Briefintroduction
Concept
VisionCentralisthenervecellinthecerebralcortexthatisinvolvedintheformationofvisiongroup.Itislocatedontheoccipitalcortexonbothsidesofthetalarfissure,thatis,theuppercuneiformgyrusandthelowerlingualgyrus.Becauseofthespecialstructureofthecortex,therearewhitefinelinesonthecrosssection,soitisalsocalledthestriatedarea.Thevisualcenterofeachhemisphereisrelatedtohalfofthevisualfieldofbotheyes,sototalblindnessoccurswhenthevisualcentersofbothhemispheresarecompletelydamaged.
Brainlocation
Thevisualareaofthehumancerebralcortexislocatedintheoccipitallobe(area17).Theleftoccipitalcortexreceivesafferentnerveprojectionsfromthetemporalretinaofthelefteyeandthenasalretinaoftherighteye.Therightoccipitalcortexreceivesafferentfiberprojectionsfromthetemporalretinaoftherighteyeandthenasalretinaofthelefteye.Theupperhalfoftheretina(thelowerquadrantofthefieldofview)isprojectedtotheupperedgeofthetalariacleft;thelowerhalf(theupperquadrantofthefieldofview)isprojectedtotheloweredgeofthetalariscleft.Therefore,damagetotheloweredgeofthetalusfissurewillappearasadefectintheupperquadrantintermsofhumanvision.Themacularareainthecenteroftheretinaisprojectedtothebackofthetaluscleft,andthemarginalareaisprojectedtothefrontofthetaluscleft.Therepresentativeareaofthemaculaisrelativelylargerinboththecortexandthelateralgeniculatebodythantheborderrepresentativearea.
Thecentralmechanismofvisionformation
Theprocessofvisionformation
Cellsareexcited,andthecellsattheleveloftheretinaarecomposedandanalyzedinaloop.Thex,Y,andWneuronsparticipateinthecontrast,orientation,distanceandothermechanismsofthesurfaceoftheextractedobject.Differentneuronshavedifferentorientations,differentspatialfrequencies,andimages.TheresponseofthevisualsystemisbasedontheperformanceoftheFourieralgorithm,andthenthevisualsystemextractsprimitiveprimitivesfromtheretinalscenetoperformsymbolgroupoperations.Inthisway,visualinformationpassesthroughvisualcells,bipolarcells,horizontalcells,andganglioncells,andpassesthroughtheopticnerve.The“serial”informationpatternistransmittedtothelateralgeniculatebodytobedecodedintoa“lattice”form,andthentransmittedbyopticradiationtodifferentfunctionalareasoftheprimaryvisualcortex,andfinallytransmittedtothecorrespondingdivisionoflaborareasofthehigh-levelarea,andintegratedindifferentcorticalareastoproduceCompletecognitionofvisualinformation.
Thecentralpathwayofvisionandcorticalpositioning
Thevisualpathwayconsistsoffourlevelsofneurons,thefirst,second,andthirdlevelsofneuronsarelocatedintheretina,andthefourthlevelofneuronsislocatedontheoutsideThegeniculatebody,fromwhichthenervefibersarefinallyterminatedintheopticcenterofthecerebralcortex.Theopticnerve,whichiscomposedofaxonsofthird-levelneurons(ganglioncells),entersthecranialcavityafterleavingtheeyeball.Itmergesintotheopticchiasmatthebottomofthethirdventricle,wherehalfofthefiberscrosstotheoppositeside.Theruleisthatthefibersfromthenasalretinaofbotheyes(thatis,thepartthatreceiveslightstimulationfromthetemporalside)crosstothecontralateralsideandascendtothecontralaterallateralgeniculatebody.Thefibersfromthetemporalretina(thatis,thepartthatreceiveslightstimulationfromthenasalside)donotcross,andascendtotheipsilaterallateralgeniculatebody.Inthisway,theleftandrighthalvesoftheentirefieldofviewareprojectedtotheoppositecerebralhemispheres.Theopticnervefibersformtheleftandrightoptictractsafterpassingthroughtheopticchiasm,andsomeofthemreachthesuperiorcolliculusofthequadruplex,andparticipateinactivitiessuchasvisualaccommodationreflex,lightreflex,andvisualmotorreflex;mostofthefibersstopatthelateralgeniculatebody.Theinternalstructureofthelateralgeniculatebodyiscomposedof6layersofnervecells,andthepartsrelatedtocentralvisionandperipheralvisioncanbeclearlydistinguished.Theopticnervefibersfromtheintersectioncorrespondingtothefoveaterminateinlayers1,4,and6,andthenon-intersectingopticnervefibersterminateinlayers2,3,and5.Thoseequivalenttothenear-peripheralareastopat1,6,2,and3layersrespectively,andthoseinthefar-peripheralareastopatlayers1,2respectively.Theendofeachopticnervefiberisdividedinto5-6smallbranches,eachofwhichterminatesinacellbodyofthelateralgeniculatebody,ratherthandendrites.Therefore,wheneveranopticnervefiberisdamaged,itispossibletodegeneratethecellsinthe2,3,and5layersontheipsilateralsideorthe1,4,and6layersonthecontralateralside.Thesynapticconnectionofthelateralgeniculatebodyisverycomplicatedandcannotbesimplyregardedasarelaystationbetweenthecerebralcortexandtheretina.