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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 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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