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).

 


Figure 1. Normal Chicken Development. From www.pbs.org/wgbh/nova/odyssey/clips.

Investigating this hypothesis, Mascetti et al. (1999) manipulated the light exposure to the eyes of chickens during embryonic development. In a dark environment, where the day 18-19 rotation would have no effect on differential light exposure to the eyes, they saw preferential left UEC for the first two post-hatch weeks (14).


Imprinting has been shown to affect the choice of UEC in young chicks. Bobbo et al (2002) presented objects in the right visual field during early development; chicks that had been to exposed to this imprinting object showed an initial preference for right UEC (4). They speculated that the amount of light the right eye receives during early development is a signal of the safety of the environment.

 

Sleep Deprivation

During the migratory season, USWS may increase in order to maintain motor function while sleeping. During the migrating season, the Swainson's thrush (Catharus ustulatus) has been observed increasing engagement in daytime USWS (10). Increases in USWS during the migratory season to aid in maintained motor vigilance has also been hypothesized in the captive White-crowned sparrow (Zonotrichia leucophrys gambelii) (25).

 

However, when chickens are deprived of all forms of sleep, upon sleep recovery USWS decreases (6). The benefits of maintaining vigilance by keeping one hemisphere active during sleep no longer outweigh the costs of USWS.