Reward a
Illustration of dopaminergic reward structures
In the language used to discuss the reward system, reward is the attractive and motivational property of a stimulus that induces appetitive behavior (also known as approach behavior) and consummatory behavior.[52] A rewarding stimulus is one that can induce the organism to approach it and choose to consume it.[52] Pleasure, learning (e.g., classical and operant conditioning), and approach behavior are the three main functions of reward.[52] As an aspect of reward, pleasure provides a definition of reward;[52] however, while all pleasurable stimuli are rewarding, not all rewarding stimuli are pleasurable (e.g., extrinsic rewards like money).[52][53] The motivational or desirable aspect of rewarding stimuli is reflected by the approach behavior that they induce, whereas the pleasure from intrinsic rewards results from consuming them after acquiring them.[52] A neuropsychological model which distinguishes these two components of an intrinsically rewarding stimulus is the incentive salience model, where "wanting" or desire (less commonly, "seeking"[54]) corresponds to appetitive or approach behavior while "liking" or pleasure corresponds to consummatory behavior.[52][4][55] In human drug addicts, "wanting" becomes dissociated with "liking" as the desire to use an addictive drug increases, while the pleasure obtained from consuming it decreases due to drug tolerance.[4]
Within the brain, dopamine functions partly as a global reward signal. An initial dopamine response to a rewarding stimulus encodes information about the salience, value, and context of a reward.[52] In the context of reward-related learning, dopamine also functions as a reward prediction error signal, that is, the degree to which the value of a reward is unexpected.[52] According to this hypothesis proposed by Montague, Dayan, and Sejnowski,[56] rewards that are expected do not produce a second phasic dopamine response in certain dopaminergic cells, but rewards that are unexpected, or greater than expected, produce a short-lasting increase in synaptic dopamine, whereas the omission of an expected reward actually causes dopamine release to drop below its background level.[52] The "prediction error" hypothesis has drawn particular interest from computational neuroscientists, because an influential computational-learning method known as temporal difference learning makes heavy use of a signal that encodes prediction error.[52] This confluence of theory and data has led to a fertile interaction between neuroscientists and computer scientists interested in machine learning.[52]
Evidence from microelectrode recordings from the brains of animals shows that dopamine neurons in the ventral tegmental area (VTA) and substantia nigra are strongly activated by a wide variety of rewarding events.[52] These reward-responsive dopamine neurons in the VTA and substantia nigra are crucial for reward-related cognition and serve as the central component of the reward system.[4][57][58] The function of dopamine varies in each axonal projection from the VTA and substantia nigra;[4] for example, the VTA–nucleus accumbens shell projection assigns incentive salience ("want") to rewarding stimuli and its associated cues, the VTA–prefrontal cortex projection updates the value of different goals in accordance with their incentive salience, the VTA–amygdala and VTA–hippocampus projections mediate the consolidation of reward-related memories, and both the VTA–nucleus accumbens core and substantia nigra–dorsal striatum pathways are involved in learning motor responses that facilitate the acquisition of rewarding stimuli.[4][59] Some activity within the VTA dopaminergic projections appears to be associated with reward prediction as well.[4][59]
Pleasure[edit]
While dopamine has a central role in causing "wanting," associated with the appetitive or approach behavioral responses to rewarding stimuli, detailed studies have shown that dopamine cannot simply be equated with hedonic "liking" or pleasure, as reflected in the consummatory behavioral response.[53] Dopamine neurotransmission is involved in some but not all aspects of pleasure-related cognition, since pleasure centers have been identified both within the dopamine system (i.e., nucleus accumbens shell) and outside the dopamine system (i.e., ventral pallidum and parabrachial nucleus).[53][55][60] For example, direct electrical stimulation of dopamine pathways, using electrodes implanted in the brain, is experienced as pleasurable, and many types of animals are willing to work to obtain it.[61] Antipsychotic drugs reduce dopamine levels and tend to cause anhedonia, a diminished ability to experience pleasure.[62] Many types of pleasurable experiences—such as sex, eating, and playing video games—increase dopamine release.[63] All addictive drugs directly or indirectly affect dopamine neurotransmission in the nucleus accumbens;[4][61] these drugs increase drug "wanting", leading to compulsive drug use, when repeatedly taken in high doses, presumably through the sensitization of incentive-salience.[55] Drugs that increase synaptic dopamine concentrations include psychostimulants such as methamphetamine and cocaine. These produce increases in "wanting" behaviors, but do not greatly alter expressions of pleasure or change levels of satiation.[55][61] However, opiate drugs such as heroin and morphine produce increases in expressions of "liking" and "wanting" behaviors.[55] Moreover, animals in which the ventral tegmental dopamine system has been rendered inactive do not seek food, and will starve to death if left to themselves, but if food is placed in their mouths they will consume it and show expressions indicative of pleasure.[64]
A clinical study from January 2019 that assessed the effect of a dopamine precursor (levodopa), dopamine antagonist (risperidone), and a placebo on reward responses to music – including the degree of pleasure experienced during musical chills, as measured by changes in electrodermal activity as well as subjective ratings – found that the manipulation of dopamine neurotransmission bidirectionally regulates pleasure cognition (specifically, the hedonic impact of music) in human subjects.[65][66] This research demonstrated that increased dopamine neurotransmission acts as a sine qua non condition for pleasurable hedonic reactions to music in humans.[65][66]
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