Theta activity as a marker of cognitive development in infancy: A longitudinal study across the first two years of life

Research shows that the theta rhythm in infant electroencephalogram indexes learning processes and is a promising candidate for a marker of early cognitive development. However, a scarcity of studies investigating the stability of individual differences in theta activity in infancy, and a large variability in analytical approaches in existing research constrain the interpretations of research findings. In our large longitudinal study, we related three different indices of frontocentral theta activity (absolute and relative power, and an index of theta modulation by novel content) at 6 and 12 months to cognitive development level, language skills, and visual attention at 24 months. We found an increase in theta power over the course of novel information encoding at 6 and 12 months, replicating prior studies. Both absolute and relative theta power, but not theta modulation index, showed a large degree of stability in individual differences from 6 to 12 months. Finally, absolute theta power at 6 and 12 months was a positive predictor of the general cognitive level, but not of specific skills (selective attention, language) at 24 months. Of note, we observed similar effects for absolute power in the alpha frequency band, suggesting that the effects are not specific to the theta frequency band. Our results support the involvement of the theta rhythm in cognitive development in infancy and point to absolute power as the potentially most sensitive index of individual differences in theta activity.