The Visual System

Part 2

The Retina

Photoreceptor cells

4 light sensitive proteins:rods with 1 type3 types of cones, each with a single type

Receptors for light detection

Receptor cells

S 15More membrane =

more photosensitive molecules =more sensitive to light

StargazingFinding your car in a dark parking lot

Choosing socks

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 andReading the Writing on the Wall

Demonstrationof

Blind Spot(diagram)

•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

“Bleaching” of photopigments

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?

Hubel and Weisel Experiments

RetinalGanglionCells

LateralGeniculateNeurons

VisualCortexNeurons

Youtube

Parallel Processing in the Cortex

• Motion sensitive

• Color and shape sensitive

Story of woman atstreet crossing

Why so much emphasis on the Visual System?