac{E_{lh} - E_{rh}}{0.5 \times (E_{lh} + E_{rh})}$$ where lh and rh are left and right hemispheres, respectively. The range of this measure is \([-2, 2]\) (although the limits will only be reached if all edges are in one hemisphere), with negative numbers indicating more edges in the right hemisphere, and a value of 0 indicating equal number of edges in each hemisphere.
The level argument specifies whether to calculate asymmetry for each
vertex, or for the whole hemisphere.