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