The Visual System Part 2 The Retina S

The Visual System Part 2

The Retina

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

Stargazing Finding your car in a dark parking lot Choosing socks

Visual Acuity • 120, 000 rods and 6, 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

Demonstration of Blind Spot (diagram)

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

“Bleaching” of photopigments

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

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

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

Why so much emphasis on the Visual System?