The (lack of) correlation between evoked and spontaneous brain oscillations: an individual difference approach

Rhythmic visual stimuli can evoke brain oscillations (i.e., the steady-state visual evoked potentials), while the brain also spontaneously generates endogenous oscillations. Previous research has suggested using rhythmic visual stimuli to entrain these spontaneous oscillations (Herrmann et al., 2016; Notbohm et al., 2016). However, it is unclear if brain oscillations induced by rhythmic stimulation are identical to spontaneous brain oscillations. This study adopted an individual difference approach to investigate whether evoked and spontaneous oscillations originate from the same neural mechanisms. If they do, a correlation across observers would be expected. We collected a large EEG dataset (N = 30), recording both evoked and spontaneous brain oscillations. For evoked oscillations, EEG activities were recorded while observers viewed periodic flickering stimuli at specific frequencies (i.e., 2Hz, 4Hz, 6Hz, …, or 30Hz). Each trial lasted 9 seconds, with 6 trials tested for each frequency. For spontaneous oscillations, resting-state EEGs were recorded for 4 minutes while observers fixated on a spot in the center of the screen. We analyzed cross-observer correlations within and between conditions at each frequency band (e.g., delta, theta, alpha, and beta). For evoked oscillations, we spit the trials into odd and even group, finding a high correlation (r=0.873, p<.01, averaged across all frequency bands). Similarly, for spontaneous oscillations, we observed a high correlation across two recording blocks (r=0.768, p<.01). These results indicate that our measurements for both evoked and spontaneous oscillations are highly reliable across individuals. However, the correlation between evoked and spontaneous oscillations was minimal (r=0.072, 95% confidence interval = [-0.30 0.45]). This lack of correlation suggests that evoked and spontaneous oscillations represent distinct forms of brain activity.