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Seeing Things 1 Eye and Brain

How Your Brain Works - Week 3 Dr. Jan Schnupp jan.schnupp@dpag.ox.ac.uk HowYourBrainWorks.net. Seeing Things 1 Eye and Brain. Light Wavelength. Optics of the Eye. Eye and Retina. The Blind Spot. Retinotopy. Adapted from drawings by Ramon y Cajal.

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Seeing Things 1 Eye and Brain

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  1. How Your Brain Works - Week 3 Dr. Jan Schnupp jan.schnupp@dpag.ox.ac.uk HowYourBrainWorks.net Seeing Things 1Eye and Brain

  2. Light Wavelength

  3. Optics of the Eye

  4. Eye and Retina

  5. The Blind Spot

  6. Retinotopy • Adapted from drawings by Ramon y Cajal

  7. The Optic Pathway: eye, optic nerve, optic chiasm, optic tract, thalamus, optic radiation, visual cortex

  8. Rene Descartes, Retinotopy and the Seat of the Soul

  9. Photoreceptors • The human eye has ca. 10 million rods and ca 120 million cones

  10. Phototransduction • Light activated rhodopsin (R) activates G-protein (G) which in turn activates phosphodiesterase (PDE) which cleaves cGMP which closes cGMP-gated Na+ channel. • What does any of this have to do with carrots?

  11. Absorption Spectra • The three different types of cones and the rods have slightly different opsins which are sensitive to different wavelengths. • “Trichromacy” theory.

  12. Sensitivity of Receptors

  13. Rod and Cone Distribution

  14. Retinal Wiring • Photoreceptors • Horizontal cells • Bipolar cells • Amacrine cells • Retinal ganglion cells

  15. Centre –Surround Receptive Fields Photo- receptors Horizontal Cell Bipolar Cell RetinalGanglion Cell

  16. On-centre and off-centre Receptive Fields • Lateral inhibition provided by photoreceptor ribbon synapses, horizontal cell synapses and amacrine cells

  17. Lateral inhibition for contrast (edge) detection

  18. RGC receptive fields as “spatial frequency filters”

  19. Difference of Gaussians Model of Retinal Ganglion Cells • The centre-surround structure of Retinal Ganglion Cells turns them into “spatial frequency filters”. Larger RGC receptive fields are tuned to “coarsely grained” structure in the visual scene, while smaller RFs are tuned to fine grain structure.

  20. Convolving a Penny with DoGs • The picture of an American cent (left) seen through large (middle) or small (right) difference of Gaussian receptive fields.

  21. The Fovea

  22. Eye muscles

  23. Eye movements • Eye-movement traces while a subject explores a picture of the bust of Nefertiti. • From "Eye Movements and Vision" by A. L. Yarbus; Plenum Press, New York; 1967

  24. Dan Simmons’ visual attention task • Count the number of passes of the white team

  25. Colour opponency

  26. Colour Opponency

  27. The Colour Wheel Yellow-Blue Red Green

  28. Why Colour Vision Does Not Work Well in Poor Light

  29. Cone mosaics • Cone mosaics for four different individuals

  30. Colour blindness Red-green channel broken Blue-yellow channel broken

  31. M cells and P cells • 90% P cells • 5% M cells • 5% non-M non-P

  32. Projections to the Lateral Geniculate Nucleus

  33. The Stepping Feet Illusion • http://www.michaelbach.de/ot/mot_feet_lin/index.html • What is going on here? • M cells are colour blind, but very sensitive to brightness (luminance) contrast. • P cells are R-G opponent • Non-M non-P cells are Y-B opponent • Only M cells project to the motion processing streams in the brain.

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