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= Dopamine Reward Prediction Error   =

Overview
Dopamine Reward Prediction Error describes a type of neural signaling that occurs when the predicted/expected value of a given reward does not match the experienced value of the reward, and how dopamine release in the brain adjusts in response to the miscalculation, regardless of whether the outcome is positive or negative for the organism. This is because dopamine is released in phases; some is released before the reward, in response to the possibility of getting a reward, and again after the reward is received and its value is appraised. The amount of dopamine being released during the second phase is determined by the real-world outcome being compared to the prediction, and then to what level the reward is determined to be satisfactory.

For example, if a very hungry organism saw a bowl of fruit on a table, they might excitedly approach the table in hopes of having a sweet snack. If this organism picked up a piece of fruit and tried to take a bite, only to discover that it is fake fruit made of plastic, it would cause a decrease of dopamine in the second phase of release due to the unsatisfactory nature of the reward. The expectation was to bite into a soft, nutritious, and sweet piece of food (the reward). The reality was biting into something that is hard and is not food (the error). The brain’s realization of the miscalculation, that it had mistaken the fake food for real food, and then adjusting the amount of dopamine it releases in response, is the prediction error signaling. The reduced release of dopamine in this event is undesirable, and prompts learning mechanisms in response, in order to avoid this deceptive and unsatisfying stimulus in future encounters.

However, if the stimulus is determined to be novel and previously unknown; the brain first uses sensory information gathered (appearance/smell/texture/temperature, etc.) about the new stimulus, and then compares it to memories of other stimuli deemed to be similar to the new stimulus. This information is then used to predict the potential value or level of reward that can be expected to be gained from the given stimulus. Once the stimulus has been encountered, the brain determines its value, and releases dopamine accordingly. After increased exposure, the novelty diminishes regardless of whether the stimulus is associated as being positive or negative, and reward prediction increases with this as well. Increased exposure breeds familiarity, because as expectations of the reward or stimulus become more reliable, the brain no longer needs to react so strongly to better imprint a memory of the novel stimulus each time it is encountered, being that it already has enough.

Neuroanatomy Involved
The dopamine reward prediction error signaling neural process uses two main pathways to function: the mesolimbic dopamine pathway and the mesocortical dopamine pathway. This requires several different parts of the brain to work together to maintain proper function. The midbrain dopamine system, specifically in the substantia nigra and ventral tegmental area (VTA), is the source for dopaminergic activity in these systems, projecting to many different areas across the brain. In the presence of a reward, the VTA projects to dopamine to dopaminergic receptors in the nucleus accumbens (NAc), hippocampus, amygdala, and the orbitofrontal cortex, which is where reward processing occurs. The Hippocampus, amygdala, and orbitofrontal cortex project the excitatory neurotransmitter, glutamine, to the nucleus accumbens. The nucleus accumbens then projects the inhibitory neurotransmitter, GABA, onto the GABAergic receptors in the ventral tegmental area, to adjust for the next phase of dopamine release. Dopaminergic projections to the amygdala and orbitofrontal cortex are what help to excite and begin the process of analyzing for the potential value of the reward.

Reward Processing
Many different aspects are analyzed in determining the prediction of a reward’s value. First, it needs to be determined what type of reward(s) are at stake. The reward can be a physical reward, social reward, an economic reward, along with many others. Food is a basic example of a physical reward. A physical reward can be anything from scoring in a sport to the experienced high of drugs. An example of a social reward would be when a person goes to socialize with their friends after working all day. A social reward can really be anything that feels reward when perceived in the context of a group. One example of an economic reward is prize money that contestants on game shows can be observed winning. It is any motivator that can be quantified in terms of money to the pursuer of the reward. The physical salience of a reward is the physical appeal and value that it is perceived to have based on a set of characteristics: size, form, color, odor, temperature, or position. Novelty salience determines whether an encountered stimulus is new or not, by comparing it to memories of similar stimuli. Surprise salience is the difference in the dopamine released in response to any previous expectation that had been firmly held, and when error occurs in this context it arouses more emotion. Motivational salience refers to why a certain reward is relevant for a certain organism, and why said organism should be pursuing the reward.

Dopamine Reward Prediction Error is a type of learning behavior that is present in many different species of animals, with multiple studies having been done on monkeys, mice, and humans. This neural process has shown to be a strong form of self-motivation in seeking out new rewards and in learning how to obtain them, while minimizing the chance of repeating past mistakes.