problem solving. rem deprivation seemed to impair declarative (i.e., factual) memory only in more complex cases, such as memories of longer stories. rem sleep apparently counteracts attempts to suppress certain thoughts. according to the dual-process hypothesis of sleep and memory, the two major phases of sleep correspond to different types of memory. "night half" studies have tested this hypothesis with memory tasks either begun before sleep and assessed in the middle of the night, or begun in the middle of the night and assessed in the morning. slow-wave sleep, part of non-rem sleep, appears to be important for declarative memory. artificial enhancement of the non-rem sleep improves the next-day recall of memorized pairs of words. tucker et al. demonstrated that a daytime nap containing solely non-rem sleep enhances declarative memory—but not procedural memory. according to the sequential hypothesis, the two types of sleep work together to consolidate memory. sleep researcher jerome siegel has observed that extreme rem deprivation does not significantly interfere with memory. one case study of an individual who had little or no rem sleep due to a shrapnel injury to the brainstem did not find the individual's memory to be impaired. antidepressants, which suppress rem sleep, show no evidence of impairing memory and may improve it. graeme mitchison and francis crick proposed in 1983 that by virtue of its inherent spontaneous activity, the function of rem sleep "is to remove certain undesirable modes of interaction in networks of cells in the cerebral cortex"—a process they characterize as "unlearning". as a result, those memories which are relevant (whose underlying neuronal substrate is strong enough to withstand such spontaneous, chaotic activation) are further strengthened, whilst weaker, transient, "noise" memory traces disintegrate. memory consolidation during paradoxical sleep is specifically correlated with the periods of rapid eye movement, which do not occur continuously. one explanation for this correlation is that the pgo electrical waves, which precede the eye movements, also influence memory. rem sleep could provide a unique opportunity for "unlearning" to occur in the basic neural networks involved in homeostasis, which are protected from this "synaptic downscaling" effect during deep sleep. neural ontogeny rem sleep prevails most after birth, and diminishes with age. according to the "ontogenetic hypothesis", rem (also known in neonates as active sleep) aids the developing brain by providing the neural stimulation that newborns need to form mature neural connections. sleep deprivation studies have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, and decreased brain mass. the strongest evidence for the ontogenetic hypothesis comes from experiments on rem deprivation, and from the development of the visual system in the lateral geniculate nucleus and primary visual cortex. defensive immobilization ioannis tsoukalas of stockholm university has hypothesized that rem sleep is an evolutionary transformation of a well-known defensive mechanism, the tonic immobility reflex. this reflex, also known as animal hypnosis or death feigning, functions as the last line of defense against an attacking predator and consists of the total immobilization of the animal so that it appears dead. tsoukalas argues that the neurophysiology and phenomenology of this reaction shows striking similarities to rem sleep; for example, both reactions exhibit brainstem control, cholinergic neurotransmission, paralysis, hippocampal theta rhythm, and thermoregulatory changes. shift of gaze according to "scanning hypothesis", the directional properties of rem sleep are related to a shift of gaze in dream imagery. against this hypothesis is that such eye movements occur in those born blind and in fetuses in spite of lack of vision. also, binocular rems are non-conjugated (i.e., the two eyes do not point in the same direction at a time) and so lack a fixation point. in support of this theory, research finds that in goal-oriented dreams, eye gaze is directed towards the dream action, determined from correlations in the eye and body movements of rem sleep behavior disorder patients who enact their dreams. oxygen supply to cornea dr. david m. maurice, an eye specialist and former adjunct professor at columbia university, proposed that rem sleep was associated with oxygen supply to the cornea, and that aqueous humor, the liquid between cornea and iris, was stagnant if not stirred. among the supportive evidence, he calculated that if aqueous humor was stagnant, oxygen from the iris had to reach the cornea by diffusion through aqueous humor, which was not sufficient. according to the theory, when the organism is awake, eye movement (or cool environmental temperature) enables the aqueous humor to circulate. when the organism is sleeping, rem provides the much needed stir to aqueous humor. this theory is consistent with the observation that fetuses, as well as eye-sealed newborn animals, spend much time in rem sleep, and that during a normal sleep, a person's rem sleep episodes become progressively longer deeper into the night. however, owls experience rem sleep, but do not move their head more than in non-rem sleep and it is well known that owls' eyes are nearly immobile. other theories another theory suggests that monoamine shutdown is required so that the monoamine receptors in the brain can recover to regain full sensitivity. the sentinel hypothesis of rem sleep was put forward by frederick snyder in 1966. it is based upon the observation that rem sleep in several mammals (the rat, the hedgehog, the rabbit, and the rhesus monkey) is followed by a brief awakening. this does not occur for either cats or humans, although humans are more likely to wake from rem sleep than from nrem sleep. snyder hypothesized that rem sleep activates an animal periodically, to scan the environment for possible predators. this hypothesis does not explain the muscle paralysis of rem sleep; however, a logical analysis might suggest that the muscle paralysis exists to prevent the animal from fully waking up unnecessarily, and allowing it to return easily to deeper sleep. jim horne, a sleep researcher at loughborough university, has suggested that rem in modern humans compensates for the reduced need for wakeful food foraging. other theories are that rem sleep warms the brain, stimulates and stabilizes the neural circuits that have not been activated during waking, or creates internal stimulation to aid development of the cns; while some argue that rem lacks any purpose, and simply results from random brain activation. furthermore, eye movements play a role in certain psychotherapies such as eye movement desensitization and reprocessing (emdr).