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