Reward System

To understand why we are driven by good food, loving touch, sexual desire, alcohol, chocolate, the internet, a flutter on the Grand National, clothes shopping, or even cocaine, we need to know about the reward system.

The reward system is one of the most important systems in the brain. It drives our behaviour. It is a group of brain structures that mediate the effects of reinforcement, i.e. desire to repeat behaviour and form habits. A reward is an appetitive stimulus given to a human or some other animal to alter its behaviour. Rewards typically serve as reinforcers. Pleasure is better than pain for motivating behaviour.

At its centre of the reward system is the striatum. It is the region of the brain that produces feelings of reward or pleasure. Functionally, the striatum coordinates multiple aspects of thinking, including movement and action planning, decision-making, motivation, reinforcement, and reward perception. It is this part of the brain that changes most noticeably as a result of addictive behaviour. Habits that have become deep ruts are a form of pathological learning.

This is an amusing TED talk on the subject of The Pleasure Trap.

The Role of Dopamine

What is the role of dopamine? Many hypotheses have been offered to try to identify the relationship between dopamine and pleasure. The first and simplest posited a direct causal connection between dopamine and the sensation of pleasure. In other words, it was thought that the increased dopamine level that accompanied a gratifying behaviour was the direct cause of its hedonic impact.

But doubts were cast on this theory by other experiments, in particular those showing that the increase in dopaminergic activity preceded the gratifying behaviour itself. A new hypothesis then arose, that dopamine acted as a facilitating factor in learning. According to this hypothesis, the amount of dopamine released by the brain prior to a behaviour is proportional to its potential for providing pleasure. Depending on whether the behaviour proves to be pleasant or unpleasant, the anticipatory dopamine level would be higher or lower the next time.

According to this same hypothesis, learning would enable this dopaminergic response to be transferred from an unconditioned stimulus (such as an open can of tuna, for your cat) to a conditioned one (the noise of the can opener). This hypothesis accords dopamine a central role in the way we learn to remember sources of gratification.

Still other studies, however, have raised questions about this role of dopamine as a modulator of learning. In particular, certain rat experiments showed that even if the animals kept pressing the lever by which they stimulated their own brains, the actual level of dopamine in their brains kept decreasing.

From these studies, a new hypothesis emerged that associated dopamine more with novelty and its ability to increase the animals’ motivation to approach the gratifying object. This “incentive value” would be a distinct component of what we commonly call “pleasure seeking.” In other words, the dopaminergic system would be necessary for wanting the gratifying object, but not for liking it or for learning to remember new sources of pleasure.

The main source of dopamine in this mid-brain area (striatum) is produced in the ventral tegmental area (VTA). It then goes to the nucleus accumbens (NAcc) in response to the sight/cue/anticipation of the reward, loading the trigger ready for action. The next action – a motor/movement activity, activated by an excitatory signal ‘go get it,’ or an inhibiting signal, such as ‘stop’, will be determined by a signal from the prefrontal cortex once it has processed the information.

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