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?

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

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 White –Black/White Only one side is “on” at a timeOnly one side is “on” at a time

Opponent Process Theory ON”“OFF” red green green red blue yellow yellow blue white black white white white 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.