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.