All addictive substances exert their effects by harnessing the powerful reward system that is normally used to guide an animal’s behavior. One simplistic way to build a reward system is to have neurons that carry positive and negative values. These reward centers would then, and have been shown to, be the primary target of the addictive substances. To find these reward centers, researchers must devise a way to stimulate a brain region and ask the animal how it likes it. How can one do this?
Olds and Milner did it, in 19542. They made a pivotal assumption that if stimulating a brain region is rewarding, the animal would like to do more of it. In this way, they can just hand over the switch to stimulate an animal’s brain region to the animal itself and see how much it presses the switch. At first, an animal may accidentally deliver the stimulation here and there, but if the stimulation is rewarding, the animal will soon to learn to press the switch more and more, forgetting to eat or sleep. Using this ingenious method, Olds and Milner found that stimulating a region in the middle of the brain called septum (which is now known to be important for stress and mood regulation) is highly addictive. Over the course of 12 hours of experiments, the rats stimulated their septa incessantly at a rate of 6-30 times a minute. These self-stimulations stopped when the researchers set the voltage of the stimulation to 0, suggesting that this unnatural obsession was nevertheless goal-directed.
The legacy of self-stimulation did not stop here. By surveying the brain regions more systematically and changing the stimulation method from electrical to chemical (e.g. cocaine), generations of researchers have generated a comprehensive map of addiction in the brain3.
- Olds, J. Self-Stimulation of the Brain. Science. 127, 315–324 (1958).
- Olds, J. & Milner, P. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J. Comp. Physiol. Psychol. 47, 419–27 (1954).
- Lüscher, C. & Malenka, R. C. Drug-evoked synaptic plasticity in addiction: from molecular changes to circuit remodeling. Neuron 69, 650–63 (2011).