06 lecture sixcortexsensory motor and association areasWord格式.docx

06 lecture sixcortexsensory motor and association areasWord格式.docx

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Thecerebralcortexistheouterlayerofneuronsor“bark”coveringthehemisphere.Inthepreviouslecture,wediscusshowthecortexcanbedividedintofourlobesbymajorsulciorfissure.Furthermore,eachoftheselobesiscomprisedofanumberofareaswhichareassociatedwithspecificfunctions.Hereweintroduceanotherclassificationschemeusedtopartitioncorticalareasintermsoffunction:

thedivisionofcorticalareasintosensory,motor,andassociationcortex.Wewillalsodiscusshowattemptingtoclassifythecortexinthiswayhascausedamajorparadigmshiftinourunderstandingofthefunctionofthecortex.

Outline

I.Inthelastlecture,welearnedthatthecerebralcortexisasheetofneuronsabout1-4millimetersthickcoveringthetwocerebralhemispheres.

A.Thecortexcanbedividedintoanumberofdifferentareasbasedonevolutionaryage,cytoarchitecture,andfunction.Brodmanndividedthecortexintoabout50areasbasedprimarilyoncytoarchitecture.

B.OneofthemostimportantoftheearlydiscoveriesinneurosciencewasthatmanyofBrodmann’sareasrepresentedfunctionallydistinctareas.

C.Lastly,welearnedthatthecortexhasavertical/modularorganizationaswell;

thus,whilethecortexisapproximatelythesamethicknessinallspecies,theincreaseinsurfaceareainthehumanbrainallowsforanincreasedinformationprocessingability,atleastinpart,becauseoftheincreaseinthenumberoffunctionalmodules.

II.Earlyneuroanatomistsidentifiedtheprimaryareasofthecortex（sensoryandmotor）,butthoughtthatmuchoftheremainingcortex（whichtheycalledassociationcortex）wasresponsibleforhigher-ordercognitiveabilitiesassociatedwithourspecies.

A.Whilethegeneralclassificationintosensory,motor,andassociationareasisstillusedinneuroscience,therehasbeenamajorparadigmshiftinhowweviewthecortexconceptually,particularlyinhowassociationcortexisdefined.

B.Oneofthemostimportantdiscoveriesmadewithinthelast40yearsorsoisthatmuchofwhatwaspreviouslycalledassociationcortexisactuallysensoryinfunction;

theseareasarenowreferredtoashigher-ordersensorycortex.forexample,inadditiontoArea17,manyothercorticalareasaredevotedtovision;

infact,inprimates,theremaybe30ormoredistinctcorticalareaswithinoccipital,parietal,andtemporallobesthataredevotedtovision.

III.Ithasbecomeclearthatmuchofthecortex,eveninhumans,isdevotedtotheprocessingofsensoryinformation（Figure6.1）.Whenprimaryandhigher-ordersensoryandmotorareasareaccountedfor,however,therearestillidentifiedareaswhichareeithermultimodalinfunction（meaningthattheyprocessinformationfrommorethanonesensorysystem）,orareneithersensorynormotorinfunction;

inmodernneuroscience,itistheseareaswhicharenowreferredtoastheassociationareasofthecortex.

A.Threemajorassociationareasarenowrecognized（Figure6.1）.

1.Prefrontalassociationcortexisinvolvedinavarietyofexecutivefunctions,forexample,anappreciationoftheconsequenceofone’sbehavior.

2.Parieto-occipital-temporalassociationcortexisprimarilymultimodal.

3.Limbicassociationcortex（partsofthefrontalandtemporallobes）isinvolvedinthehigher-orderelaborationofemotion,memory,andothercognitivefunctions.

B.Asmoredata,particularlyonthehumanbrain,arecollected,thesedefinitionsmayagainchange.Inmodernneuroscience,technologyisprovidingnoninvasivedetailedimagesofthehumanbrainwithsuchtechniquesasfunctionalmagneticresonanceimaging（fMRI）,whichcanshowwhatareasofthebrainareactivewhenanindividualperformsvarioustasks,providingneurosciencewithvaluableinformationaboutthefunctionalorganizationofthehumancortex.

IV.Anumberofotherparadigmshavealsobeenchallengedbymodernneuroscience.

A.Forexample,historically,itwasbelievedthatafewfunctions,forexamplelanguage,appearedtoberelatedonlytoonehemisphere.Forotherfunctions,towasgenerallybelievedthatthehemisphereshadsimilarfunctionsrelatingtothetwosidesofthebody.Wenowknowthatbothhemispheresplayaroleinlanguageandthattheprocessingofinformationisnowidenticalinthetwohemispheres,althoughingeneral,onehemispheredoesprocessinformationfromorcontroltheoppositesideofthebody.

B.Forexample,ithasbeenlongknowthateachhemisphereisinvolvedininitiatingmotormovementorinprocessingsensoryinformationfromtheopposite（alsocalledcontralateral）halfofthebodyorworld.

1.Themotorcortexontherightcontrolsthemovementoftheleftsideofthebodybecausethecorticospinaltractcrossesthemidlineofthebrainbeforeitentersthespinalcord（seeFigure4.1）.

