Light exposure of the chick embryo influences monocular sleep

Together with some aquatic mammals, several species of birds, such as the domestic fowl, exhibit a particular behavioural and electrophysiological state called 'unihemispheric sleep', in which one cerebral hemisphere is awake and the other is sleeping. Slow-wave sleep in one hemisphere is associated with closure of the contralateral eye, whilst the eye contralateral to the awake hemisphere is open; closure of both eyes, in contrast, is associated with bihemispheric slow-wave sleep or with REM sleep. During the last few days of incubation the chick embryo is turned in the egg so that it occludes its left eye, whereas light entering through the shell can stimulate the right eye. We found that in the first 2 days after hatching chicks coming from eggs incubated in the light and reared with an imprinting object slept mostly with their right-eye open, whereas those coming from eggs incubated in the dark slept mostly with their left-eye open. In chicks reared without an imprinting object, in contrast, the effect of incubation extended over the first 2 weeks in females, with dark-incubated chicks showing more pronounced left-eye closure than light-incubated chicks, whereas the effect of incubation was similar to that observed in chicks reared with the imprinting object during the first 2 days post-hatch in males and more variable from day to day thereafter. Asymmetric light stimulation of embryos can thus modulate, though it does not generate, the left-right direction of eye opening during post-hatching monocular sleep; apparently, such a modulation depends on the emotional/motivational state of the animal associated with the outcomes of a naturally occurring visual imprinting.