Sleep and Dreaming

The age-old, common-sense explanation of sleep is that it results from reduced brain activity, induced by fatigue. Until 1950 scientists working on sleep thought that the awake state was maintained by active sensory stimulation and that accordingly, the brain fell asleep when, with fatigue, sensory stimulation decreased. In the late 1940's and early 1950's, a series of startling discoveries disproved this notion. First, Giuseppe Moruzzi and Horace Magoun found that severing the ascending sensory pathways of cats did not interfere with either sleep or wakefulness. By contrast, lesions of the reticular formation that did not interfere with the ascending sensory pathways resulted in a behavioral stupor and a continuous EEG pattern resembling sleep. Moruzzi and Magoun concluded that although reticular activity could be increased by collaterals from the specific sensory systems, the forebrain is kept awake not by direct input from sensory pathways, but by tonic activity in pathways from the reticular formation to the cortex, According to this view, sleep results from the reduction in activity in the reticular formation, wakefulness by the return of activity.