What does dopamine do?
(1) The initial response of dopamine neurons (orange) to any external event consists of a short, unspecific, prediction-dependent excitation. The excitation is elicited by the occurrence, and thus the detection, of any sufficiently strong, impactful and thus salient event, including punishers, stimuli resembling rewards (in particular in a reward context; stimulus and context generalization) and novel stimuli.
(2) The strongest dopamine response (red) consists of the biphasic reward prediction error (RPE) signal that is elicited by primary rewards and reward-predicting stimuli.
=> Note the time scale: the RPE response lasts less than 1 second. <=Â
(3) After these two initial responses, dopamine neurons show slower excitations or inhibitions (blue) with a large number of sufficiently strong events and processes, including sensory stimuli and movements, reflecting an underlying general function in arousal and behavioural activation (with the possible exception of risky rewards). In specific cases, backpropaging RPEs and slowly increasing reward expectation can elicit similar ramping activity. Recording methods require subsecond timescales to distinguish between dopamine changes (1), (2) and (3).
(4) Beyond these phasic responses, tonic, or very slowly changing, levels of dopamine (light blue) are required to enable movements and a large variety of cognitive processes that are not encoded by phasic dopamine changes. Most non-RPE deficits from impaired dopamine transmission reflect these tonic or slowly changing dopamine levels.
Left: opamine neurons show more than the fast phasic reward prediction error (RPE) signal ('there is more to dopamine than reward'). Preceding the RPE signal is an unspecific salience signal. Center and right: while dopamine neurons do not process specific movement processes tested with well-controlled arm and eye movements, subpopulations of them are heterogeneously activated or inhibited at various task events with movements engaging plenty of muscles and sensory receptors in rodents (center) and monkeys (right). These slower dopamine activities seem to reflect general behavioural activation. These behavioural relationships unfold in the sub-second to second range, thus requiring high temporal resolution for their distinction.
Neuronal responses to external events can have multiple components (center), as shown in frontal cortex (top left). An external reward is registered in the brain first by its physical impact and then by its value. Dopamine responses show similar two components (right). Their first component reflects salience and is enhanced by several factors, which explains some non-reward responses (blue box). The benefit is early reward detection (maroon box).
