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Psy393: Cognitive Neuroscience Psy393:Psy393: CognitiveCognitive NeuroscienceNeuroscience Prof.Prof. AndersonAnderson DepartmentDepartment ofof PsychologyPsychology WeekWeek 33 ThThee EEye:ye: ProofProof forfor thethe existenceexistence ofof God?God? AndAnd thenthen therethere waswas lightlight OpticsOptics PerceptionPerception Absorption EyeEye isis receiverreceiver notnot sendersender Plato Euclid SendSend && receivereceive Echolocation Sonar PerceptionPerception isis relativerelative PerceptionPerception notnot ofof environmentenvironment (realism)(realism) butbut ofof ourour interpretationinterpretation (relativism)(relativism) • Butterflies see ElectromagneticElectromagnetic spectrumspectrum ultraviolet markings • Snakes see infra- red waves TheThe firstfirst stagestage ofof transformationtransformation Retinal sensitivity to “visible light” 400-700 nanometers (nm is 1/100 millionth of a meter) AccomodationAccomodation StartStart withwith gettinggetting thingsthings inin focusfocus onon thethe retinaretina Do it all the time unconsciously AccomodationAccomodation Changes is lens curvature though muscles Far-sightedness in the elderly More convex Retina:Retina: RodsRods && ConesCones NotNot justjust morphologicalmorphological differencesdifferences 22 typestypes ofof visionvision Photopic Diurnal Colour Bright light Scotopic Nocturnal Monochromatic Dim light PredatorPredator vsvs preyprey DistributionDistribution ofof rodsrods && conescones 55 millionmillion conescones perper retinaretina Fovea 1% are in the fovea, 99% in periphery 120120 millionmillion rodsrods perper retinaretina None in fovea Rods:conesRods:cones 20:1 in periphery Periphery Retina:Retina: ItsIts allall backwardsbackwards Epithelial layer PigmentPigment epitheliumepithelium atat Back backback ofof eyeeye NourishmentNourishment ofof photoreceptorsphotoreceptors LightLight mustmust passpass throughthrough neuralneural machinerymachinery Front WhWhyy ddonon’’tt wewe seesee ththee cells/bloodcells/blood vesselsvessels inin ourour eyes?eyes? ImagesImages stabilizedstabilized onon retinaretina disappeardisappear HowHow’’ss thatthat forfor perception!perception! TheThe retinalretinal ““blackblack holehole”” TheThe blindblind spotspot GanglionGanglion cellscells——>optic>optic nervenerve exitexit eyeeye From fovea FillingFilling inin thethe blindblind spotspot ““fillingfilling--inin”” thethe blindblind spot?spot? RamachandranRamachandran exampleexample InferenceInference oror perceptionperception Transduction:Transduction: LightLight toto energyenergy Visual pigment molecules Opsin & retinal Retinal Light reactive chemical Absorbs a single photon! Isomerization: Morphing Change in shape of ion channels Change in membrane potential Electricity! Amplification: 1 pigment molecule —> cascade of million others Perception of light Can perceive a single rod activation Photoreceptor VisualVisual pigmentspigments NotNot allall pigmentspigments areare createdcreated equalequal RodsRods vsvs ConesCones DarkDark adaptationadaptation ChangesChanges inin sensitivitysensitivity toto lightlight relatedrelated toto differencedifference inin timetime toto pigmentpigment regenerationregeneration TimecourseTimecourse parallelsparallels lightlight sensitivitysensitivity inin darkdark adaptationadaptation curvecurve Dar Time Light DarkDark adaptation:adaptation: SwitchingSwitching visualvisual systemssystems DarkDark adaptationadaptation curvecurve SwitchSwitch fromfrom photopicphotopic toto scotopicscotopic visionvision MaxMax adaptadapt ConesCones Test fovea 3-5 min RodsRods Rod monochromat 25-30 min 25-30 min “Racoon” vision? InIn livingliving colour:colour: SpectralSpectral sensitivitysensitivity ““MonochromaticMonochromatic”” lightlight Threshold 1 wavelength MethodMethod ofof adjustmentadjustment Cones Fovea (cones) Periphery (rods) After dark adaptation SensitivitySensitivity == 1/threshold1/threshold Spectral sensitivity curve Need less photons Overall cone sensitivity Need more photons 11 rod,rod, 33 typestypes ofof conescones 3 cone pigments types Absorption spectra Short (S) 419 nm Medium (M) 531 nm Long (L) 558 1 rod pigment Btwn S & M (green-blue) S ML Not color specific E.g., blue, green, red Maximally responsive to these colours Spectral sensitivity associated with absorption spectra Weighted towards long wavelength cones Most prominent Convergence:Convergence: AcuityAcuity vsvs SensitivitySensitivity Tradeoffs: Power vs grace Less light needed for rod receptors Also, differential convergence on to neurons Rod:ganglion cell, 120:1 Cone:ganglion cell, 6:1 Decreases threshold for ganglion response FovealFoveal andand peripheralperipheral visionvision DifferentialDifferential convergenceconvergence WhyWhy peripheryperiphery isis blurryblurry relativerelative toto fovea?fovea? Fovea:Fovea: AllAll conescones MostMost acuteacute ButBut leastleast sensitivesensitive WhatWhat thethe useuse ofof sensitivitysensitivity ifif youyou cantcant telltell