THERE has been shown to exist in the median cephalic portion of the brain stem of cats1 and monkeys2 an area comprising the midbrain tegmentum, the subthalamus and hypothalamus and the medial portion of the thalamus which subserves a specialized function in the response of the animal to sensory stimuli. This area can be readily excited by peripheral stimulation, such excitation, or that produced by direct stimulation with a probing electrode, arousing the sleeping animal to wakefulness and altering its electroencephalogram in a manner consistent with such a change. The evidence suggests that this activating system is of particular importance in initiating and maintaining an alert or attentive state in the animal, rendering it capable of reacting adaptively to stimuli in a way impossible in sleep.
Lindsley and associates3 have observed that lesions placed so as to destroy a significant portion of this area in cats were
FRENCH JD, MAGOUN HW. EFFECTS OF CHRONIC LESIONS IN CENTRAL CEPHALIC BRAIN STEM OF MONKEYS. AMA Arch NeurPsych. 1952;68(5):591-604. doi:10.1001/archneurpsyc.1952.02320230017002