how human memory works

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HowHumanMemoryWorksbyRichardC.Mohs,PhD

BrowsethearticleHowHumanMemoryWorks

Humanmemoryisacomplex,brainwideprocessthatisessentialtowhoweare.Learnaboutencoding,thebrain,andshortandlongtermmemory.Seemorebrainpictures.PublicationsInternational,Ltd.

HowHumanMemoryWorksOverview

Themoreyouknowaboutyourmemory,thebetteryou'llunderstandhowyoucanimproveit.Here'sabasicoverviewofhowyourmemoryworksandhowagingaffectsyourabilitytoremember.

Yourbaby'sfirstcry...thetasteofyourgrandmother'smolassescookies...thescentofanoceanbreeze.Thesearememoriesthatmakeuptheongoingexperienceofyourlifetheyprovideyouwithasenseofself.They'rewhatmakeyoufeelcomfortablewithfamiliarpeopleandsurroundings,tieyourpastwithyourpresent,andprovideaframeworkforthefuture.Inaprofoundway,itisourcollectivesetofmemoriesour"memory"asawholethatmakesuswhoweare.

Mostpeopletalkaboutmemoryasifitwereathingtheyhave,likebadeyesoragoodheadofhair.Butyourmemorydoesn'texistinthewayapartofyourbodyexistsit'snota"thing"youcantouch.It'saconceptthatreferstotheprocessofremembering.

Inthepast,manyexpertswerefondofdescribingmemoryasasortoftinyfilingcabinetfullofindividualmemoryfoldersinwhichinformationisstoredaway.Otherslikenedmemorytoaneuralsupercomputerwedgedunderthehumanscalp.Buttoday,expertsbelievethatmemoryisfarmorecomplexandelusivethanthatandthatitislocatednotinoneparticularplaceinthebrainbutisinsteadabrainwideprocess.

Doyourememberwhatyouhadforbreakfastthismorning?Iftheimageofabigplateoffriedeggsandbaconpoppedintoyourmind,youdidn'tdredgeitupfromsomeoutofthewayneuralalleyway.Instead,thatmemorywastheresultofanincrediblycomplexconstructivepoweronethateachofuspossessesthatreassembleddisparatememoryimpressionsfromaweblikepatternofcellsscatteredthroughoutthebrain.Your"memory"isreallymadeupofagroupofsystemsthateachplaya

differentroleincreating,storing,andrecallingyourmemories.Whenthebrainprocessesinformationnormally,allofthesedifferentsystemsworktogetherperfectlytoprovidecohesivethought.

Whatseemstobeasinglememoryisactuallyacomplexconstruction.Ifyouthinkofanobjectsay,apenyourbrainretrievestheobject'sname,itsshape,itsfunction,thesoundwhenitscratchesacrossthepage.Eachpartofthememoryofwhata"pen"iscomesfromadifferentregionofthebrain.Theentireimageof"pen"isactivelyreconstructedbythebrainfrommanydifferentareas.Neurologistsareonlybeginningtounderstandhowthepartsarereassembledintoacoherentwhole.

Ifyou'reridingabike,thememoryofhowtooperatethebikecomesfromonesetofbraincellsthememoryofhowtogetfromheretotheendoftheblockcomesfromanotherthememoryofbikingsafetyrulesfromanotherandthatnervousfeelingyougetwhenacarveersdangerouslyclose,fromstillanother.Yetyou'reneverawareoftheseseparatementalexperiences,northatthey'recomingfromalldifferentpartsofyourbrain,becausetheyallworktogethersowell.Infact,expertstellusthereisnofirmdistinctionbetweenhowyourememberandhowyouthink.

Thisdoesn'tmeanthatscientistshavefiguredoutexactlyhowthesystemworks.Theystilldon'tfullyunderstandexactlyhowyourememberorwhatoccursduringrecall.Thesearchforhowthebrainorganizesmemoriesandwherethosememoriesareacquiredandstoredhasbeenaneverendingquestamongbrainresearchersfordecades.Still,thereisenoughinformationtomakesomeeducatedguesses.Theprocessofmemorybeginswithencoding,thenproceedstostorageand,eventually,retrieval.

