June 22, 2021

I, Science

The science magazine of Imperial College

Expectation of reward and pleasure plays an important role in motivating behaviour. Many of the decisions we make every day depend on our prediction of positive outcomes. The neurological basis of expectation and reward is complex and elusive but research in subjects ranging from bees to humans indicates that the neurotransmitter dopamine has a critical role.

Dopamine has numerous functions in the brain, including important roles in cognition, addiction, reward, motivation, attention, memory and modulation of movement. It is the degeneration of neurons which use dopamine as their main signalling molecule that leads to Pakinson’s Disease.

Professor Wolfram Schultz of Cambridge University found that dopamine activity is increased in anticipation of reward. When a cue from the environment indicates that a reward is on its way, dopamine is released in response. This also happens when you receive an unexpected reward. If the reward does not meet with expectations, dopamine activity is downregulated.

It is tempting to relate this pattern of activity to our emotional responses. High levels of dopamine may be associated with the feelings of anticipation we would experience when a reward is imminent. The drop in dopamine levels could also go some way to explain the feelings of frustration and disappointment we experience when a reward is not delivered. However, as Professor Schultz points out, “It is unclear whether dopamine activity is involved in pleasure at all, and [it is] a very contentious issue”.

Reward is often used to facilitate learning. From Pavlov’s dogs to the excitement of chatting someone up, actions which lead to positive outcomes are reinforced. Reward-based learning requires a means of coding for deviation from the predicted reward. The dopamine response pattern to stimuli, unexpected rewards and unfulfilled predictions, has emerged as a likely candidate. Activity in dopaminergic neurons is believed to signal that a subject’s estimate of the value of a reward is in error and also the magnitude of this error. Dopamine is released when an unpredicted reward is received. If the event that triggers a reward happens often enough, dopamine activity in your brain will be triggered, strengthening the association so that eventually you receive a buzz, not just when the event happens, but when it is predicted. It is thought that this adjusts the strength of connections in the brain until the estimate is accurate.

Dopamine cells connect to many parts of the brain, including the prefrontal cortex, where having the correct levels of dopamine is critical for focusing. Positive expectations increase the level of dopamine in the brain, which improves your ability to focus. This makes evolutionary sense – the greater the resulting reward, the more attention you want to give to the task.

Expectation can also affect our physiological responses. Expectation of pain can lead to benign stimuli being perceived as painful and the mysterious benefits of placebo drugs can, in part, be attributed to the effects of positive expectations. In 2002 researchers from the Pacific Parkinson’s Research Centre proposed the placebo-reward hypothesis after noting substantial release of dopamine in the brains of Parkinson’s Disease patients in response to placebo. High placebo responses are associated with greater dopamine activity, and imaging has shown placebo-induced activation of the reward circuitry in Parkinson’s Disease, depression and pain.

Our behaviour is often governed by our estimates of reward and pain so understanding how expectations are formed may bring us closer to understanding human action.