vision – our most dominant sense. vision purpose of the visual system –transform light energy...
TRANSCRIPT
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 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.
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
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
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.
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.
Retina• Light-sensitive surface with cells that
convert light energy to neural impulses
• At the back of the eyeball
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)
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
Optic Nerve
• The nerve that carries visual information from the eye to the occipital lobes of the brain
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.
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
• 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
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.