THE BRAIN FROM TOP TO BOTTOM

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Pleasure-Seeking Behaviour

Pleasure and Drugs

Avoiding Pain

The experiment that revealed the existence of the reward circuit in the 1950s was the result of the work of two researchers, Olds and Milner. This experiment has been repeated several times since, with all sorts of variations, but it basically works as follows.

Electrodes are implanted in the nucleus accumbens of a rat’s brain. When the rat presses a lever, it stimulates its own nucleus accumbens. Once the rat discovers the trick, it keeps stimulating itself endlessly, without even stopping to eat. The direct stimulation of this circuit is so powerful that the animal forgets its own basic needs.

This is exactly what happens when someone takes drugs.

THE REWARD CIRCUIT

Eating, drinking, having sex, and displaying maternal behaviour are all activities that are essential for the survival of the individual and the species. In the course of evolution, natural selection has associated strong feelings of satisfaction with these behaviours that meet such basic needs. A veritable reward circuit evolved to encourage these behaviours. Subsequently, this circuit expanded to encourage us to repeat other pleasurable experiences that we learn in the course of our lives.

The reward circuit is at the heart of our mental activity and guides all our behaviours. This circuit is complex, but it contains a central link that seems to play a fundamental role.

This link consists of the nerve connections between two particular small groups of neurons. One of these groups is located in the ventral tegmental area (VTA), and the other in the nucleus accumbens.

The chemical messenger that makes the connections between these two groups of neurons is dopamine. This is the site where most drugs act and cause dependencies.