Onthenextpage,you'lllearnhowencodingworksandthebrainactivityinvolvedinretrievingamemory.

MemoryEncoding

BrainImageGallery

LaunchVideo

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Thetypicalbrainhasabout100trillionsynapses,whicharethepointswherenervecellsinthehumanbrainconnectwithothercells.PublicationsInternational,Ltd.

Encodingisthefirststepincreatingamemory.It'sabiologicalphenomenon,rootedinthesenses,thatbeginswithperception.Consider,forexample,thememoryofthefirstpersonyoueverfellinlovewith.Whenyoumetthatperson,yourvisualsystemlikelyregisteredphysicalfeatures,suchasthecoloroftheireyesandhair.Yourauditorysystemmayhavepickedupthesoundoftheirlaugh.Youprobablynoticedthescentoftheirperfumeorcologne.Youmayevenhavefeltthetouchoftheirhand.Eachoftheseseparatesensationstraveledtothepartofyourbraincalledthehippocampus,whichintegratedtheseperceptionsastheywereoccurringintoonesingleexperienceyourexperienceofthatspecificperson.

Expertsbelievethatthehippocampus,alongwithanotherpartofthebraincalledthefrontalcortex,isresponsibleforanalyzingthesevarioussensoryinputsanddecidingifthey'reworthremembering.Iftheyare,theymaybecomepartofyourlongtermmemory.Asindicatedearlier,thesevariousbitsofinformationarethenstoredindifferentpartsofthebrain.Howthesebitsandpiecesarelateridentifiedandretrievedtoformacohesivememory,however,isnotyetknown.

Althoughamemorybeginswithperception,itisencodedandstoredusingthelanguageofelectricityandchemicals.Here'showitworks:Nervecellsconnectwithothercellsatapointcalledasynapse.Alltheactioninyourbrainoccursatthesesynapses,whereelectricalpulsescarryingmessagesleapacrossgapsbetweencells.

Theelectricalfiringofapulseacrossthegaptriggersthereleaseofchemicalmessengerscalledneurotransmitters.Theseneurotransmittersdiffuseacrossthespacesbetweencells,attachingthemselvestoneighboringcells.Eachbraincellcanformthousandsoflinkslikethis,givingatypicalbrainabout100trillionsynapses.Thepartsofthebraincellsthatreceivetheseelectricimpulsesarecalleddendrites,featherytipsofbraincellsthatreachouttoneighboringbraincells.

Theconnectionsbetweenbraincellsaren'tsetinconcretetheychangeallthetime.Braincellsworktogetherinanetwork,organizingthemselvesintogroupsthatspecializeindifferentkindsofinformationprocessing.Asonebraincellsendssignalstoanother,thesynapsebetweenthetwogetsstronger.Themoresignalssentbetweenthem,thestrongertheconnectiongrows.Thus,witheachnewexperience,yourbrainslightlyrewiresitsphysicalstructure.Infact,howyouuseyourbrainhelpsdeterminehowyourbrainisorganized.Itisthisflexibility,whichscientistscallplasticity,thatcanhelpyourbrainrewireitselfifitiseverdamaged.

Asyoulearnandexperiencetheworldandchangesoccuratthesynapsesanddendrites,moreconnectionsinyourbrainarecreated.Thebrainorganizesandreorganizesitselfinresponsetoyourexperiences,formingmemoriestriggeredbytheeffectsofoutsideinputpromptedbyexperience,education,ortraining.

