Vision – our most dominant sense

VisionPurpose 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

• Light can be described as both a particle and a wave.

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

• Visible light has wavelengths from about 400nm to 700nm.

• Wavelength of light is related to its perceived color.

The Visual System: The Structure of the

Visual System

Module 9: Sensation

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

• A ring of muscle tissue that forms the colored portion of the eye; creates a hole in the center of the iris (pupil)

• Regulates the size of the pupil by changing its size--allowing more or less light to enter the eye

Parts of the Eye - Iris

Pupil

• The adjustable opening in the center of the eye that controls the amount of light entering the eye (surrounded by the iris)

• In bright conditions the iris expands, making the pupil smaller.

• In dark conditions the iris contracts, making the pupil larger.

Parts of the Eye - Pupil

Lens• A transparent structure behind the pupil;

focuses the image on the back of the eye (retina)

• Muscles that change the thickness of the lens change how the light is bent thereby focusing the image

• Glasses or contacts correct problems in the lens’ ability to focus.

Parts of the Eye - Lens

Nearsighted - Myopia

Farsighted - Hyperopia

Retina• Light-sensitive surface with cells that

convert light energy to neural impulses

• At the back of the eyeball

Parts of the Eye - Retina

Receptor Cells

• These cells are present in every sensory system to change (transduce) some other form of energy into neural impulses.

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

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

• Visual receptor cells located in the retina

• Can only detect black and white

• Respond to less light than do cones

Rods

• Visual receptor cells 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’s eyes look straight ahead

• C will move various colored pieces of paper in A’s peripheral vision

• A will guess the color – note if the person is guessing correctly they are doing it wrong

You will need to write up the results

• You will need to write the following

1. Procedures – what did you do

2. Results – correct guess versus bad

3. Your conclusion

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

• 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

Fovea

• The central focal point of the retina

• The spot where vision is best (most detailed)

Parts of the Eye - Fovea

Visual Processing in the Retina

Optic Nerve

• The nerve that carries visual information from the eye to the occipital lobes of the brain

Parts of the Eye – Optic Nerve

Cookie Monster Experiment

• What happened? Why

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, so there is a small blind spot in vision.

Parts of the Eye – Blind Spot

The Visual System: Color Vision

Module 9: Sensation

Color Vision

• Our visual system interprets differences in the wavelength of light as color.

• Rods are color blind, but with the cones we can see different colors.

• This difference occurs because we have only one type of rod but three types of cones.

Color Vision

• There are two theories of color vision:

–Trichromatic Theory

–Opponent-Process Theory

Trichromatic (three-color) Theory

• Theory of color vision that says cones are “tuned” to be sensitive to red, green and blue light

• All the colors we see are a combination of these three colors.

• Similar to the design of a color TV

How do we see color?• Trichromatic (three color) Theory

–three different retinal color receptors•Red green blue

Can you see what is in the middle?

Red-Green Color Blindness

• Opponent-Process Theory- Vision from opposing pairs of color receptors- only one “side” ON at a time

Opponent-Process Theory

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

• Only one side is “on” at a timeOnly one side is “on” at a time

Opponent Process Theory

ON” “OFF”red greengreen red blue yellow yellow blue black whitewhitewhite white black

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

• In inherited and found more in males

Afterimage Effect

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

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.