Mammalian-like developmental changes are present in the neural signatures of avian sleep

Summary

Despite their phylogenetic differences and distinct pallial structures, mammals and birds share many features of sleep structure, most prominently, the existence of distinct stages of rapid eye movement (REM) and slow wave sleep (SWS).^1–4 However, the structure of sleep does not remain constant during lifetime, but undergoes radical changes during development.⁵ Our current understanding of these age-related variations in neural signature of sleep is limited to studies in a small number of mammalian species.^5–7 How does the neural signature of sleep evolve during brain development in an avian brain? To answer this, we recorded multi-channel EEG from developing juvenile and adult zebra finches for several nights. We compared the structure of sleep in adults and juveniles using the ratio of slow oscillations to gamma oscillations (“depth of sleep”, DOS). We observed a significantly larger DOS, as well as less-frequent state transitions in juveniles compared to adults. Furthermore, multichannel EEG revealed local slow oscillations, in addition to global (wide-spread) slow waves. The occurrence of these local waves was correlated with DOS and was higher in frontal sites. In addition, we observed that the functional connectivity increased rapidly during development and was significantly higher in adults than in juveniles. Complementarily, a graph theory analysis of highly-correlated networks revealed numerous small networks in juveniles compared to adults, underscoring the lower connectivity observed in juveniles. Overall, our results reveal significant changes in the neural signature of sleep during development in an avian brain that are remarkably similar to age-dependent changes that occur in mammals.