Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Week 3.

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Psy393: Psy393: Cognitive Cognitive Neuroscience Neuroscience Prof. Anderson Prof. Anderson Department of Psychology Department of Psychology Week 3 Week 3

Transcript of Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Week 3.

Psy393: Psy393: Cognitive Cognitive

NeuroscienceNeuroscienceProf. AndersonProf. Anderson

Department of PsychologyDepartment of Psychology

Week 3Week 3

The Eye: The Eye: Proof for the existence of Proof for the existence of

God?God?

And then there was lightAnd then there was light

OpticsOptics PerceptionPerception

AbsorptionAbsorption Eye is receiver Eye is receiver

not sendernot sender PlatoPlato EuclidEuclid

Send & receiveSend & receive EcholocationEcholocation SonarSonar

Perception is relativePerception is relative Perception not of environment (realism) but Perception not of environment (realism) but

of our interpretation (relativism)of our interpretation (relativism) Electromagnetic spectrumElectromagnetic spectrum

The first stage of transformationThe first stage of transformation Retinal sensitivity to “visible light” Retinal sensitivity to “visible light” 400-700 nanometers (nm is 1/100 millionth of a meter)400-700 nanometers (nm is 1/100 millionth of a meter)

• Butterflies see ultraviolet markings

• Snakes see infra-red waves

AccomodationAccomodation Start with getting things in Start with getting things in

focus on the retinafocus on the retina Do it all the time Do it all the time

unconsciouslyunconsciously AccomodationAccomodation

Changes is lens curvature Changes is lens curvature though musclesthough muscles

Far-sightedness in the elderlyFar-sightedness in the elderly More convex

Retina: Rods & ConesRetina: Rods & Cones Not just morphological differencesNot just morphological differences

2 types of vision2 types of vision PhotopicPhotopic

DiurnalDiurnal ColourColour Bright lightBright light

ScotopicScotopic NocturnalNocturnal MonochromaticMonochromatic Dim lightDim light

Predator vs preyPredator vs prey

Distribution of rods & Distribution of rods & conescones

5 million cones per retina5 million cones per retina 1% are in the fovea, 99% in 1% are in the fovea, 99% in

peripheryperiphery 120 million rods per retina120 million rods per retina

None in foveaNone in fovea Rods:conesRods:cones

20:1 in periphery20:1 in periphery

Fovea

Periphery

Retina: Its all backwardsRetina: Its all backwards

Pigment Pigment epithelium at epithelium at back of eyeback of eye Nourishment of Nourishment of

photoreceptorsphotoreceptors Light must pass Light must pass

through neural through neural machinerymachinery

Epithelial layer

Front

Back

Why don’t we see the Why don’t we see the cells/blood vessels in our cells/blood vessels in our

eyes?eyes? Images stabilized on Images stabilized on

retina disappearretina disappear How’s that for How’s that for

perception!perception!

The retinal “black hole”The retinal “black hole”

The blind spotThe blind spot Ganglion cells—>optic nerve exit eyeGanglion cells—>optic nerve exit eye

From fovea

Filling in the blind spotFilling in the blind spot

““filling-in” the blind spot?filling-in” the blind spot? Ramachandran exampleRamachandran example

Inference or perceptionInference or perception

Transduction: Light to Transduction: Light to energyenergy

Visual pigment moleculesVisual pigment molecules Opsin & retinalOpsin & retinal

RetinalRetinal Light reactive chemicalLight reactive chemical Absorbs a single photon!Absorbs a single photon! Isomerization: MorphingIsomerization: Morphing Change in shape of ion channelsChange in shape of ion channels Change in membrane potential Change in membrane potential Electricity!Electricity!

Amplification: Amplification: 1 pigment molecule —> cascade of 1 pigment molecule —> cascade of

million others million others Perception of light Perception of light

Can perceive a single rod activationCan perceive a single rod activation

Photoreceptor

Visual pigmentsVisual pigments Not all pigments are created equalNot all pigments are created equal

Rods vs ConesRods vs Cones Dark adaptation Dark adaptation

Changes in sensitivity to light related to Changes in sensitivity to light related to difference in time to pigment difference in time to pigment regenerationregeneration

Timecourse parallelsTimecourse parallelslight sensitivity in light sensitivity in dark adaptation curvedark adaptation curve

LightTime

Dark

Dark adaptation: Dark adaptation: Switching visual systemsSwitching visual systems

Dark adaptation curveDark adaptation curve Switch from photopic to scotopic visionSwitch from photopic to scotopic vision

Max adaptMax adapt ConesCones

Test foveaTest fovea 3-5 min3-5 min

RodsRods Rod Rod

monochromatmonochromat 25-30 min25-30 min

“Racoon” vision?

