Visualizing average connectivity
Bilateral FC was computed between a pair of electrodes, which depended
on the distance between the electrodes. Following previous literature
(Kayser & Tenke, 2015), the inter-electrode distance was computed in
angular space, as the Euclidean distance between the two locations on
the scalp, in elevation and azimuth angle space. Specifically, we
estimated the inter-electrode distance as \(cos(\Delta\theta)\), with\(\Delta\theta\) being the angular separation between the seed electrode
and the electrode under consideration. We estimated the FC spread, using
the FC with inter-electrode distance profile from the seed electrode to
the neighboring electrodes, by averaging FC between electrode pairs
within a defined inter-electrode distance intervals (angular separation
of [60⁰ ‒ 120⁰] or [-0.5 ‒ 0.5] in \(cos(\Delta\theta)\) space)
covering the range of -1 to 1 with 0.25 bin width. The term connectivity
spread here is based on the topography of the connectivity fall-off
profile in the local neighborhood w.r.t the seed electrode. The
terminology was based on ‘field spread’ which refers to the spatial
blurring effect of the skull on the distribution of electric potential
over scalp with EEG (Nunez et al. , 1997). The connectivity spread
along with the bootstrapped median SEM were visualized with errorbars in
Figure 2d, 3d, 4d, 5d, Supplementary Figure 4d, 5d, 6d and 7d.
Bootstrapped median SEM were estimated by the standard deviation across
the bootstrapped medians in 1000 iterations.
The FC was averaged across the left and right electrode groups for
better visual representation in Figure 2b, 4b and 5b and the FC versus
inter-electrode distance plots shown in 2c, 4c and 5c. The scalp maps
were averaged by taking the mean of the FC map of the left electrode
group and the mirror image of the FC map of the right electrode group.
The FC versus distance profiles were simply averaged across the two
electrode groups.