Early development of electrophysiological activity: contribution of periodic and aperiodic components of the EEG signal
Brain function rapidly changes in the first two years of life. In the last decades, resting-state EEG has been widely used to explore those changes. Previous studies have focused on the relative power of the signal in canonical frequency bands (i.e., theta, alpha, beta, gamma). However, EEG power is a mixture of a 1/f-like background power (aperiodic) in combination with narrow peaks that appear over that curve (periodic activity; e.g., alpha peak). Therefore, the relative power may capture both, aperiodic and periodic brain activity, misleading actual periodic changes in infancy. For this reason, we explored the early developmental trajectory of the relative power in the canonical frequency bands from infancy to toddlerhood and compared it to changes in periodic activity in a longitudinal study with three waves of data collection at ages 6, 9, and 16 to 18 months. Finally, we analyzed whether the periodic activity and/or aperiodic components of the EEG contributed to explaining age changes in relative power. We found that relative power and periodic activity trajectories differed in this period in all the frequency bands but alpha, and we replicated an increment of alpha peak frequency. We found that age changes in aperiodic parameters (exponent and offset) depend on the frequency range. More importantly, only alpha relative power was directly related to periodic activity but other frequency bands were predicted also by aperiodic components. This suggests that relative power is capturing the developmental changes of the aperiodic brain activity and, therefore, more fine-grained measures are needed.