In living colour: In living colour: Spectral sensitivitySpectral sensitivity

““Monochromatic” lightMonochromatic” light 1 wavelength1 wavelength

Method of adjustmentMethod of adjustment Fovea (cones)Fovea (cones) Periphery (rods)Periphery (rods)

After dark adaptationAfter dark adaptation

Sensitivity = 1/thresholdSensitivity = 1/threshold

Spectral sensitivity curve

Threshold

Cones

Need less photons

Need more photons

Overall cone sensitivity

1 rod, 3 types of cones1 rod, 3 types of cones 3 cone pigments types3 cone pigments types

Short (S)Short (S) 419 nm419 nm

Medium (M)Medium (M) 531 nm531 nm

Long (L)Long (L) 558558

1 rod pigment1 rod pigment Btwn S & M (green-blue)Btwn S & M (green-blue)

Not color specificNot color specific E.g., blue, green, redE.g., blue, green, red Maximally responsive to these coloursMaximally responsive to these colours

Spectral sensitivity associated with absorption spectraSpectral sensitivity associated with absorption spectra Weighted towards long wavelength conesWeighted towards long wavelength cones Most prominentMost prominent

Absorption spectra

S M L

Convergence: Convergence: Acuity vs Sensitivity Acuity vs Sensitivity

Tradeoffs: Power vs graceTradeoffs: Power vs grace Less light needed for rod receptorsLess light needed for rod receptors Also, differential convergence on to Also, differential convergence on to

neuronsneurons Rod:ganglion cell, 120:1Rod:ganglion cell, 120:1 Cone:ganglion cell, 6:1Cone:ganglion cell, 6:1

Decreases threshold for ganglion Decreases threshold for ganglion responseresponse

Foveal and peripheral Foveal and peripheral visionvision

Differential convergenceDifferential convergence Why periphery is blurry relative to Why periphery is blurry relative to

fovea?fovea? Fovea: All conesFovea: All cones

Most acuteMost acute But least sensitiveBut least sensitive

What the use of sensitivity if you cant What the use of sensitivity if you cant tell what it is?tell what it is? FoveationFoveation

Z C H S K E T D K F L F G L A D N X

Neural transformationNeural transformation Convergence is allows transformation of informationConvergence is allows transformation of information Different forms of convergence allow diversity in responseDifferent forms of convergence allow diversity in response Up in the CNS circuits get more complexUp in the CNS circuits get more complex

Thousands of interconnected neuronsThousands of interconnected neurons Electrical engineeringElectrical engineering

Neural circuit designs: Neural circuit designs: ExcitationExcitation

““feature” detectorsfeature” detectors

No convergenceNo convergence ConvergenceConvergence

Responsive to line lengthResponsive to line length But not unique to line But not unique to line

lengthlength

Output of red neuron

Preferred response

Neural circuit designs: Neural circuit designs: Excitation & inhibitionExcitation & inhibition

More complex response propertiesMore complex response properties

Preferred response (cell likes medium sized lines!

Transformation of Transformation of information in ganglion information in ganglion

cellscells Between Between

photoreceptors photoreceptors and ganglion and ganglion cells cells HorizontalHorizontal BipolarBipolar AmacrineAmacrine

Pattern of Pattern of convergence convergence btwn these cellsbtwn these cells

Receptive fieldsReceptive fields

Area of space (retina for vision) that Area of space (retina for vision) that when stimulated influences a when stimulated influences a neurons firing rateneurons firing rate

Receptive field propertiesReceptive field properties The features of a stimulus that increase The features of a stimulus that increase

a neurons firing ratea neurons firing rate

Receptive fieldsReceptive fields A neuron’s window onto the worldA neuron’s window onto the world Classical definition:Classical definition:

