Scientists have long known that two brain structures lying below the rostrum of the corpus callosum, called septal nuclei (SNs), play a significant role in human pleasure response. This part of the brain interacts with many other elements of the limbic system, which regulates fear expression and other forms of emotional response. Studies show that in some animals, most notably rats, electrical stimulation of SNs can motivate self-stimulation, causing them to perform such behaviors as manipulating levers or returning to regions of their housing that administer further electrical stimulation. Furthermore, connections between the SNs and portions of the brain dedicated to olfaction and memory retention have also been discovered.
Several other neural structures have been found to play a role in governing the brain's emotional responses, however, not just the SNs of rats and humans. In fact, when laboratory rats had electrical stimulation applied to their habenular nuclei, pleasure responses shifted by 30 percent, whereas the same electrical shock applied to the SNs produced a lesser result. While scientists remain convinced that SNs play a role in the brain's regulation of fear, sadness, joy, and pleasure response, scientists now believe that other neural structures may respond more forcefully to stimulation—even if the voltage of the shock administered doesn't change—than the septal nuclei.