Fig. 1. Schematic of the main aims and methods of the study.

a Schematic of different interaction schemes between spontaneous and evoked activity. With no spontaneous-evoked correlation, evoked amplitudes are identical regardless of the level of prestimulus spontaneous activity. With positive correlation, higher prestimulus spontaneous activity leads to greater evoked amplitudes. With negative correlation, higher prestimulus spontaneous activity leads to lower evoked amplitudes. b Aims of the study. The study aims to assess which electrophysiological processes exhibit relationships between spontaneous prestimulus and evoked poststimulus activity. The study considers electrophysiological dynamics in the time and frequency domains, further classifying frequency domain evoked power as reflecting oscillations or scale-free (fractal) dynamics. Each of these electrophysiological parameters is associated with different physiological processes. c Methodology for assessing spontaneous-evoked correlation. In the method of pseudotrials, trials and pseudotrials are split into prestimulus high and low conditions. They are then normalized relative to the mean prestimulus period, and the pseudotrial time courses are subtracted: any difference is indicative of a relationship between spontaneous and evoked activity. In the method of trial-to-trial variability, a negative correlation results in a reduction of the trial-to-trial standard deviation.