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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.

Unihemispheric sleep is an aspect of cerebral lateralization of birds. During sleep, domestic chicks show brief periods during which one eye is open whilst the other remains shut. In this study, time spent in sleeping and in monocular-unihemispheric sleep (Mo-Un sleep) was investigated following the monocular learning of a spatial discrimination task. Two groups of experimental chicks from day 8 to day 11 post-hatching were trained in a spatial paradigm based on geometrical and topographical clues. One group performed the task with left eye open (LE-chicks), whilst another group performed the task with the right eye open (RE-chicks). LE-chick learned the task, whilst RE-chicks were unable to learn. Time spent in binocular sleep and right Mo-Un sleep (right eye closed and left hemisphere sleeping) was equal in both groups of chicks. Time spent in left Mo-Un sleep (left eye closed and right hemisphere sleeping) was significantly higher in LE-chicks than in RE-chicks. Laterality index reveals that LE-chicks had a significant bias towards more left Mo-Un sleep at any recording day, whilst RE-chicks showed a significant bias towards more right Mo-Un sleep at day 8 and 9 but not at days 10 and 11. RE-chick bias at days 8 and 9 could be attributed to a recovery process in left hemisphere connected to its activation/use effect during trials whilst recovery would be absent at days 10 and 11. LE-chicks bias would be associated with the formation of a spatial memory trace and with a recovery process in right hemisphere.

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 contra-lateral to the open eye exhibited an EEG with fast waves typical of wakefulness, whereas the hemisphere contra-lateral to the closed eye exhibited an EEG typical of slow wave sleep. We investigated the pattern of monocular-unihemispheric sleep (Mo-Un sleep; i.e. selective preferential closure/opening of the left or right eye during sleep) following three types of visual learning tasks. The first group of chicks was submitted to a colour discrimination task (1), the second group to a spatial discrimination task with colour as a conspicuous, but irrelevant, cue (2), the third group to a spatial task without colour cue. After learning, the amount of binocular sleep and Mo-Un sleep patterns were recorded. The first and the second group of chicks exhibited more right Mo-Un sleep (right eye-closure/left unihemispheric sleep), suggesting that this pattern may be connected with prevalent engagement of left hemisphere during training trials. The third group showed a significant more left Mo-Un sleep (left eye-closure/right unihemispheric sleep) which would be associated with a prevalent engagement of right hemisphere during trials. Chicks of the control groups, did not learn the task, but were submitted to an equal number of trials. Controls of tasks 1 and 2 showed more left Mo-Un sleep suggesting a dominance of right hemisphere during exposure trials. Instead there was no eye-closure bias in controls of task 3, suggesting an absence of hemispheric dominance during trials. It is suggested that the Mo-Un sleep pattern may be a type of local sleep associated with a process of functional recovery in the hemisphere which was mainly engaged during training trials.

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