Thesechangesarereinforcedwithuse,sothatasyoulearnandpracticenewinformation,intricatecircuitsofknowledgeandmemoryarebuiltinthebrain.Ifyouplayapieceofmusicoverandover,forexample,therepeatedfiringofcertaincellsinacertainorderinyourbrainmakesiteasiertorepeatthisfiringlateron.Theresult:Yougetbetteratplayingthemusic.Youcanplayitfaster,withfewermistakes.Practiceitlongenoughandyouwillplayitperfectly.Yetifyoustoppracticingforseveralweeksandthentrytoplaythepiece,youmaynoticethattheresultisnolongerperfect.Yourbrainhasalreadybeguntoforgetwhatyouonceknewsowell.

Toproperlyencodeamemory,youmustfirstbepayingattention.Sinceyoucannotpayattentiontoeverythingallthetime,mostofwhatyouencountereverydayissimplyfilteredout,andonlyafewstimulipassintoyourconsciousawareness.Ifyourememberedeverysinglethingthatyounoticed,yourmemorywouldbefullbeforeyouevenleftthehouseinthemorning.Whatscientistsaren'tsureaboutiswhetherstimuliarescreenedoutduringthesensoryinputstageoronlyafterthebrainprocessesitssignificance.Whatwedoknowisthathowyoupayattentiontoinformationmaybethemostimportantfactorinhowmuchofityouactuallyremember.

Thenextpageprovidesdetailsonhowinformationisstoredinshorttermandlongtermmemory.

ShortandLongTermMemory

Onceamemoryiscreated,itmustbestored(nomatterhowbriefly).Manyexpertsthinktherearethreewayswestorememories:firstinthesensorystagetheninshorttermmemoryandultimately,forsomememories,inlongtermmemory.Becausethereisnoneedforustomaintaineverythinginourbrain,thedifferentstagesofhumanmemoryfunctionasasortoffilterthathelpstoprotectusfromthefloodofinformationthatwe'reconfrontedwithonadailybasis.

Thecreationofamemorybeginswithitsperception:Theregistrationofinformationduringperceptionoccursinthebriefsensorystagethatusuallylastsonlyafractionofasecond.It'syoursensorymemorythatallowsaperceptionsuchasavisualpattern,asound,oratouchtolingerforabriefmomentafterthestimulationisover.

Afterthatfirstflicker,thesensationisstoredinshorttermmemory.Shorttermmemoryhasafairlylimitedcapacityitcanholdaboutsevenitemsfornomorethan20or30secondsatatime.Youmaybeabletoincreasethiscapacitysomewhatbyusingvariousmemorystrategies.Forexample,atendigitnumbersuchas8005840392maybetoomuchforyourshorttermmemorytohold.Butdividedintochunks,asinatelephonenumber,8005840392mayactuallystayinyourshorttermmemorylongenoughforyoutodialthetelephone.Likewise,byrepeatingthenumbertoyourself,youcankeepresettingtheshorttermmemoryclock.

Importantinformationisgraduallytransferredfromshorttermmemoryintolongtermmemory.Themoretheinformationisrepeatedorused,themorelikelyitistoeventuallyendupinlongtermmemory,ortobe"retained."(That'swhystudyinghelpspeopletoperformbetterontests.)Unlikesensoryandshorttermmemory,whicharelimitedanddecayrapidly,longtermmemorycanstoreunlimitedamountsofinformationindefinitely.

Peopletendtomoreeasilystorematerialonsubjectsthattheyalreadyknowsomethingabout,sincetheinformationhasmoremeaningtothemandcanbementallyconnectedtorelatedinformationthatisalreadystoredintheirlongtermmemory.That'swhysomeonewhohasanaveragememorymaybeabletorememberagreaterdepthofinformationaboutoneparticularsubject.

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Mostpeoplethinkoflongtermmemorywhentheythinkof"memory"itselfbutmostexpertsbelieveinformationmustfirstpassthroughsensoryandshorttermmemorybeforeitcanbestoredasalongtermmemory.Tolearnhowinformationmakesitswayoutoflongtermmemory,seethenextpage.Wewillexplorehowmemoriesarerecalledandwhathappenswhenamemorycannotberetrievedaphenomenonyoumightcall"forgetting."

