## 3 Results
### Supplementary Analyses
#### 3.7.2 Supplementary Analyses: Alpha \"Responder\" and \"Non-Responder\" Groups  
Previous investigations have suggested that the alpha and other time-frequency measures are subject to individual differences [31; 46]. As such, it is possible that the attenuation effect might only be visible in a subset of individuals, so-called \"responders\". To ensure that our across-participant results were not unduly blunted, we partitioned our sample according to \"responders\" and \"non-responders\", where a \"respondor\" was defined as a participant who showed a significant target-related alpha attenuation effect in at least 2/5 occipitoparietal recording electrodes during the 1 Hz slow calibration, where the alpha attenuation effect is most visible [19]. With this criterion, we identified 9 responders and 15 non-responders in our sample (Figure 7). Two non-responders showed significant differences in target versus non-target alpha activity, though in the opposite direction. Across the recording sites in the slow calibration, the alpha attenuation effect was most often significant at electrode P4, followed closely by Pz. Because of the prominence of the attenuation effect at P4, we decided to use that site for this particular supplementary analysis, in an attempt to maximize group differences.  
All responders demonstrated significant target-related alpha attenuation effects at P4 in the slow calibration. However, these effects dissipated in the test conditions, where only 3/9, 1/9, and 2/9 responders demonstrated significant effects in the no jitter, small, and large jitter conditions. One additional responder demonstrated a significant target-related increase in alpha activity during the no jitter calibration. For non-responders, 2/15 individuals showed significant alpha effects during the slow calibration, but both were in the wrong direction. The only significant instances of target-related alpha attenuation observed in the non-respondor group were in the no jitter (1/15) and large jitter condition calibrations (2/15). Even so, there were actually more numerous cases of significant inverse effects in both the no jitter (2/15) and large jitter (3/15) conditions. Non-responders showed no significant effects in the small jitter condition.  
Using the responder and non-respondor designations, we re-ran key across-participant condition comparisons for alpha attenuation measures in the separate groups. As expected, non-responders showed no significant target effects in any of the three test conditions (all \(p\) values \(\geq\) 0.256), and no significant effect of condition on target versus non-target alpha differences in either the pooled signal or Pz (both \(\chi^{2}(2)\) values \(\leq\) 2.533; both \(p\) values \(\geq\) 0.282). For responders, there again were no significant differences between target and non-target alpha responses in the no jitter and large jitter conditions (all \(p\) values \(\geq\) 0.110). However, in the small jitter condition, target effects were trending in both signal sources (both \(p\) values \(\leq\) 0.066). Responders demonstrated no significant effect of condition on alpha attenuation distributions in either signal source (both \(\chi^{2}(2)\) values \(\leq 1.556\); both \(p\) values \(\geq 0.459\)).  
Figure 7: Differences between responder and non-responder alpha attenuation estimates. (**A**) Alpha attenuation estimates at site P4 during the slow calibration. Solid lines denote significant target versus non-target differences; dashed lines signify a lack of significant change. (**B**) Box and whisker plots of alpha attenuation estimates (median target alpha z-scores minus median non-target z-scores) across tests conditions, separated according to responder and non-responder groups. Alpha attenuation appears to be somewhat more pronounced for responders across the test conditions, but there are no clear effects of jitter on alpha attenuation in either group.