2.Likewise,Area17orprimaryvisualcortexontheright“sees”theleftorcontralateralhalfofvisualspace.

C.Butmodernneurosciencehasrevealedimportant,andsometimessubtle,distinctionsincognitivefunctionbetweenthehemispheres;

forexample,moodisaltereddifferentlydependingonwhetheranindividualsustainsdamagetotheleftorrighthemisphere.

D.Neurosciencehasalsoshownusthatthehippocampusontheleftsideofthebrainismoreinvolvedinbeingabletorememberwordsandepisodesinyourlife,whileyourrighthippocampusisinvolvedmoreinspatialmemory.

E.Informationinthetwohemispheresis“coordinated,”inpart,byprojectionsofthecorpuscallosum,the300millionaxonsthatconnectacorticalareainonehemispherewiththesameareaintheotherhemisphere.Essentially,wehavetwobrainsinourhead.

1.Thiswasdiscoveredinindividualswithepilepsywhohavethecorpuscallosumseveredorcuttopreventthespreadofseizures.

2.Thissurgeryresultsinanindividualwithtwoseparatefunctioninghemisphere;

suchpatientsshowverylittlelossoffunctionbecausetheirprojectionpathwaysarestillintact.Theyhavetobetestedverycarefullytodetectanysubtlechangeinfunction.

FurtherReading:

Anyrecentneurosciencetextwillcovermostofthebasicinformationcoveredinthislecture,althoughinmuchgreaterdetail.TheauthoritativebookinneuroscienceisPrinciplesofNeuralScience,editedbyE.R.Kandel,J.H.Schwartz,andT.M.Jessell.

QuestionstoConsider:

1.Eventhoughthecortexcanbedividedintosensory,motor,andassociationcortex,onsingleareabringsalloftheinformationtogether?

Yetourexperienceseems“unified.”Thisisknownasthe“binding”problem.Whydoyousupposeitcontinuestochallengeneuroscientists?

2.Whatdoyouthinkaboutthefactwehave,essentially,twobrainsinourheads?

LectureSix—Transcript

Helloagain.Inthelastlecturewelearnedthatthecerebralcortexisjustasheer,anoutermantleofcalls,orcoveringofgraymatter,overthehemisphere.Whenwelookatasectionthroughthecortex,weseethatitisjustthisouterpartofthecortex.It’sthelightareashereonourpicture.Withineachofthelobeswealsotalkedaboutdifferentstructuresthatwerelocatedwithinthelobesordifferentareas.Alotofmotorareashereonthefrontallobe,butalsoverycomplexfunctionshere,somatosensorybackhere,visualhere,auditoryhereandothercomplexfunctionsinalloftheselobes.

Now,withineachoftheselobeswesawthattherewasahorizontalorganizationthatwereferredtobyBrodmannareas.SoBrodmanndividedtocortexupintothose50differentareas.Thisjustshowsthelateralsurfaceofthecortex.YoucanseethesearejustthenumbersthatwereoriginallygivenbyBrodmann.Thisallowsuptotalktoeachother.But,thereisanotherreallyimportantdiscoverythathappened.AndthatisthatoneofthemostimportantearlydiscoverieswasnowmadebyBrodmann,butlater,thateachoftheseareasactuallyrepresentsafunctionallydistinctareaofthebrainwithadifferentfunction.

Now,Brodmannjustcutupthecortexandlookedatthewaythecellswereorganized.Soallhemadehisnumberingsystembasedonwerethedifferencesinthewaythecellswereorganizedintolayers.Hedidn’thaveanywayofknowingthatwewouldlaterinneurosciencediscoverthateachofhisareaswouldhaveadifferentfunction.Now,thatgivesasortofhorizontalorganizationothecortex.But,ifweweretolookinanyspecificareaofthecortexandtotakeoutatripofcortexthrough,say,thesixlayersinneocortex,wewoulddiscoverthatthereisaverticalorganization.Inadrawingwehavemadehere,itshowsyouthatthisisjustonegyrushere,thiswouldbeasulcus.Youwouldtakeastripofcortexaboutonetotwosquaremillimeters;

youtakeitoutandlookatit.Neocortexgoesthroughallsixofthelayers.Thislittletinystripisanindependentlyfunctioningmodule.Howallthesemodulesareconnectedtoeachother,understandingthatisoneofthegoalsofmodernneuroscience.

Whilethecortexisthesamethicknessinallspeciesthathaveacortex,theincreaseinthesurfaceareainthehumanbrainallowsforanincreasednumberofareasandanincreasednumberoftheselittlefunctionalmodulesandtheirinterconnectionswitheachother.Thisiswhatisallowedtheincreasedinformation-processingabilitythatwehaveashumanbeings.Thisisjustsocriticallyimportanttounderstand.Now,welookedinthefirstlectureatancientpeoples.Ancientpeoplesdidn’tevenseemtohaveaconceptionthatthebrainwasimportantatall.Imadethecommentthatthebrainwasremovedinpharaohs,forexample.Thepharaohcouldentertheafterlifewithoutabrain,butnowwit