of the primary neural activity in the scalp region [104]. In the present study, surface Laplacian was computed on the grand mean waveforms representing ERO total theta amplitude for each subgroup and for each task condition separately. The surface Laplacian was calculated at a scalp location based on the weighted average of the measured potentials at neighboring electrodes, where the electrodes closer to the index location get more weight. Topographic maps representing mean CSD potential within 300–500 ms (μV/r2, where r = head radius in cm) were created. Z-scores were calculated for each scalp map by keeping the mean SL/CSD values of all 61 scalp electrodes as zero (‘0’) and the values above and below the mean were represented in SD units (z-scores). Then the z-score converted CSD maps were also plotted in order to compare topographic features regardless of the amplitude differences across the maps. Positivity or the source (red/orange) in the CSD maps represents increased ERO amplitude or synchronization, whereas negativity or the sink (blue/cyan) indicates decreased amplitude or desynchronization [105].
