The Visual System

Part 2

The Retina

The Retina

• Photoreceptors– Rods– Cones

• Bipolar cells• Ganglion Cells

– axons converge at optic disk– axons constitute optic nerve

• Horizontal cells• Amacrine cells

Importance of abundantmembrane

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

StargazingFinding your car in a dark parking lot

Choosing socks

“Bleaching” of photopigments

The Dark Current

Phototransduction• In the DARK, rod is depolarized due to influx

of Na+ (called dark current)• In the LIGHT, rod is hyperpolarized• cGMP keeps Na+/Ca++ channels open• Light results in decrease of cGMP and thus

closure of ion channels, and• Hyperpolarization of cell, and• Reduced release of Neurotransmitter Glutamate

Hubel and Weisel Experiments

RetinalGanglionCells

LateralGeniculateNeurons

VisualCortexNeurons

YouTubeVideos

Properties of Cortical Neurons• Simple

– respond to stationary bar of light in certain orientation

• Complex– respond to moving

bar of light in certain orientation

• Hypercomplex– respond to moving

bar of light of a certain length in a certain orientation

Parallel Processing in the Cortex

• Motion sensitive

• Color and shape sensitive

Story of woman atstreet crossing

Why so much emphasis on the Visual System?