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UT PSY 394U - Study Notes

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MULTIPLE REWARD SIGNALS IN THE BRAINWolfram SchultzThe fundamental biological importance of rewards has created an increasing interest in theneuronal processing of reward information. The suggestion that the mechanisms underlyingdrug addiction might involve natural reward systems has also stimulated interest. This articlefocuses on recent neurophysiological studies in primates that have revealed that neurons in alimited number of brain structures carry specific signals about past and future rewards. Thisresearch provides the first step towards an understanding of how rewards influence behaviourbefore they are received and how the brain might use reward information to control learning andgoal-directed behaviour.REVIEWSNATURE REVIEWS | NEUROSCIENCE VOLUME 1 | DECEMBER 2000 | 199The fundamental role of reward in the survival and well-being of biological agents ranges from the control ofvegetative functions to the organization of voluntary,GOAL-DIRECTED BEHAVIOUR. The control of behaviourrequires the extraction of reward information from alarge variety of stimuli and events. This informationconcerns the presence and values of rewards,their pre-dictability and accessibility,and the numerous methodsand costs associated with attaining them.Various experimental approaches including brainlesions,psychopharmacology,electrical self-stimulationand the administration of addictive drugs,have helpedto determine the crucial structures involved in rewardprocessing1–4.In addition,physiological methods such asin vivo microdialysis,voltammetry5–9and neural imag-ing10–12have been used to probe the structures and neu-rotransmitters that are involved in processing rewardinformation in the brain. However, I believe that thetemporal constraints imposed by the nature of thereward signals themselves might be best met by studyingthe activity of single neurons in behaving animals, and itis this approach that forms the basis of this article.Here,I describe how neurons detect rewards,learn to predictfuture rewards from past experience and use rewardinformation to learn, choose, prepare and execute goal-directed behaviour (FIG.1).I also attempt to place theprocessing of drug rewards within a general frameworkof neuronal reward mechanisms.Behavioural functions of rewardsGiven the dynamic nature of the interactions betweencomplex organisms and the environment,it is not sur-prising that specific neural mechanisms have evolvedthat not only detect the presence of rewarding stimulibut also predict their occurrence on the basis of repre-sentations formed by past experience.Through thesemechanisms,rewards have come to be implicit or explic-it goals for increasingly voluntary and intentional formsof behaviour that are likely to lead to the acquisition ofgoal objects.Rewards have several basic functions.A commonview is that rewards induce subjective feelings of plea-sure and contribute to positive emotions.Unfortunately,this function can only be investigated with difficulty inexperimental animals. Rewards can also act as positiveREINFORCERS by increasing the frequency and intensity ofbehaviour that leads to the acquisition of goal objects,as described in CLASSICAL and INSTRUMENTAL CONDITIONINGPROCEDURES.Rewards can also maintain learned behav-iour by preventing EXTINCTION13.The rate of learningdepends on the discrepancy between the occurrenceof reward and the predicted occurrence of reward,theso-called ‘reward prediction error’14–16(BOX 1).Rewards can also act as goals in their own right andcan therefore elicit approach and consummatory behav-iour.Objects that signal rewards are labelled with posi-tive MOTIVATIONAL VALUE because they will elicit effortfulGOAL-DIRECTED BEHAVIOURBehaviour controlled byrepresentation of a goal or anunderstanding of a causalrelationship between behaviourand attainment of a goal.REINFORCERSPositive reinforcers (rewards)increase the frequency ofbehaviour leading to theiracquisition.Negative reinforcers(punishers) decrease thefrequency of behaviour leadingto their encounter and increasethe frequency of behaviourleading to their avoidance.Institute of Physiology andProgram in Neuroscience,University of Fribourg,CH-1700 Fribourg,Switzerland.e-mail:[email protected]© 2000 Macmillan Magazines Ltd200 | DECEMBER 2000 | VOLUME 1 www.nature.com/reviews/neuroREVIEWSReward detection and perceptionAlthough there are no specialized peripheral receptorsfor rewards,neurons in several brain structures seemto be particularly sensitive to rewarding events asopposed to motivationally neutral events that are sig-nalled through the same sensory modalities. A promi-nent example are the dopamine neurons in the parscompacta of substantia nigra and the medially adjoin-ing ventral tegmental area (groups A8,A9 and A10).In various behavioural situations, including classicaland instrumental conditioning, most dopamine neu-rons show short,phasic activation in a rather homo-geneous fashion after the presentation of liquid andsolid rewards,and visual or auditory stimuli that pre-dict reward17–20(FIG.2a, b).These phasic neural responsesare common (70–80% of neurons) in medial tegmen-tal regions that project to the nucleus accumbens andfrontal cortex but are also found in intermediate andlateral sectors that project to the caudate andputamen20.These same dopamine neurons are alsoactivated by novel or intense stimuli that have atten-tional and rewarding properties20–22.By contrast,onlya few dopamine neurons show phasic activations topunishers (conditioned aversive visual or auditorystimuli)23,24.Thus,the phasic response of A8,A9 andA10 dopamine neurons seems to preferentially reportrewarding and,to a lesser extent,attention-inducingevents.The observation of a phasic dopamine response tonovel attention-inducing stimuli led to the suggestionthat this response might reflect the salient attention-inducing properties of rewards rather than their posi-tive reinforcing aspects25,26.However,dopamine neu-rons are rarely activated by strong attention-generatingevents such as aversive stimuli23,24and they aredepressed rather than activated by the omission ofa reward, which presumably also generates atten-tion20,27–29.Of course,the possibility that the dopamineactivation might encode a specific form of attentionthat is only associated with rewarding events cannot yetbe completely ruled out.A closer examination of the properties of the phasicdopamine response suggests that it might encode areward prediction


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