Local sleep in songbirds: Different simultaneous sleep states across the avian pallium

Abstract

SummaryWakefulness and sleep have often been treated as distinct and global brain states. However, an emerging body of evidence on the local regulation of sleep stages challenges this conventional view. Apart from unihemispheric sleep, the current data that support local variations of neural oscillations during sleep are focused on the homeostatic regulation of local sleep, i.e., the role preceding awake activity. Here, to examine local differences in brain activity during natural sleep, we recorded the electroencephalogram (EEG) and the local field potential (LFP) across multiple sites within the avian pallium of zebra finches without perturbing the previous awake state. We scored the sleep stages independently in each pallial site and found that the overall percentage of slow wave sleep (SWS) was significantly higher, and that SWS durations were significantly longer, in the deeper mesopallium sites compared to most sites on the pallial surface. We analyzed the likelihood of observing simultaneous sleep states across electrodes, and found that on average, sleep stages are not pallium-wide phenomena, but rather deviate widely across electrode sites. Importantly, deeper electrode sites had a dominant role in defining the temporal aspects of sleep state congruence. Altogether, these findings show that local regulation of sleep oscillations also occurs in the avian brain without prior awake recruitment of specific pallial circuits and in the absence of mammalian cortical neural architecture.