The Eye: A Window into Perception and Neural Transformation

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

2. The Eye: Proof for the existence of God?

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

4. Perception is relative • Perception not of environment (realism) but of our interpretation (relativism) • Electromagnetic spectrum • Butterflies see ultraviolet markings • Snakes see infra-red waves • The first stage of transformation • Retinal sensitivity to “visible light” • 400-700 nanometers (nm is 1/100 millionth of a meter)

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

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

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

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

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

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

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

12. Transduction: Light to energy • 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

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

14. Dark adaptation: Switching visual systems • Dark adaptation curve • Switch from photopic to scotopic vision • Max adapt • Cones • Test fovea • 3-5 min • Rods • Rod monochromat • 25-30 min “Racoon” vision?

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

16. 1 rod, 3 types of cones • 3 cone pigments types • Short (S) • 419 nm • Medium (M) • 531 nm • Long (L) • 558 • 1 rod pigment • Btwn S & M (green-blue) • 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 Absorption spectra S M L

17. Convergence: Acuity vs Sensitivity • 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

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

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

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

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

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

23. Receptive fields • Area of space (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

24. Receptive fields • 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

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

26. Receptive field: Ganglion cells • Restricted portion of space • Small receptive fields (RF) • Convergence from photoreceptors • RF properties • 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 On-cell

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

28. Ghosts in the machine

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

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

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

32. Lateral inhibition and perception • Less inhibition from lighter side • More inhibition from darker side

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

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

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

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