1. The Visual System Part 2

2. The Retina

3. Photoreceptor cells 4 light sensitive proteins: rods with 1 type 3 types of cones, each with a single type Receptors for light detection Receptor cells S 15 More membrane = more photosensitive molecules = more sensitive to light

5. Stargazing Finding your car in a dark parking lot Choosing socks

6. Visual Acuity 120,000,000 rods and 6,000,000 cones per retina 1,200,000 retinal ganglion cells & axons 105:1 convergence ratio Fovea: cones only, 1:1 highest visual acuity but poor sensitivity Counting fingers and Reading the Writing on the Wall

8. Demonstration of Blind Spot (diagram)

9. Differences in spectral sensitivity due to 4 types of opsin. Rods have 1 type; Each type of cone has another type. Trichromatic Theory of Color Vision Web-based color deficiency test “Blue” cones“Green” cones“Red” cones

10. “Bleaching” of photopigments

11. The cascade of events initiated by light in a photoreceptor. The result of the cascade is to close a cGMP-gated sodium channel, reducing the inward current and hyperpolarizing the membrane. Source: Kandel, Schwartz & Jessell, Principles of Neural Science, 4th Edn What is the advantage of this multistep process?

12. Hubel and Weisel Experiments Retinal Ganglion Cells Lateral Geniculate Neurons Visual Cortex Neurons Youtube

14. Parallel Processing in the Cortex Motion sensitive Color and shape sensitive Story of woman at street crossing

15. Why so much emphasis on the Visual System?