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

The Eye: Proof for the existence of God?

And then there was light Optics Optics Perception Perception Absorption Absorption Eye is receiver not sender Eye is receiver not sender Plato Plato Euclid Euclid Send & receive Send & receive Echolocation Echolocation Sonar Sonar

Perception is relative Perception not of environment (realism) but of our interpretation (relativism) Perception not of environment (realism) but of our interpretation (relativism) Electromagnetic spectrum Electromagnetic spectrum The first stage of transformation The first stage of transformation Retinal sensitivity to “visible light” Retinal sensitivity to “visible light” nanometers (nm is 1/100 millionth of a meter) nanometers (nm is 1/100 millionth of a meter) Butterflies see ultraviolet markings Snakes see infra- red waves

Accomodation Start with getting things in focus on the retina Start with getting things in focus on the retina Do it all the time unconsciously Do it all the time unconsciously Accomodation Accomodation Changes is lens curvature though muscles Changes is lens curvature though muscles Far-sightedness in the elderly Far-sightedness in the elderly More convex

Retina: Rods & Cones Not just morphological differences Not just morphological differences 2 types of vision 2 types of vision Photopic Photopic Diurnal Diurnal Colour Colour Bright light Bright light Scotopic Scotopic Nocturnal Nocturnal Monochromatic Monochromatic Dim light Dim light Predator vs prey Predator vs prey

Distribution of rods & cones 5 million cones per retina 5 million cones per retina 1% are in the fovea, 99% in periphery 1% are in the fovea, 99% in periphery 120 million rods per retina 120 million rods per retina None in fovea None in fovea Rods:cones Rods:cones 20:1 in periphery 20:1 in periphery Fovea Periphery

Retina: Its all backwards Pigment epithelium at back of eye Pigment epithelium at back of eye Nourishment of photoreceptors Nourishment of photoreceptors Light must pass through neural machinery Light must pass through neural machinery Epithelial layer Front Back

Why don’t we see the cells/blood vessels in our eyes? Images stabilized on retina disappear Images stabilized on retina disappear How’s that for perception! How’s that for perception!

The retinal “black hole” The blind spot The blind spot Ganglion cells—>optic nerve exit eye Ganglion cells—>optic nerve exit eye From fovea

Filling in the blind spot “filling-in” the blind spot? “filling-in” the blind spot? Ramachandran example Ramachandran example Inference or perception Inference or perception

Transduction: Light to energy Visual pigment molecules Visual pigment molecules Opsin & retinal Opsin & retinal Retinal Retinal Light reactive chemical Light reactive chemical Absorbs a single photon! Absorbs a single photon! Isomerization: Morphing Isomerization: Morphing Change in shape of ion channels Change in shape of ion channels Change in membrane potential Change in membrane potential Electricity! Electricity! Amplification: Amplification: 1 pigment molecule —> cascade of million others 1 pigment molecule —> cascade of million others Perception of light Perception of light Can perceive a single rod activation Can perceive a single rod activation Photoreceptor

Visual pigments Not all pigments are created equal Not all pigments are created equal Rods vs Cones Rods vs Cones Dark adaptation Dark adaptation Changes in sensitivity to light related to difference in time to pigment regeneration Changes in sensitivity to light related to difference in time to pigment regeneration Timecourse parallels Timecourse parallels light sensitivity in dark adaptation curve Light Time Dark

Dark adaptation: Switching visual systems Dark adaptation curve Dark adaptation curve Switch from photopic to scotopic vision Switch from photopic to scotopic vision Max adapt Max adapt Cones Cones Test fovea Test fovea 3-5 min 3-5 min Rods Rods Rod monochromat Rod monochromat min min “Racoon” vision?

In living colour: Spectral sensitivity “Monochromatic” light “Monochromatic” light 1 wavelength 1 wavelength Method of adjustment Method of adjustment Fovea (cones) Fovea (cones) Periphery (rods) Periphery (rods) After dark adaptation After dark adaptation Sensitivity = 1/threshold Sensitivity = 1/threshold Spectral sensitivity curve Threshold Cones Need less photons Need more photons Overall cone sensitivity

1 rod, 3 types of cones 3 cone pigments types 3 cone pigments types Short (S) Short (S) 419 nm 419 nm Medium (M) Medium (M) 531 nm 531 nm Long (L) Long (L) rod pigment 1 rod pigment Btwn S & M (green-blue) Btwn S & M (green-blue) Not color specific Not color specific E.g., blue, green, red E.g., blue, green, red Maximally responsive to these colours Maximally responsive to these colours Spectral sensitivity associated with absorption spectra Spectral sensitivity associated with absorption spectra Weighted towards long wavelength cones Weighted towards long wavelength cones Most prominent Most prominent Absorption spectra S ML

Convergence: Acuity vs Sensitivity Tradeoffs: Power vs grace Tradeoffs: Power vs grace Less light needed for rod receptors Less light needed for rod receptors Also, differential convergence on to neurons Also, differential convergence on to neurons Rod:ganglion cell, 120:1 Rod:ganglion cell, 120:1 Cone:ganglion cell, 6:1 Cone:ganglion cell, 6:1 Decreases threshold for ganglion response Decreases threshold for ganglion response