MemoryRetrieval

Whenyouwanttoremembersomething,youretrievetheinformationonanunconsciouslevel,bringingitintoyourconsciousmindatwill.Whilemostpeoplethinktheyhaveeithera"bad"ora"good"memory,infact,mostpeoplearefairlygoodatrememberingsometypesofthingsandnotsogoodatrememberingothers.Ifyoudohavetroublerememberingsomethingassumingyoudon'thaveaphysicaldiseaseit'susuallynotthefaultofyourentirememorysystembutaninefficientcomponentofonepartofyourmemorysystem.

Let'slookathowyourememberwhereyouputyoureyeglasses.Whenyougotobedatnight,youmustregisterwhereyouplaceyoureyeglasses:Youmustpayattentionwhileyousetthemonyourbedsidetable.Youmustbeawareofwhereyouareputtingthem,oryouwon'tbeabletoremembertheirlocationthefollowingmorning.Next,thisinformationisretained,readytoberetrievedatalaterdate.Ifthesystemisworkingproperly,whenyouwakeupinthemorningyouwillrememberexactlywhereyouleftyoureyeglasses.

Ifyou'veforgottenwheretheyare,oneofseveralthingscouldhavehappened:

Youmaynothaveregisteredclearlywhereyouputthemdowntobeginwith.

Youmaynothaveretainedwhatyouregistered.

Youmaynotbeabletoretrievethememoryaccurately.

Therefore,ifyouwanttostopforgettingwhereyouleftyoureyeglasses,youwillhavetoworkonmakingsurethatallthreestagesoftherememberingprocessareworkingproperly.

Ifyou'veforgottensomething,itmaybebecauseyoudidn'tencodeitveryeffectively,becauseyouweredistractedwhileencodingshouldhavetakenplace,orbecauseyou'rehavingtroubleretrievingit.Ifyou've"forgotten"whereyouputyoureyeglasses,youmaynothavereallyforgottenatallinstead,thelocationofyoureyeglassesmayneverhavegottenintoyourmemoryinthefirstplace.Forexample,youprobablywouldsaythatyouknowwhatafivedollarbilllookslike,butmostofthetimesthatyou'veseenone,you'venotreallyencodeditsappearance,sothatifyoutriedtodescribeit,youprobablycouldn't.

Distractionsthatoccurwhileyou'retryingtoremembersomethingcanreallygetinthewayofencodingmemories.Ifyou'retryingtoreadabusinessreportinthemiddleofabusyairport,youmaythinkyou'rerememberingwhatyouread,butyoumaynothaveeffectivelysaveditinyourmemory.

Finally,youmayforgetbecauseyou'resimplyhavingtroubleretrievingthememory.Ifyou'veevertriedtoremembersomethingonetimeandcouldn't,butthenlateryourememberthatsameitem,itcouldbethattherewasamismatchbetweenretrievalcuesandtheencodingoftheinformationyouweresearchingfor.

Aswegetolder,memoryproblemstendtoincrease.Inthenextsection,youwilllearnhowagingcanaffectmemory.

EffectsofAgingonMemory

Thereyouareatabusinessfunctionandyouseeacolleagueacrosstheroom.Asyouwalkover,yousuddenlyrealizeyoucan'tremembertheperson'sname.Oddsareyou'renotsuddenlydevelopingAlzheimer'sdisease,althoughmanypeoplejumptothatconclusion.You'resimplyexperiencingabreakdownoftheassemblyprocessofmemoryabreakdownthatmanyofusbegintoexperienceinour20sandthattendstogetworseaswereachour50s.Thisagedependentlossoffunctionappearsinmanyanimals,anditbeginswiththeonsetofsexualmaturity.

