Ontogeny
The development of unihemishperic slow-wave sleep (USWS) has been studied
primarily by using the behavioral assay, unilateral eye closure (UEC)
during sleep, as the measure of USWS (as opposed to direct measurement
of coritcal rhythms by EEG).
Hemispheric Dominance
The tendency of one hemisphere to perform better than the other on
visual tasks has been observed in homing pigeons (Columbia livia)
and the European robin (Erithacus rubecula). In these waking
state experiments, the left hemisphere was shown to be dominant in both
of the species (21, 26).
The same hemispheric dominance has been noted during USWS.In two week
old domestic chickens, Rogers and Chaffey (1994) observed a preference
toward left UEC during USWS (27). This
implies a preference for maintaining vigilance in the left hemisphere.
During the last few days (18 and 19) of incubation in normal chick development,
the embryo turns in the egg so that the left eye is occluded by pressing
this eye against its body. This leaves the right eye open for stimulation
from light entering through the shell. This is concurrent with the establishment
of functional visual connections in the cortex. The difference in light
exposure to the eyes may drive the hemispheric dominance observed in
USWS (4).
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