Vision

Presented By: Jenna, Jeff, and Olivia

Essential QuestionsWhat parts of the visual system

create and process images?What are some theories related

to color vision and what do they state?

Name some common visual disorders and what causes them.

My head is

spinning just from reading these!

Structure of the Eye

StructureIris: Colored part of the eye; contracts and

expands retina in response to light and protects it

Pupil: Hole in the center of the iris where light passes through

Cornea: Clear, dome-shaped surface that covers eye; first (and most powerful) lens

Lens: keeps images in focus (adjustable for close work/distance); inverts images and sends to brain

Retina: film over the eye; converts light into electrical signals to send to the brain

Blind Spot: location where the optic nerve exits the eye; anything that falls in this area is not seen

Bipolar cells: cells in the retina that connect receptors to ganglion cells

Ganglion cells: axons of these cells form the optic nerve; located in the retina

Fovea: area of the retina that controls central vision

Optic Chiasm: where the optic nerve fibers from each eye meet

Optic Nerve: group of neurons that transfer visual information from the retina to the brain

Visual Cortex: area in the brain that processes vision, and combines it in order to create an image

Forebrain: receives and processes visual information (contains the visual cortex)

Occipital Lobe: located in the forebrain; contains the visual cortex

Thalamus: processes visions; information from the optic nerves is relayed here, and then to the cerebral cortex

Structures that help with vision, but aren’t totally located in the eye

PhotoreceptorsLight sensitive cells (neurons) in the retina

that convert light energy to neural impulses◦Furthest that light travels in the visual system

Two types:◦Rods: most sensitive to light, but not to color;

120 million◦Cones: provide eye’s color sensitivity; 6-7

millionVisual Phototransduction: process through

which photoreceptors convert light into electrical signals in the rods and cones of the retina

Visual Processing1. Images enter through lens and are inverted

◦info on viewer’s left is taken in on right side of the eye

2. Eye sends info to optic chiasmus ◦info on left side sent to be right cerebral hemisphere, processed by right occipital lobe

3. Neurons of cerebral cortex pass their separate analyses of the image into parietal lobe and temporal lobe◦assembled and integrated w/ emotions, memories into meaningfulness

Eye DisordersNearsighted: inability to see objects

that are far away; eyes have difficulty refracting light

Farsighted: inability to see objects that are nearby; eye ball is either too short or lens cannot become round

Blindness: inability to seeCataracts: clouding of the lensGlaucoma: increased pressure within

the eye

Some Functions of the EyeAccommodation: when

eye muscles change to focus on something close or far away◦ Normally in lens, pupil

size, and movement of eyes in opposite directions to keep binocular vision

Dark Adaptation: chemical and physical changes undergone in eye to make it possible to see in relative darkness ◦ Includes dilating pupil, and

increasing activity in rods

More Functions of the EyeParallel Processing:

ability of brain to simultaneously process incoming messages of different quality (multitasking for your eyes)◦ Important for vision

because brain first divides vision into color, motion, shape, depth then synthesizes it

Acuity: clearness of vision◦ Usually depends on

focused retina, and how well brain interprets images

What is color?Color/ Hue: sensation created in the brain

from wavelengths of light and visual information gathered from the eyes

Color is a sensation, so it does not exist outside of the brain

Only exists in the individual mindCreated when light waves are translated

by photoreceptors into neural impulses, and are transferred to specialized areas of the brain to be processed

Electromagnetic SpectrumThe range of electromagnetic

energyIncludes:

◦Radio waves◦Microwaves◦Infrared◦Visible Light◦Ultraviolet◦X-Rays◦Gamma Rays

More information about the Electromagnetic Spectrum

Classifications in the electromagnetic spectrum depend on the wavelength of a wave as well as its amplitude◦Wavelength: length of a light wave ◦Amplitude: strength/ intensity of something

(usually a stimulus)◦Peak/ Crest: Top of the wave◦Trough: bottom of the wave◦Frequency: Speed of the wave

The only section of the electromagnetic spectrum that is visible to us is the small, and properly named, section called “Visible Light”

Visible SpectrumSmall section of the Electromagnetic Spectrum

that our eyes are sensitive toLight waves with different wavelengths within the

Visible Spectrum, create different colorsWaves come from Radiant and/or Reflected Light

◦ Radiant light: visible energy emitted by something◦ Reflected light: energy reflected by objects

The eye extracts information from the wavelength of a light ray in order for the brain to process color

Visible SpectrumVisual experiences of color, form,

position, and depth, are all based on processing one stream of sensory information in multiple parts of the visual cortex

Since color only exists within the brain, perception of a color can vary in saturation and brightness◦Saturation: purity of a color◦Brightness: psychological sensation that is

based on a light wave’s intensity

Theories on viewing colorsTrichromatic theory: idea that colors

are sensed by three different types of cones sensitive to light in red, blue, and green wavelengths◦explains earliest stages of color

sensation◦also called Young-Helmholtz theory

Another theoryOpponent-process theory: idea that

cells in visual system process colors in complementary pairs, such as red and green or yellow and blue◦explains color sensation from bipolar

cells onward in visual system

AfterimagesSensations that linger after

stimulus is removed ◦negative afterimages appear in

reversed colors

Monocular CuesDepth perceived in each

individual eye Types: relative size, interposition,

relative clarity, texture gradient, relative height, relative motion, light, shadow

Binocular CuesDepth perceived using both eyes Types: Retinal disparity,

Convergence◦Retinal disparity: the difference

between the visual images that each eye perceives because of the different angles viewed

◦Convergence: coordinated movement of eyes inward to focus on objects close to us

Optical Illusions (Fun ways to mess with your eyes!)

THE END