Mark Walton, Jerylin Gan & Paul Phillips
In:- Neural Basis of Motivational and Cognitive Control – MIT Press (2011)
Keywords: dopamine, midbrain, basal ganglia, nucleus accumbens, opioid neurotransmitters
The release of dopamine into the striatum and particularly the nucleus accumbens is closely related to the subjective evaluation of sensory inputs, and to the subsequent selection of behaviour and actions.
The authors start by referring to a distinction between the evaluation of reward, and the process of deciding to obtain, and then acting to obtain a reward. It is suggested that much twentieth century research fell short in not paying attention to the internal motivation of subjects. The authors acknowledge that several regions of the brain may be implicated; their emphasis here is concentrated on the striatum, particularly the nucleus accumbens and also dopamine projections.
The dopamine projection to the nucleus accumbens come from the ventral tegmental area (VTA) in the midbrain. Dopamine is a modulatory neurotransmitter often associated with the modulation of the excitatory neurotransmitter, glutamine. There is particularly dense innervation of the striatum by dopamine. Release of dopamine and availability of dopamine receptors in the nucleus accumbens is associated with drug addiction and also with compulsive shopping, eating and gambling.
A good deal of past research has concentrate on the role of dopamine in the selection of isolated rewards, rather than the more realistic situation of subjects assessing competing rewards and associated uncertainty as to the costs and probabilities of obtaining particular rewards. Recent studies, however, point to a correlation between the firing of dopamine neurons and the size and probability of particular rewards. Some studies also suggest a connection between dopamine activity and the timing of future rewards. Dopamine is seen as important in allowing the subject to exert the effort needed to obtain a particular reward. Dopamine release is viewed by the authors as facilitating, but not controlling, responses that seek potentially costly rewards. It is seen as a motivation to seek novel options and potential future rewards.
Evidence suggests that dopamine is involved in signalling the availability of reward. This is partly related to the prediction of reward, but also to actions directed towards gaining rewards. Additionally, the release of dopamine from the VTA can increase the probability of a reward being sought. In situations where there is conditioning, dopamine release can change from being directly related to the arrival of the reward, to being merely something that predicts the future probability of the reward. Dopamine activity can also increase where a reward is either above or below the predicted level, being thus an indicator for error predictions. The authors see dopamine in the nucleus accumbens as being important in making reward predictions when the subject is encountering an uncertain environment. However, this is viewed as only one influence on the subject’s actions.Tags: basal ganglia, dopamine, nucleus accumbens, opioid neurotransnitters Posted by