Visual lateralization and monocular sleep in the domestic chick.

Behavioural sleep during the first 2 weeks of life was investigated in female chicks reared with an imprinting object or in social (visual) isolation. Binocular sleep tended to decrease and monocular sleep to increase with age in both rearing conditions. In chicks reared with an imprinted object. during the first week, monocular sleep with either right or left eye closure occurred with approximately the same frequency, except that on day 5 in which right eye closure dominated; during the second week, however, there was a clear bias towards more monocular sleep with left eye closure. During the second week, the pattern of monocular sleep was similar in both rearing conditions, but during the first week chicks reared with the imprinting object showed relatively more right eye closure compared to chicks reared without the imprinting object, an effect that might tentatively be associated with consolidation of imprinting memories in the left hemisphere. Binocular sleep occurred in all four body postures adopted by chicks during sleep: standing sleep, sleep with bill forward, sleep with bill on the ground, and sleep with head on the ground. Monocular sleep, in contrast, only occurred when chicks adopted the bill forward posture. When the colour of the imprinting object was suddenly changed on day 8, a striking shift towards predominant right eye closure during monocular sleep was observed. The same occurred when the imprinting object was suddenly removed from the home-cage on day 8, but not with other types of changes (i.e., when a novel different object was inserted into the home-cage or when a novel-coloured imprinting object was inserted into the home-cage together with the original one). It is argued that this phenomenon could be associated with right hemisphere involvement in response to novelty.