**3. Results**

Figure 2b shows a sample of bandpass-filtered EEG activity collected after on-screen instructions required the subject to covertly move his/her attention to a peripheral location. Xi was observed during covert attention tasks (up and down), execution and imagination of leg movements, imagination of someone else's lifting and releasing a small object, and imagination of self-releasing a grasped object. It was absent in the other ideomotor tasks for this study (i.e., no specific departure from background spectral distribution). Note that xi had not been previously observed in studies of rest with eyes opened or closed, or in a variety of sensorimotor and social coordination tasks (e.g., [56]). The green brainwave that appeared during the covert attention tasks is uncharacteristic of waking EEG in two respects: It has an unusually sustained duration that is infrequently observed during mental activities (compare with, e.g., rest eye opened, with its fast succession of spatiotemporal patterns, see also [56,57,62] for reference dynamics generally lasting one or two cycles). Further, its topography remains several centimeters away from the alpha rhythms. For comparison, Figure 2c provides the dynamics of alpha during an eye opened and eye closed task in a different subject. Note the ample oscillations dominated by blue and magenta color that become sustained and ample at eye closure.

**Figure 2.** The transition to xi activity during covert shifts of attention is compared to the onset of alpha during closure of the eyes: (**a**) shows a colorimetric map with two electrodes of interest, CP3 (xi, electrode colored green) and PO3 (lateralized variant left alpha, electrode colored blue, see [63]), underlined by yellow circles. (**b**) shows a sample task during which a subject held his attention at the top of the computer screen while fixating the screen's center (i.e., fovea centrally located, and attentional spotlight maintained several degrees away vertically, verified by the absence of saccades or eye movement in EOG traces). Note the onset of ample waves with green color, about 1.5 s after instruction, and sustained for seconds. For comparison, (**c**) shows the 10 Hz band during quiet rest with eye open (left of the marker) and with eye closed (right) in another subject. Note the characteristic low amplitude, moment-to-moment variability in spatiotemporal patterns (color change) prior to eye closure, and the large amplitude activity in posterior locations (colored blue and magenta) afterwards. (**b**) is filtered in the 10–13 Hz band, (**c**) in the 8–12 Hz, in agreemen<sup>t</sup> with each target activity's spectral distribution.

The spatial discrepancy between xi and alpha cannot be attributed to irregular electrode positioning or idiosyncratic orientation of the alpha generators shifting the forward projection of alpha generators on the scalp: Analysis within subject confirms that they are two distinct phenomena. They were found to co-occur at the rough timescale of entire tasks, for instance, during covert shift of attention upward (Figure 3a), and they differed in both spatial and spectral distribution as shown by their different color and peak frequencies. They also exhibited distinct temporal organization: Although some alpha patterns were intermittently present in the tasks where xi was discovered, alpha and xi occurred at different times (Figure 3b,c).

**Figure 3.** Xi and Alpha differ spectrally and temporally. (**a**) shows a colorimetric spectrum during covert attentional shift upward. Note that in this single subject, the peak of alpha at 10.62 Hz differs from the peak of xi at 11.29 Hz. (**b**) shows a sample bandpass filtered EEG in the 8–13 Hz. Alpha and xi patterns are outlined (other patterns obscured for simplicity). Note that the patterns are not co-occurring in time, as if their functional processes excluded each other. To aid segmentation, signal envelope is plotted in (**c**).