Foveal and peripheral vision Differential convergence Differential convergence Why periphery is blurry relative to fovea? Why periphery is blurry relative to fovea? Fovea: All cones Fovea: All cones Most acute Most acute But least sensitive But least sensitive What the use of sensitivity if you cant tell what it is? What the use of sensitivity if you cant tell what it is? Foveation Foveation Z C H S K E T D K F L F G L A D N X

Neural transformation Convergence is allows transformation of information Convergence is allows transformation of information Different forms of convergence allow diversity in response Different forms of convergence allow diversity in response Up in the CNS circuits get more complex Up in the CNS circuits get more complex Thousands of interconnected neurons Thousands of interconnected neurons Electrical engineering Electrical engineering

Neural circuit designs: Excitation “feature” detectors “feature” detectors No convergence No convergence Convergence Convergence Responsive to line length Responsive to line length But not unique to line length But not unique to line length Output of red neuron Preferred response

Neural circuit designs: Excitation & inhibition More complex response properties More complex response properties Preferred response (cell likes medium sized lines!

Transformation of information in ganglion cells Between photoreceptors and ganglion cells Between photoreceptors and ganglion cells Horizontal Horizontal Bipolar Bipolar Amacrine Amacrine Pattern of convergence btwn these cells Pattern of convergence btwn these cells

Receptive fields Area of space (retina for vision) that when stimulated influences a neurons firing rate Area of space (retina for vision) that when stimulated influences a neurons firing rate Receptive field properties Receptive field properties The features of a stimulus that increase a neurons firing rate The features of a stimulus that increase a neurons firing rate

Receptive fields A neuron’s window onto the world A neuron’s window onto the world Classical definition: Classical definition: Region of sensory surface (retina for vision) that when stimulated influences a neurons firing rate Region of sensory surface (retina for vision) that when stimulated influences a neurons firing rate Receptive field properties Receptive field properties The features of a stimulus that increase a neurons firing rate The features of a stimulus that increase a neurons firing rate Simple: spot of light Simple: spot of light Complex: A friends Complex: A friendsface

Journey through the visual system RF properties tell us about the development of perception RF properties tell us about the development of perception Like the development of complex behaviour Like the development of complex behaviour Early versus later stages Early versus later stages

Receptive field: Ganglion cells Restricted portion of space Restricted portion of space Small receptive fields (RF) Small receptive fields (RF) Convergence from photoreceptors Convergence from photoreceptors RF properties RF properties Center-surround antagonism Center-surround antagonism On-cells (on center, off surround) On-cells (on center, off surround) Off-cells (off center, on surround) Off-cells (off center, on surround) What is it for? What is it for? Enhancing contrast Enhancing contrast Goal: Detection of change Goal: Detection of change On-cell

Lateral inhibition Center-surround Center-surround Amacrine & horizontal cells Amacrine & horizontal cells Lateral network that allows cross-talk Lateral network that allows cross-talk Transformation of information Transformation of information Spots of light at photoreceptors Spots of light at photoreceptors Center-surround at ganglion cells Center-surround at ganglion cells

Ghosts in the machine

Lateral inhibition and perception Experience of light is diminished by summation of inhibitory influences Experience of light is diminished by summation of inhibitory influences Result: Illusory Dark spots Result: Illusory Dark spots Reduced inhibition Maximal inhibition

Lateral inhibition and perception Mach bands Mach bands Dark and light bands at contrast borders Dark and light bands at contrast borders Hyper-realism Hyper-realism Perceptual contrast effects in renaissance drawings Perceptual contrast effects in renaissance drawings Raphael’s Madonna

Lateral inhibition and perception Mach bands Mach bands Transformation from physical to perceptual energies Transformation from physical to perceptual energies Transformation takes place in ganglion cells Transformation takes place in ganglion cells Objective Physical reality Subjective perception

Lateral inhibition and perception Less inhibition from lighter side Less inhibition from lighter side More inhibition from darker side More inhibition from darker side

Simultaneous contrast Perception of lightness is influenced by more than just lateral inhibition (LI) Perception of lightness is influenced by more than just lateral inhibition (LI) Lightness perception is achieved by ganglion cells alone Lightness perception is achieved by ganglion cells alone

Other higher-order contrast effects LI at ganglion cell insufficient to explain illusory perceptions of lightness LI at ganglion cell insufficient to explain illusory perceptions of lightness Your visual system “reasons” Your visual system “reasons” Perceptual rationalization Perceptual rationalization Hypothesis testing Hypothesis testing Dichoptic viewing Dichoptic viewing White’s illusion White’s illusion Opposite of what would be predicted from LI Opposite of what would be predicted from LI “belongingness” “belongingness”

Does understanding the retina explain vision? World projected on retina = vision? World projected on retina = vision? No No Why? Why? Illusory lightness Illusory lightness Can’t be accounted for by retina alone Can’t be accounted for by retina alone Retinal representations of world is local Retinal representations of world is local Bits of lightness and darkness Bits of lightness and darkness Need sharing of information Need sharing of information

It takes a village … Its all about sharing of information Its all about sharing of information Retina has no global “representation” of Brad Pitt Retina has no global “representation” of Brad Pitt Photoreceptor A doesn’t talk with photoreceptor B Photoreceptor A doesn’t talk with photoreceptor B Respond to small spots of light Respond to small spots of light A B