VISION From Light to Sight
Objective To describe how the receptor cells for vision respond to the physical energy of light waves and are located in the retina of the eye.
Focus Questions What is the visible spectrum? What are the key structures of the eye and their functions? What are rods and cones, and how do their functions differ?
What We See Light is only one kind of electromagnetic energy. Electromagnetic energy sources differ in wavelength. Humans are only capable of seeing only a miniscule portion of electromagnetic energy.
How We See Light waves reflected from an object pass through the cornea, pupil and lens.
What happens when you look at an object? Scattered light from the object enters through the cornea. light The light is projected onto the retina. The retina sends messages to the brain through the optic nerve. nerve The brain interprets the object. brain
Cornea A clear membrane covering the visible part of the eye that helps gather and direct incoming light.
Pupil The opening In the middle of the iris that changes size to let in different amount of light.
Lens A transparent structure located behind the pupil that actively focuses, or bends, light as it enters the eye.
Some Of Us See Life Through This Lens.
Iris The colored part of the eye, which is the muscle that controls the size of the pupil.
What happens when you look at an object again? Scattered light from the object enters through the cornea. light The light is projected onto the retina. The retina sends messages to the brain through the optic nerve. nerve The brain interprets the object. brain
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.
Retina A thin light-sensitive membrane located at the back of the eye that contains the sensory receptors for vision.
Retina – back of the eye.
Rods and Cones Sensory receptors for light that are contained within the retina.
Visual receptor cells located in the retina Can detect sharp images and color Need more light than the rods Cones
Visual receptor cells located in the retina Can only detect black and white Highly sensitive to light Rods
Fovea The central focal point of the retina. The spot where vision is best (most detailed).
Fovea – composed entirely of cones
Fovea - Example There are no rods in the fovea. Images that do not fall on the fovea tend to be perceived as blurry or indistinct. For example, focus your eyes of the word For at the beginning of this sentence. In contrast to the sharpness of the letters in For, the words to the left and right will become comparatively blurry. The image of the outlying words is striking the peripheral areas of the retina, where rods are more prevalent and there are very few cones. (handout)
The Blind Spot
The Other Blind Spot
The Blind Spot - Definition An area of the retina without rods and cones, where the optic nerve leaves the eye (known as the optic disc), producing a small gap in the field of vision. If you don’t believe me just ask the spider (handout).
Processing Visual Information
Key Themes Signals from the rods and cones undergo preliminary processing in the retina before they are transmitted to the brain.
Key Questions What are the bipolar and ganglion cells and how do their functions differ? How is visual information transmitted from the retina to the brain? What properties of light correspond to color perceptions, and how is color vision explained?
Ganglion Cells Where preliminary processing occurs. Found in the retina where specialized neurons connect to bipolar cells. Made possible by a bit of brain tissue that migrates to the eye during fetal development.
Ganglion cells = purple bulbs
Bipolar Cells In the retina, they are the specialized neurons that connect the rods and cones with the ganglion cells.
Bipolar Cells = Gold bulbs
From Eye to Brain Rods and cones receive light. They signal the bipolar cells. Then the bipolar cells funnel to the ganglion cells. Finally the ganglion cells (all bundled together make up the optical nerve) process the information and send it to the brain.
The Optic Chiasm The point in the brain where the optic nerve fibers from each eye meet and partly cross over to the opposite side of the brain.
The Optic Chiasm Where the left and right optic nerves meet. Most of the optic nerve axons project straight to the thalamus. From the thalamus, the impulses are then sent straight to the visual cortex where they are decoded and interpreted.
Color Vision Our experience of color involves three properties of the light wave. HueSaturationBrightness
My Favorite Colors
Hue The property of wavelength of light known as color. Different wavelengths correspond to our subjective experiences of different colors. 700 nanometers is considered to be pure red. 400 nanometers is pure violet.
Saturation The property of color that corresponds to the purity of the lightwave.
Pure red, produced by a single wavelength, is more saturated than pink (red plus white).
Brightness The perceived intensity of a color which corresponds to the amplitude of the light wave. The higher the amplitude the greater the brightness.
Trichromatic Theory of Color Vision Sensation of color results because cones in the retina are especially sensitive to red light, green light or blue light.
Color Blindness One of several inherited forms of color deficiencies in which an individual cannot distinguish between certain colors. Want to take the test?
Test Plate
What Number Do You See?
Trace a line from one “x” to the other.
Opponent-Process Theory Color vision is the product of opposing pairs of color receptors, red/green, blue/yellow, and black/white. When one member of the pair is stimulated, the other is inhibited.
Afterimage A visual experience that occurs after the original source of stimulation is no longer present.
Afterimage – Page 95
The End
Afterimage handout
Pillow Fight
Too cool for eye protection
Not Injured
From a sparkler on the 4 th of July
PLEASE protect your eyes.