Region of sensory surface (retina for vision) that when Region of sensory surface (retina for vision) that when stimulated influences a neurons firing ratestimulated influences a neurons firing rate

Receptive field propertiesReceptive field properties The features of a stimulus that increase a neurons firing The features of a stimulus that increase a neurons firing

raterate Simple: spot of lightSimple: spot of light Complex: A friends Complex: A friends

faceface

Journey through the Journey through the visual systemvisual system

RF properties tell us about the RF properties tell us about the development of perceptiondevelopment of perception Like the development of complex Like the development of complex

behaviourbehaviour Early versus later stagesEarly versus later stages

Receptive field: Ganglion Receptive field: Ganglion cellscells

Restricted portion of space Restricted portion of space Small receptive fields (RF)Small receptive fields (RF)

Convergence from Convergence from photoreceptorsphotoreceptors

RF propertiesRF properties Center-surround antagonismCenter-surround antagonism On-cells (on center, off On-cells (on center, off

surround)surround) Off-cells (off center, on Off-cells (off center, on

surround)surround)

What is it for?What is it for? Enhancing contrastEnhancing contrast Goal: Detection of changeGoal: Detection of change

On-cell

Lateral inhibitionLateral inhibition Center-surroundCenter-surround

Amacrine & horizontal cellsAmacrine & horizontal cells

Lateral network that Lateral network that allows cross-talkallows cross-talk

Transformation of Transformation of informationinformation Spots of light at Spots of light at

photoreceptorsphotoreceptors Center-surround at Center-surround at

ganglion cellsganglion cells

Ghosts in the machineGhosts in the machine

Lateral inhibition and Lateral inhibition and perceptionperception

Experience of Experience of light is light is diminished by diminished by summation of summation of inhibitory inhibitory influencesinfluences

Result: Illusory Result: Illusory Dark spotsDark spots Reduced inhibition

Maximal inhibition

Lateral inhibition and Lateral inhibition and perceptionperception

Mach bandsMach bands Dark and light bands at contrast bordersDark and light bands at contrast borders

Hyper-realismHyper-realism Perceptual contrast effects in renaissance drawingsPerceptual contrast effects in renaissance drawingsRaphael’s Madonna

Lateral inhibition and Lateral inhibition and perceptionperception

Mach bandsMach bands

Transformation from physical to perceptual Transformation from physical to perceptual energiesenergies

Transformation takes place in ganglion cellsTransformation takes place in ganglion cells

Objective Physical reality

Subjective perception

Lateral inhibition and Lateral inhibition and perceptionperception

Less inhibition Less inhibition from lighter sidefrom lighter side

More inhibition More inhibition from darker side from darker side

Simultaneous contrastSimultaneous contrast Perception of lightness is influenced by Perception of lightness is influenced by

more than just lateral inhibition (LI)more than just lateral inhibition (LI) Lightness perception is achieved by Lightness perception is achieved by

ganglion cells aloneganglion cells alone

Other higher-order contrast Other higher-order contrast effectseffects

LI at ganglion cell insufficient to explain LI at ganglion cell insufficient to explain illusory perceptions of lightnessillusory perceptions of lightness

Your visual system “reasons”Your visual system “reasons” Perceptual rationalization Perceptual rationalization Hypothesis testingHypothesis testing

Dichoptic viewingDichoptic viewing

White’s illusionWhite’s illusion Opposite of Opposite of

what would be what would be predicted from predicted from LILI

““belongingnessbelongingness””

Does understanding the Does understanding the retina explain vision?retina explain vision?

World projected on retina = vision?World projected on retina = vision? NoNo

Why? Why? Illusory lightnessIllusory lightness

Can’t be accounted for by retina aloneCan’t be accounted for by retina alone Retinal representations of world is localRetinal representations of world is local

Bits of lightness and darknessBits of lightness and darkness Need sharing of informationNeed sharing of information

It takes a village …It takes a village … Its all about sharing of Its all about sharing of

informationinformation Retina has no global Retina has no global

“representation” of “representation” of Brad PittBrad Pitt

Photoreceptor A Photoreceptor A doesn’t talk with doesn’t talk with photoreceptor Bphotoreceptor B

Respond to small spots Respond to small spots of lightof light

A

B