The figure above displays the brain-stimulation current levels maintained by two rats over 360 minutes and 240 minutes, respectively. Five-minute periods in which lever-presses produced brain stimulation were alternated with five-minute periods during which there was no stimulation. During stimulation periods pressing one of two levers delivered a brief train of pulses and increased the stimulation current by a small amount, whereas pressing the other lever delivered an identical pulse train and decreased the stimulation current by a small amount. By switching levers occasionally, each rat was able to keep the stimulation current within relatively narrow bounds.