User:Polvinod/Unihemispheric slow-wave sleep draft

NOTE: The following is a draft under construction.

Unihemispheric slow-wave sleep (USWS), also termed as asymmetric slow-wave sleep (ASWS), is characterized by slow-wave activity in one hemisphere of the brain while a low voltage EEG, characteristic of wakefulness, is present in the other. The phenomenon has been observed in a number of terrestrial, aquatic as well as avian species. Unique physiology, including differential release of the neurotransmitter acetylcholine has been linked to the phenomenon. USWS offers species exhibiting a number of benefits, including the ability to rest in areas of high predation or during long migratory flights. The behavior remains an important research topic because USWS is possibly the first animal behavior which uses different regions of the brain to simultaneously control sleep and wakefulness.

Northern Fur Seal (Callorhinus ursinus) exhibits unihemispheric slow-wave sleep in aquatic environments.

Species Exhibiting USWS

Aquatic Mammals

Cetaceans

Of all the cetacean species, USWS has been found to be exhibited in the following four species:

• Bottlenose dolphin (tursiops truncates)
• Porpoise (phocoena phocoena)
• Amazon river dolphin (inia geoffrensis)
• Pilot whale (globicephala scammoni)
• Beluga whale (delphinapterus leucus)

Pinnipeds

A major difference between the pinnipeds and the cetaceans is that while the cetaceans solely sleep in water, pinnipeds are capable of sleeping on either land or water. In addition, pinnipeds that exhibit USWS do so at a higher rate while sleeping in water than on land. Though no USWS has been observed in true seals, four different species eared seals have been found to exhibit USWS including:

• Northern fur seal (Callorhinus ursinus)

Significant research has been done illustrating that the northern fur seal can alternate between BSWS and USWS depending on its location while sleeping. While on land, 69% of all SWS is present bilaterally; however, when sleep takes place in water, 68% of all SWS is found with interhemispheric EEG asymmetry, indicating USWS.

• Steller sea lion (Eumetopias jubatus)
• Southern sea lion (Otari bryonia)

Sirenia

The final order of aquatic mammals, sirenia include three species of manatees. Experiments have only exhibited USWS in the Amazonian manatee (trichechus inunguis).

Birds

The common swift (apus apus) was the best candidate for research as to whether birds exhibiting USWS can potentially sleep in flight as it directly observed during their flight habits at night. Still, evidence for USWS is strictly circumstantial and based on the notion that if swifts must sleep to survive, they must do so via aerial roosting as little other time is spent sleeping in a nest.

Multiple other species of birds have also been found to exhibit USWS including:

• White-crowned sparrow (zonotrichia leucophrys gambelii)
• Mallard (Anas platyrhynchos).
• Domestic chicken (Gallus gallus domesticus),
• Northern bobwhite (Colinus virginianus),
• Japanese quail (Coturnix japonica)
• Glaucous-winged Gull (Larus glaucescens)
• Orange-fronted parakeet (Aratinga canicularis)
• Common Blackbird (Turdus merula)

Benefits

Many species of birds and marine mammals have advantages due to the capability to undergo unihemispheric slow-wave sleep including, but not limited to an increased chance of the evasion of potential predators and ability to sleep during migration. Unihemispheric sleep aids in the visual vigilance of the environment, the preservation of movement, and in Cetaceans, the controlling of the respiratory system.

Adaptation to high-risk predation

Most species of birds are able to detect approaching predators during unihemispheric slow-wave sleep. During flight, birds maintain visual vigilance while sleeping using USWS by keeping one eye open. The utilization of unihemispheric slow-wave sleep by avian species is directly proportional to the risk of predation. In other words, the usage of USWS of certain species of birds increases as the risk of predation increases.

Survival of the fittest adaptation

The evolution of both cetaceans and birds may have involved some mechanisms in order to help increase the chance of avoiding predators. Certain species, especially species of birds, that acquired the ability to perform unihemispheric slow-wave sleep had an advantage and were more likely to escape their potential predators over certain species that lacked the ability.

Regulation based on surroundings

Birds can sleep more efficiently by sleeping with both hemispheres simultaneously (bi-hemispheric slow wave sleep) when in safe conditions, but will increase the usage of USWS if the environment has a potential to become dangerous. It is more beneficial to sleep using both hemispheres; however, the positives of unihemispheric slow-wave sleep prevail over its negatives under extreme conditions. While in unihemispheric slow-wave sleep, birds will sleep with one open eye towards the direction from which predators are more likely to approach. When birds do this in a flock, this is called the "group edge effect".

The "group edge effect" describes how birds that are at the edge of the flock are more alert and are scanning for predators for often. These birds are more at risk then the birds in the center of the flock and are required to be on the lookout for both their own safety and the safety of the flock as a whole. These birds will spend more time in unihemispheric slow-wave sleep than the birds towards the center. Since USWS allows for the one eye to still be open, the cerebral hemisphere that undergoes slow-wave sleep varies depending on the position of the bird relative to the rest of the flock. If the bird's left side is facing outward, the left hemisphere will be in slow-wave sleep; if the bird's right side is facing outward, the right hemisphere will be in slow-wave sleep. This is because the eyes are contra-lateral to the left and right hemispheres of the cerebral cortex. The open eye of the bird is always directed towards the outside of the group, in the direction predators could potentially attack from.

