Vision Our most dominant sense

Our Essential Questions

• What are the major parts of the eye?• How does the eye translate light into

neural impulses?

Vision

Purpose of the visual system–transform light energy into an

electro-chemical neural response–represent characteristics of objects

in our environment such as size, color, shape, and location

Light: The Visual Stimulus

Light: The Visual Stimulus

• Wavelength of a light is the distance of one complete cycle of the wave.

• Visible light : 400nm - 700nm.• Wavelength of light is related to its

perceived color

The Structure of the Visual System

So how does this stimulus (light) transform into messages in our brain?

Cornea• The clear bulge on the front of the eyeball• Begins to focus the light by bending it

toward a central focal point• Protects the eye

Parts of the Eye – Cornea

Iris• Colored portion of the eye

–Does color affect vision?• A ring of muscle tissue

that regulates the size of the pupil –Allows more or less light to

enter the eye

Parts of the Eye - Iris

Pupil

• Opening in the center of the eye • Controls the amount of light entering the

eye –bright conditions - iris expands, pupil gets

smaller–dark conditions - iris contracts, pupil gets

larger

Parts of the Eye - Pupil

Lens• A transparent structure behind the pupil• Focuses the image on the back of the eye

–Muscles change the thickness of the lens change how light is bent focuses the image

• Glasses or contacts correct problems

Parts of the Eye - Lens

Retina• At the back of the eyeball• Light-sensitive surface with cells that

convert light energy to neural impulses–This is where the magic happens!

Parts of the Eye - Retina

Fovea

• The central focal point of the retina• The spot where vision is best (most

detailed)

Parts of the Eye - Fovea

Receptor Cells

• In sight they change light into neural impulses the brain can understand

• Visual system has two types of receptor cells – rods and cones

Distribution of Rods and Cones

• Cones—concentrated in center of eye (fovea)– approx. 6 million

• Rods—concentrated in periphery – approx. 120 million

• Blind spot—region with no rods or cones

Differences Between Rods and Cones

• Cones– allow us to see in bright light– allow us to see fine spatial detail– allow us to see different colors

• Rods– allow us to see in dim light– can not see fine spatial detail– can not see different colors

Receptive Fields and Rod vs. Cone Visual Acuity

• Cones—in the fovea, one cone often synapse onto only a single ganglion cell

• Rods—the axons of many rods synapse onto one ganglion cell

• This allows rods to be more sensitive in dim light, but it also reduces visual acuity

Let’s Review

• Cone Characteristics• Rod Characteristics

• Located in the retina

• Can only detect black and white

• Respond to less light than do cones

Rods

• Located in the retina

• Can detect sharp images and color

• Need more light than the rods

• Many cones are clustered in the fovea

Cones

Let’s do an experiment now

• What do you see in your peripheral vision (that’s the stuff on the side)?

Get into groups of 3

• Pick an A, B, and C

The Experiment• A will look straight ahead• B will look A in the eyes – to make

sure that A doesn’t cheat!• C will move various colored pieces of

paper in A’s peripheral vision• A will guess the color

–Note: if the person is consistently guessing correctly then they are cheating!

Write up the results…

1. Results – correct guess versus bad

2. Your conclusion- What do your results tell you about

our vision?- How do the different kinds of receptor

cells affect our vision?

Distribution of Rods and Cones

• Cones—concentrated in center of eye (fovea)– approx. 6 million

• Rods—concentrated in periphery – approx. 120 million

• Blind spot—region with no rods or cones

Let’s Compare…

Cones–allow us to see

in bright light–allow us to see

fine spatial detail

–allow us to see different colors

Rods–allow us to see

in dim light–can not see fine

spatial detail–can not see

different colors

Visual Processing in the Retina

Optic Nerve

• The nerve that carries visual information from eye occipital lobes

Parts of the Eye – Optic Nerve

Blind Spot• The point at which the optic nerve travels through the retina to exit the eye

• There are no rods and cones at this point

Blind Spot

Parts of the Eye – Blind Spot

What do you see in your blind spot?

The Visual System: Color Vision

How do we see color?

Color Vision

• Differences in wavelength of light = color• Rods are color blind, but cones can see

different colors–We have only one type of rod but three types

of cones

Color Vision

• Two theories of color vision:–Trichromatic Theory–Opponent-Process Theory

Trichromatic (3-Color) Theory

• Cones are “tuned” to be sensitive to red, green and blue light

• All the colors we see are a combination of these 3 colors

• Similar to the design of a color TV

Opponent-Process Theory

• Sensory receptors in the retina come in pairs:–Red/Green–Yellow/Blue–Black/White

•Only one side is “on” at a time

Opponent Process Theory

ON” “OFF”red greengreen red blue yellow yellow blue black whitewhite black

Opponent-Process Theory

• If one sensor is stimulated, the other is inhibited

• If one sensor is over-stimulated, and fatigues, the paired sensor will be activated, causing an afterimage

Afterimage Effect

Can you see what is in the middle?

Red-Green Color Blindness

Color Deficient Vision

• People who lack one of the three types of cones

• Usually the red or green receptors are missing

• Usually referred to as color blindness• Inherited and found more in males

Overview of Visual System

• The eye is like a camera; instead of using film to catch the light, we have rods and cones.

• Cones allow us to see fine spatial detail and color but cannot function well in dim light.

Overview of Visual System

• Rods enable us to see in dim light but at the loss of color and fine spatial detail.

• Our color vision is based on the presence of 3 types of cones, each maximally sensitive to a different range of wavelengths.