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

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.