Wesawearlierinthischapterthatasyoulearnandremember,yourbraindoesn'tchangeitsoverallstructureorgrowwholenewbatchesofnervecellsit'stheconnectionsbetweencellsthatchangeasyoulearn.Yoursynapsesarereinforced,andcellsmakemoreandstrongerconnectionswitheachother.Butasyoubegintoage,thesesynapsesbegintofalter,whichbeginstoaffecthoweasilyyoucanretrievememories.

Researchershaveseveraltheoriesaboutwhat'sbehindthisdeterioration,butmostsuspectthatagingcausesmajorcelllossinatinyregioninthefrontofthebrainthatleadstoadropintheproductionofaneurotransmittercalledacetylcholine.Acetylcholineisvitaltolearningandmemory.

Inaddition,somepartsofthebrainthatareessentialtomemoryarehighlyvulnerabletoaging.Onearea,calledthehippocampus,loses5percentofitsnervecellswitheachpassingdecadeforatotallossof20percentbythetimeyoureachyour80s.Inaddition,thebrainitselfshrinksandbecomeslessefficientasyouage.

Ofcourse,otherthingscanhappentoyourbraintospeedupthisdecline.Youmayhaveinheritedsomeunhealthygenes,youmighthavebeenexposedtopoisons,orperhapsyousmokedordranktoomuch.Allthesethingsspeedupmemorydecline.

Soyoucanseethatasyouage,somephysicalchangesinthebraincanmakeitmoredifficulttorememberefficiently.Thegoodnewsisthatthisdoesn'tmeanthatmemorylossanddementiaareinevitable.Whilesomespecificabilitiesdodeclinewithage,overallmemoryremainsstrongformostpeoplethroughouttheir70s.Infact,researchshowsthattheaverage70yearoldperformsaswelloncertaincognitivetestsasdomany20yearolds,andmanypeopleintheir60sand70sscoresignificantlybetterinverbalintelligencethandoyoungerpeople.

Studiesalsohaveshownthatmanyofthememoryproblemsexperiencedbyolderpeoplecanbelessenedorevenreversed.Studiesofnursinghomepopulationsshowthatpatientswereabletomakesignificantimprovementsinmemorywhengivenrewardsandchallenges.Physicalexerciseandmentalstimulationalsocanreallyimprovementalfunction.

Evidencefromanimalstudiessuggeststhatstimulatingthebraincanstopcellsfromshrinkingandcanevenincreasebrainsizeinsomecases.Studiesshowthatratslivinginenrichedenvironmentswithlotsoftoysandchallengeshavelargerouterbrainswithlarger,healthierbraincells.Andanimalsgivenlotsofmental

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exercisehavemoredendrites,whichallowtheircellstocommunicatewitheachother.Researchhasshownthat,inourlateryears,astimulatingenvironmentencouragesthegrowthofthesedendrites,whileadullenvironmentimpedesit.

Theimportantpointtorememberisthatasyouage,youmaynotlearnorrememberasquicklyasyoudidwhenyouwereinschoolbutyouwilllikelylearnandremembernearlyaswell.Inmanycases,anolderperson'sbrainmaybelesseffectivenotbecauseofastructuralororganicproblembutsimplyasaresultoflackofuse.

ABOUTTHEAUTHORS:

RichardC.Mohs,Ph.D.,hasbeenvicechairmanoftheDepartmentofPsychiatryattheMountSinaiSchoolofMedicineandassociatechiefofstaffforresearchattheBronxVeteransAffairsMedicalCenter.Theauthororcoauthorofmorethan300scientificpapers,Dr.Mohshasconductednumerousresearchstudiesonaging,Alzheimer'sdisease,andcognitivefunction.

CarolTurkingtonisafreelancewriterwhospecializesinthefieldsofhealthandpsychology.AformereditorandwriterfortheDukeUniversityMedicalCenterandtheAmericanPsychologicalAssociation,shehasmorethan40bookstohercredit,includingTheMemoryandMemoryDisordersSourcebookTheEncyclopediaofMemoryandMemoryDisordersandTheBrainEncyclopedia.

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