Each night, you place your head on your pillow, close your eyes, and, barring insomnia, you lose consciousness of the world around you, drifting into blissful oblivion. Eventually, you reawaken, only to hallucinate in a paralyzed body. These hallucinations are called dreams. Why do dreams happen, and what function do they serve? Although these are simple questions, the purpose of dreaming (and to a large extent, sleep itself) is far from understood.
Sleep takes place in several stages. Each repeated cycle of stages takes roughly 90 minutes to complete. During the first four stages, brainwaves recorded with EEG gradually slow in frequency and increase in amplitude. “Spindle” activity detected in the EEG may represent the efforts of the thalamus and cortex to suppress sensory processing. Oddly enough, semantic processing (i.e., understanding words) can still take place during the first two stages of sleep. This is evidenced by the observation of the N400 response, a neural signature of semantic processing in EEG recordings during stage 2 sleep. Stages 3 and 4 are dominated by strong delta activity, slow brainwaves which occur at 1 – 4 cycles per second, which indicate strong neural synchrony. It is believed that consciousness is minimal in delta sleep because there is less functional differentiation of the cortex as compared with wakefulness.
After stage 4, a bizarre transition occurs: EEG recordings become almost indistinguishable from the awake brain, the eyes move rapidly behind their shut lids, and the mind hallucinates whilst the skeletal muscles remain paralyzed. This is known as rapid eye movement (REM) sleep, and it is the stage of sleep during which dreams are most frequently reported. The physiological correlates of dreaming have been well studied, but the purpose of dreaming is almost entirely unknown. Theories attempting to explain dreaming range from the romantic to the utterly banal. Some hypothesize that dreams, such as those that commonly occur during REM sleep, give us deeper insights into ourselves, perhaps as a form of natural, internal psychotherapy. A less romantic view of REM sleep is that it has been selected for by evolution simply to jostle the ocular fluid, which provides the lens of the eye with oxygen, since the lens lacks capillaries and its tissue dies if the fluid stagnates.
Both views may be partially correct. It is possible that REM sleep evolved to fulfill a low level physiological purpose, and dreaming occurred as a side effect: the neocortex’s attempt to interpret neural noise as meaningful sensory input. Spatial patterns of brainwave activity, known as ponto-geniculo-occipital (PGO) waves, occur prior to and during REM sleep and may be signals that generate dream imagery. PGO waves are recorded invasively, starting in the region of the brainstem known as the pons then moving through parts of the thalamus and visual cortex. A direct association has yet to be made between the brainstem and dreaming; however, patients with lesions to the parietal lobe often stop experiencing dreams.
Dreaming may indeed represent a final frontier for neuroscience and psychology. Despite being so intimate, we arguably understand less about dreams than we do about the far reaches of outer space. If you are a young person interested in brain science, you may one day be among the first researchers to discover the true purpose of REM sleep and dreaming!
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