The Ascending Reticular Activating System

Discovery of the ascending reticular activating system (ARAS) can be attributed to work done in research neuroscientist Horace Magoun’s laboratory. Before this finding, most scientists would focus on the diencephalon (and anterior midbrain) but not more caudally. Stimulation of the medial bulbar reticular formation in the pontine and midbrain tegmentum resulted disappearance of synchronized discharge and low-voltage fast activity. The effects were mediated by a thalamic projection system. This finding was a dramatic departure from the early philosophers’ ascription of the awake soul to the ventricles (Galen), lumbosacral cord (Plato), pineal gland (Descartes), and even from more modern nineteenth- and twentieth-century hypotheses that the corpus striatum or periaqueductal gray matter housed the “seat of awareness.” Magoun and his collaborators closed in on its true location in the cephalic brainstem—clinicians and neuropathologists would soon follow.