Surfacing for air while sleeping in marine mammals

Unihemispheric slow-wave sleep seems to allow the simultaneous sleeping and surfacing to breathe of aquatic mammals including both dolphins and seals. Bottlenose dolphins are one specific species of cetaceans that have been proven experimentally to use USWS in order to maintain both swimming patterns and the surfacing for air while sleeping over the course of five days.

Ability to rest during long flights

When migrating, birds may undergo unihemispheric slow-wave sleep in order to simultaneously sleep while visually navigating flight.

Physiology

In most animals, slow-wave sleep (SWS) is characterized by high amplitude, low frequency electroencephalography (EEG) readings. This is also known as desynchronized state of the brain, or deep sleep. In USWS, only one hemisphere exhibits the deep sleep EEG while the other hemisphere exhibits an EEG typical of wakefulness with a low amplitude and high frequency. There also exists instances in which hemispheres are in transitional stages of sleep, but they have not been the subject of study due to their ambiguous nature. USWS represents the first known behavior in which one part of the brain controls sleep while another part controls wakefulness.

Role of Acetylcholine

Due to the origin of USWS in the brain, neurotransmitters are believed to be involved in its regulation. The neurotransmitter acetylcholine has been linked to hemispheric activation in northern fur seals. Acetylcholine is released in nearly the same amounts per hemisphere in bilateral slow-wave sleep. However, in USWS, the maximal release of the cortical acetylcholine neurotransmitter is lateralized to the hemisphere exhibiting an EEG trace resembling wakefulness. The hemisphere exhibiting SWS is marked by minimal release of acetylcholine. This model of acetylcholine release has been further discovered in additional species such as the bottlenose dolphin.

Eye Opening

In domestic chicks and other species of birds exhibiting USWS, one eye remained open contra-lateral to the "awake" hemisphere. The closed eye was shown to be contra-lateral to the hemisphere engaging in slow-wave sleep. Learning tasks, such as those including predator recognition, demonstrated the open eye could be preferential. This has also been shown to be the favored behavior of white whale, although inconsistencies have arisen directly relating the sleeping hemisphere and open eye. Keeping one eye open aids birds to engage in USWS while mid-flight as well as to observe predators in their vicinity.

Thermoregulation

Brain temperature has been shown to drop when a sleeping EEG is exhibited in one or both hemispheres. This decrease in temperature has been linked to a method to thermoregulate and conserve energy while maintaining the vigilance of USWS. The thermoregulation has been demonstrated in dolphins and is believed to be conserved among species exhibiting USWS.

Anatomical Variations

Smaller Corpus Callosum

USWS requires hemisphereic separation to isolate the cerebral hemispheres enough to ensure one can engage in SWS while other is awake. The corpus callosum is the anatomical structure in the mammalian brain which allows for interhemispheric communication. Cetaceans have been observed to have a smaller corpus callosum when compared to other mammals. Similarly birds lack a corpus callosum all together and have only few means of interhemispheric connections. Other evidence contradicts this potential role; sagital transsections of the corpus callosum have been found to result in strictly bihemispheric sleep. As a result it seems this anatomical difference, though well correlated, does not directly explain the existence of USWS.

Noradrenergic Diffuse Modulatory System Variations

A promissing method of identifying the neuroanatomical structures responsible for USWS is continuing comparisons of brains that exhibit USWS with those that do not. Some studies have shown induced asynchrous SWS in non USWS exhibiting animals as a result of sagital transsections of subcortical regions, including the lower brain stem, while leaving the corpus callosum intact. Other comparisons found that mammals exhibiting USWS have a larger posterior commissure and increased decussation of ascending fibers from the locus coeruleus in the brainstem. This is consistent with the fact that one form for neuromodulation, the noradrenergic diffuse modulatory system present in the locus coeruleus, is involved in regulating arousal, attention, and sleep-wake cycles.

Complete Crossing of the Optic Nerve

Complete crossing of the nerves at the optic chiasm in birds has also stimulated research. Complete decussation of the optic tract has been seen as a method of ensuring the open eye strictly activates the ipsilateral hemisphere. Some evidence indicates that this alone is not enough as blindness would theoretically prevent USWS if retinal nerve stimuli was the sole player. However, USWS was still exhibited in blinded birds despite the absence of visual input.

Future Research

Recent studies have illustrated that the white-crowned sparrow, as well as other passerines, have the capability of sleeping most significantly during the migratory season while in flight. However, the sleep patterns in this study were observed during migratory restlessness in capacity which might not be equal to that of free-flying birds. Free-flying birds might be able to spend some time sleeping while in non-migratory flight as well when in unobstructed sky as opposed to in controlled captive conditions. To truly determine if birds can sleep in flight, recordings of brain activity must take place during flight instead of after landing. A method of recording brain activity in pigeons during flight has recently proven promising in that it could obtain an EEG of each hemisphere but for relatively short periods of time. Coupled with simulated windtunnels in a controlled setting, these new methods of measuring brain activity could elucidate the truth behind whether or not birds sleep during flight.
Additionally, based on research elucidating the role of acetylcholine in control of USWS, additional neurotransmitters are being researched to understand their roles in the asymmetric sleep model