Local sleep: a spatial learning task enhances sleep in the right hemisphere of domestic chicks (Gallus gallus)

During sleep, domestic chicks (Gallus gallus) show brief and transient periods during which one eye is open while the other remains shut. Electrophysiological recordings showed that the hemisphere contralateral to the open eye exhibited an EEG with fast waves typical of wakefulness, whereas the hemisphere contralateral to the closed eye exhibited an electroencephalogram (EEG) typical of slow-wave sleep. We investigated the time spent in sleep and in monocular-unihemispheric sleep (Mo-Un sleep) following the learning of a spatial discrimination task. A group of experimental chicks from days 8 to 11 post-hatching were trained singly to select one container among four, having a hole on the top (making food available) and positioned in a corner of a rectangular arena. Chicks of the control group did not learn the task because all four containers had a hole on the top and therefore chicks could randomly select any one of them. Experimental and control chicks underwent the same number of trials. Experimental chicks had more total time spent sleeping than control chicks. Experimental chicks spent more time in left Mo-Un sleep, which would be connected with a dominance of the right hemisphere during learning trials. Control chicks showed no eye closure bias at days 8 and 9; however, a slight bias for more right eye closure at days 10 and 11 was observed, suggesting that there was an absence of hemispheric dominance during the first 2 days of control trials and a dominance of the right hemisphere during the last 2 days of control trials. Overall, chicks that learned the spatial task slept significantly more than chicks that were exposed to the experimental paradigm but did not learn the task. This suggests that the Mo-Un sleep pattern showed by experimental chicks is a type of local sleep associated with a process of functional recovery and/or with consolidation of memory in the right hemisphere, which would be mainly engaged during training trials.