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1、Whatisdopamine-andisittoblameforouraddictions?Thekeydifferencebetweenhumanbrainsandthoseofotheranimalsisthesizeandcomplexityofourcerebralcortex,thebrainsouterlayerofneuraltissue.Wethereforetendtofocusourattentiononthisarea,believingthatouruniquementallifeisduetothismasterpieceofevolution.However,weo
2、ftenignorethebitsthatarenearlyidenticalbetweenhumansandanimals,suchasthetinygroupofbraincellsthatusethechemicaldopaminetocommunicatewithotherbraincells.Dopamineisoftendescribedasthebrain,spleasurechemica,butitisactuallyinvolvedinalargenumberofphysicalandmentalprocesses.Itisusedbyaclusterofneuronsint
3、hemidbraintotransmitmessagestootherneurons.Thedopamineneuronsaresmallinnumber(0.0006%oftheneuronsinthehumanbrain)andtheyareobservedinallmammalsandevenusimpleanimalssuchasturtles.Inthe1950s,researchersdiscoveredthatratsappearedtoenjoythestimulationofthenervebundlethatlinksthedopamineneuronswiththeirt
4、argetsintheforebrain.Theratswouldlearntopressaleverforthiskindofstimulation,and,leftunchecked,woulddosothousandsoftimesinaday.Asimilar(andwhollyunethical)experimentwasperformedin1970onahumanpatient.Liketherats,thepatientlearnedtopressabuttontostimulatethedopaminenervebundle,pressingthebuttonupto1500
5、timesoverthecourseofathree-hoursessionandreportingfeelingsofpleasureduringthestimulation.Sincethen,studieshaveshownthatthedopaminesystemcanbeactivatedbyawiderangeofpleasantexperiences,suchaseating,sexualintercourse,gettingrevenge,winningvideogames,listeningtomusic,earningmoneyandreadingfunnycartoons
6、.Thedopaminesystemalsorespondsrobustlytoaddictivedrugs,includingopiates,alcoholandcocaine.Thesedrugscanevokestrongeractivationthannaturalrewardsand,unlikenaturalrewards,theydonotcausesatiety.Astraightforwardinterpretationofthesefactsisthatthedopaminesystemisapleasurepathwayinthebrain.Thispotentially
7、explainswhyanimalsandpeoplewouldbewillingtopressbuttonsorpushleverstoactivatethedopamineneurons.Itmightalsoexplainwhysomedrugsaresoaddictive.Thestrongandprolongedactivationinducedbydrugscanactasa“super-reward，makingdrugsevenmoredesirable.However,manymentaleventsoccurnearthetimeofareward,includingcha
8、ngesinmotivation,arousal,attention,emotion,andlearning.Forexample,imaginepassingbyavendingmachinethatofferssweets.Ifyouaremotivatedbyhunger,yourattentionwillbedrawntothemachineandyouwillbecomemorealertasyouapproachit.Onceyouhaveeatenthesweets,youexperiencepleasure,yourbrainlearnstoassociatethevendin
9、gmachinewithreward,andyourhungerdecreases.Itislikelythatthedopaminesystemisinvolvedinmanyoftheseprocessesratherthanjustpleasureperse.Oneofthemostimportantaspectsofdopaminefunctionislearning.Researchersbelievethatdopamineneuronschangetheiractivitywhenexpectationsaboutrewarddonotmatchthereality,signal
10、linga,rewardpredictionerror*thatdriveslearning.Forinstance,dopamineneuronsareactivatedbyunanticipatedrewards,buttheyaresuppressedwhenexpectedrewardsfailtomaterialise.Eventsfollowedbyincreasesindopamineactivationbecomeassociatedwithreward,andthosethatarefollowedbydecreasesarelinkedwithdisappointment.
11、Iftheenvironmentisunchanging,allourbrainsneedtodotoobtainrewardistoengageinactionsthatactivatethedopamineneuronsandavoidtheonesthatsuppressthem.Itishighlyunlikelythatwehavemuchawarenessofthelearningthatdopamineactivationinduces,suchasmakingusattachedtothingsweunknowinglyassociatewithdopamineactivati
12、on.Thislackofawarenessmightexplainwhypeopleoftenmakeseeminglyirrationalormaladaptivechoices.Imagineadrugaddicttakingcocaine.Becausethepleasurefromcocainedoesnotsatiatelikeanaturalreward,thedopamineactivation,andhencedrug-inducedlearning,occurswitheachpuffofthecrackpipe,makingtheactualpipeanobjectwhi
13、chtheaddictisdrawnto.Canbrainresearchbeusedtoovercometheeffectsofdopamineinaddiction?Neuroscientistsareactivelypursuingthecreationofdrugsthatblockthelearninginducedbydopamineinaddiction.However,theyhavehadlimitedsuccess,foritisdifficulttocreateadrugthatblocksthelearningwithoutalsoblockingotherfuncti
14、onsofdopamine,suchasfeelingalert,motivatedandhappy.Dopamine-inducedlearningiscertainlynotthewholestorybehindaddiction,butitdoessuggestthatweshouldconsiderwhetheraddictionissomethingthathumanreasoningonitsowncanovercome.Thesamemightverywellalsoapplytoothereverydayfailuresofwillpower,suchasovereating.
15、Ourspecialcerebralcortexmaybeincontrolofouractions,butourprimitivedopaminesystemmayverywellserveasitsteacher.Source:http:Questions1-10Dothefollowingstatementsagreewiththeclaimsofthewriterinthepassage?YesifthestatementagreeswiththeclaimsofthewriterNoifthestatementcontradictstheclaimsofthewriterNotGiv
16、enifitisimpossibletosaywhatthewriterthinksaboutthis1. Therearemanywaysinwhichhumanbrainsdifferfromthebrainsofotheranimals.2. Thecellsinthebrainthatmakeuseofdopamineasameansofcommunicationareuniquetohumans.3. Dopaminecanbecharacterisedinotherwaysthanassimplyachemicalinvolvedinthesensationofpleasure.4. Theexperimentcarriedoutonahumansubjectin1970wasunethicalbecausei
