Cumulative Record. Chapter 10: The Experimental Analysis of Behavior. Quote 24

In one of Olds’ experiments, a rat presses a lever to give itself mild electrical stimulation in the anterior hypothalamus. When every response is so “reinforced,” behavior is sustained in strength for long periods of time . . . Although there remain some puzzling differences between behavior so reinforced and behavior reinforced with food, Olds’ discovery in an important step toward our understanding of the physiological mechanisms involved in the operation of the environmental variable. (pp. 156-157)