Excerpts from "Sleep in marine mammals" L.M. Mukhametov Experimental
Brain Research Supplement 8......
"Unihemispheric slow wave sleep is the main type of sleep in the dolphin
brain. The synchronized EEG patterns in one hemisphere may be accompanied
by a desynchronized pattern in the other. The intermediate EEG
synchronization (stage 2) could be observed both bilaterally and
unilaterally. Delta sleep (stage 3) in dolphins can only be
unihemispheric. We have never recorded bilateral delta waves in dolphin
brain during natural sleep. Unilateral sleep episodes can last more than
2h."
The author stresses "natural" sleep because bilateral delta waves can be
evoked by certain pharmacological agents such as Diazepam.
"Unihemispheric sleep was observed in all dolphins of the two species (21
bottlenose dolphins and 9 porpoises), in which the electrodes were
implanted in both hemispheres. The state with the asynchrony of EEG slow
waves in the two hemispheres occupied about 30%-40% of recording time.
Simultaneous recordings from the parietal, occipital, and frontal fields of
the hemisphere confirm that a hemisphere is always synchronized or
desynchronized as a unit. Records from thalamic nuclei demonstrate that
the thalamus can also generate slow wave activity unilaterally and
concurrently with ipsilateral neocortex. Thus, unilateral slow wave sleep
is not only a cortical, but also a subcortical phenomenon."
Mukhametov provides a figure in this paper which indicates the time that
each hemisphere spends in each stage of sleep over a 24hr period. 7 periods
of unilateral delta synchronization are indicated giving a total of about 4
hours (about 2 hours a hemisphere). The sleep times for the two hemispheres
is commonly unequal, but while individuals may demonstrate a particular
cerebral dominance with respect to sleep, there seems no pattern of left or
right dominance that is uniform across the species.
It has been observed that "sleeping" dolphins will have either one or two
eyes open at any time and that there appears to be no correlation with
which hemisphere is wakeful. Observations of the optic chiasm in dolphins
(Jacobs MS, Morgane PJ, Mcfarland WL, (1975) Degeneration of the visual
pathways in the bottlenose dolphin. Brain Res 88:346-352) suggest that
its complete decussation allows either eye to act as a "sentinel" eye
during hemispheric sleep.
Investigations of sleep in seals is interesting too. One study of the Cape
fur seal (Arctocephalus pusillus) found that across a 24hr recording time,
4.6% of this time was spent in paradoxical sleep (absent in dolphins) and
27.4% was spent in slow-wave sleep. 40% of the latter slow-wave sleep was
demonstrated to be unilateral. (Lyamin OI, Chetyrbok IS, (1992) Neurosci
Letters 143:263-).
A study of northern fur seals (Callorhinus ursinus) showed that "the
relative amount of interhemispheric slow wave asymmetry was higher during
sleep in water comparing to that on land" (Liamin OI, Mukhametov LM,
Poliakova IG, (1986) Zh Vyssh Nerv Deiat 36(6):1039-). When sleeping at
the water surface, these seals are observed to maintain their position by
using just one of their flippers. Some of the interesting mail to
bionet.neuroscience that has appeared recently on dolphins and split brains
has wondered how much circuitry (if any) must be duplicated in a unilateral
sleeper. Probably tricks like these of the seal and adaptations involving
the decussation of sensory afferents (as with dolphin vision above) allow a
'lone' hemisphere to tackle the problems of sleep in a marine environment
without duplication. I'd be interested in what kinds of functional
asymmetries exist in the brains of these animals and see how the temporary
absence of a unilateral function is reconciled during unihemispheric sleep
eg Can a unilaterally sleeping dolphin produce 'meaningful' vocalizations
and respond to other dolphin signals in a normal way?
I think that these observations of sleep in the marine mammals further
suggest that sleep isn't just an evolutionary adaptation but a critical
aspect of our physiology. I've heard that one blind species of marine
mammal has adapted to a particularly harsh environment (one where the
opportunity to sleep is severely compromised) by sleeping in 90 second
periods throughout the day, and clocks up a total of 7 hours sleep every
24! The same conditions that let vision slip from the species'
physiological repertoire, couldn't stifle the physiological requirement for
sleep.
