Pre-stimulus EEG analysis
Following the original study’s protocol for pre-stimulus data, we calculated peak alpha frequency from the electrode that showed the highest amplitude in the alpha band range for each participant. We detected no significant correlations between peak alpha frequency and flash-fusion thresholds (r = -0.29, P>0.05) (Fig 3, middle), or between pre-stimulus peak alpha power and flash-fusion thresholds (Pearson’s r = 0.18, P>0.05).
Having initially failed to replicate Samaha & Postle’s original result with the pre-stimulus dataset, we included a second analysis, post-hoc. Previous findings (Pfurtscheller et al., 1996; Busch et al., 2009; Ruzzoli et al., 2019) have indicated that high alpha power may be indicative of suboptimal performance or the absence of sensory processing, and a suppressed alpha power may be indicative of focused attention. Therefore, selecting the electrodes with highest amplitudes during task performance to extract peak alpha frequency may not be the most informative for pre-stimulus data. We therefore selected the electrode that had the highest power amplitude in the alpha band range in the eyes-closed data and used this same electrode to measure alpha levels in the pre-stimulus data: electrode B7, located in the occipital region. Two participants did not show a peak in the alpha band in pre-stimulus recordings for electrode B7 and we therefore excluded these participants from the respective analyses. This time, we found a moderate correlation between flash-fusion thresholds and pre-stimulus peak alpha frequency (n = 29, r = -0.42, P<0.01) (Fig 3, right).
Figure 3: Correlations between flash fusion thresholds and peak alpha frequency. Left: we found a moderate correlation between flash fusion threshold and peak alpha frequency extracted from resting state EEG-recording. Middle: Peak alpha frequency extracted from the electrode that has the highest amplitude in the alpha band for pre-stimulus EEG-recording was not correlated with flash fusion thresholds. We found a moderate correlation between flash fusion threshold and peak alpha frequency extracted from electrode B7 for pe-stimulus EEG-recording.