whatwhat itit is?is? Z C H S K E T D K F L F G L A D N X FoveationFoveation NeuralNeural transformationtransformation ConvergenceConvergence isis allowsallows transformationtransformation ofof informationinformation DifferentDifferent formsforms ofof convergenceconvergence allowallow diversitydiversity inin responseresponse UpUp inin thethe CNSCNS circuitscircuits getget moremore complexcomplex Thousands of interconnected neurons Electrical engineering NeuralNeural circuitcircuit designs:designs: ExcitationExcitation ““featurefeature”” detectorsdetectors Output of red neuron Preferred response NoNo convergenceconvergence ConvergenceConvergence Responsive to line length But not unique to line length NeuralNeural circuitcircuit designs:designs: ExcitationExcitation && inhibitioninhibition MoreMore complexcomplex responseresponse propertiesproperties Preferred response (cell likes medium sized lines! TransformationTransformation ofof informationinformation inin ganglionganglion cellscells BetweenBetween photoreceptorsphotoreceptors andand ganglionganglion cellscells Horizontal Bipolar Amacrine PatternPattern ofof convergenceconvergence btwnbtwn thesethese cellscells ReceptiveReceptive fieldsfields AreaArea ofof spacespace (retina(retina forfor vision)vision) thatthat whenwhen stimulatedstimulated influencesinfluences aa neuronsneurons firingfiring raterate ReceptiveReceptive fieldfield propertiesproperties TheThe featuresfeatures ofof aa stimulusstimulus thatthat increaseincrease aa neuronsneurons firingfiring raterate ReceptiveReceptive fieldsfields A neuron’s window onto the world Classical definition: Region of sensory surface (retina for vision) that when stimulated influences a neurons firing rate Receptive field properties The features of a stimulus that increase a neurons firing rate Simple: spot of light Complex: A friends face JourneyJourney throughthrough thethe visualvisual systemsystem RFRF propertiesproperties telltell usus aboutabout thethe developmentdevelopment ofof perceptionperception LikeLike thethe developmentdevelopment ofof complexcomplex behaviourbehaviour Early versus later stages ReceptiveReceptive field:field: GanglionGanglion cellscells Restricted portion of space Small receptive fields (RF) Convergence from photoreceptors RF properties On-cell Center-surround antagonism On-cells (on center, off surround) Off-cells (off center, on surround) What is it for? Enhancing contrast Goal: Detection of change LateralLateral inhibitioninhibition Center-surround Amacrine & horizontal cells Lateral network that allows cross-talk Transformation of information Spots of light at photoreceptors Center-surround at ganglion cells GhostsGhosts inin thethe machinemachine LateralLateral inhibitioninhibition andand perceptionperception ExperienceExperience ofof lightlight isis diminisheddiminished byby summationsummation ofof inhibitoryinhibitory influencesinfluences Maximal inhibition Result:Result: IllusoryIllusory DarkDark spotsspots Reduced inhibition LateralLateral inhibitioninhibition andand perceptionperception Mach bands Dark and light bands at contrast borders Hyper-realism Raphael’s Madonna Perceptual contrast effects in renaissance drawings LateralLateral inhibitioninhibition andand perceptionperception MachMach bandsbands Objective Subjective Physical perception reality TransformationTransformation fromfrom physicalphysical toto perceptualperceptual energiesenergies TransformationTransformation takestakes placeplace inin ganglionganglion cellscells LateralLateral inhibitioninhibition andand perceptionperception LessLess inhibitioninhibition fromfrom lighterlighter sideside MoreMore inhibitioninhibition fromfrom darkerdarker sideside SimultaneousSimultaneous contrastcontrast PerceptionPerception ofof lightnesslightness isis influencedinfluenced byby moremore thanthan justjust laterallateral inhibitioninhibition (LI)(LI) LightnessLightness perceptionperception isis achievedachieved byby ganglionganglion cellscells alonealone OtherOther higherhigher--orderorder contrastcontrast effectseffects LILI atat ganglionganglion cellcell insufficientinsufficient toto explainexplain illusoryillusory perceptionsperceptions ofof lightnesslightness White’s illusion Opposite of whatwhat would be predicted from L “belongingness” Your visual system “reasons” Perceptual rationalization Hypothesis testing Dichoptic viewing DoesDoes understandingunderstanding thethe retinaretina explainexplain vision?vision? WorldWorld projectedprojected onon retinaretina == vision?vision? NoNo Why?Why? IllusoryIllusory lightnesslightness Can’t be accounted for by retina alone RetinalRetinal representationsrepresentations ofof worldworld isis locallocal Bits of lightness and darkness Need sharing of information ItIt takestakes aa villagevillage …… Its all about sharing of information A Retina has no global “representation” of Brad Pitt Photoreceptor A doesn’t talk with photoreceptor B Respond to small spots of B light.
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