1. The Human Visual System Vision Class 2006-7 The main topics covered here were: photoreceptors and ganglion cells in the retina. The photoreceptors are the input, the ganglion are the output to the brain. We also discussed the role of the ganglion cells in edge detection.

2. The Human Brain: Overall View Visual information goes from the eyes via the LGN in the thalamus to the back of the cortex

4. Each half of the visual field goes to a different hemisphere of the brain

5. Visual pathway

7. Hemifield neglect

8. Eye

9. Recording

10. Recording from a Neuron

11. Neurons Purkinje cells from cerebellum, dendrites showing calcium concentration

12. Synaptic connection

13. Synapse

14. Receptors Density - Fovea

19. Photoreceptors

21. Photo response Retinal (the chromophore) + opsin (rodopsin, cone-opsin) 11-cis retinal + photon → all trans retinal Activates transducin (a G- protein) Activates phosphodiesterase Breaks cGMP cGMP keep ionic channels open, now some close (one retinal → 500 cGMP) Cell hyperpolarization

22. Image Capture Huge dynamic range 10 -8 – 10 +6 μW/cm 2 Photons: poisson process. Noisy at low levels For low light: large receptors, slow integration Rods/cones, local adaptation,change of amplitude and time constant, motion deblur

23. Dynamic Range

25. Visual receptor types

27. Retina Mosaic

28. Color Mixing

32. Edge Detection by Z.C. Peak in f’(x) z.c. in f’’(x) Noise: Asin(wx) > -w 2 cos(wx) Convolve with a Gaussian (optimal) d 2 /dx 2 (G*f) = d 2 /dx 2 (G) * f In 2-D: d 2 /dx 2 + d 2 /dy 2

33. Edge Detection by Z.C.

35. Z.C. at Two Scales

39. Wheel Edges 4 Resolutions

40. END