Visual System: Sight

The Stimulus Input: Light Energy Electromagnetic spectrum--Energy that includes radio waves, X-rays, microwaves, and visible light Visible spectrum--Energy in the form of light visible to the human eye Light can be described as a particle and a wave that varies in hue (color) and intensity (amplitude) Humans can detect about 5 million different hues. Shorter Wavelengths Longer Wavelengths

The Stimulus Input: Light Energy Hue is the color of light as determined by the wavelength of the light energy Remember ROY G. BIV --> red, orange, yellow, green, blue, indigo and violet The eye can detect 7 million separate hues The brightness of light as determined by the height (or amplitude) of the wave The taller the wave, the brighter the color

Afterimage Stare at the red dot in the green square and count to forty. Then stare at the white square and tell me what you see. You should see a greenish/blue dot in a reddish/purple background. That is called an “afterimage”.

The Visual Pathway Cool = Cornea People = Pupil Like = Lens Fish = Fovea (Rods & Cones) Because = Bipolar Cells Gangsters = Ganglion Cells Never = Optic Nerve Cheat = Optic Chiasm The = Thalamus Officers = Occipital Lobe

Primary Visual Pathway: Thalamus processes info about form, color, brightness & depth Secondary Visual Pathway: Midbrain processes info about the location of an object

Visual Information Processing Vision is the most complex, best developed and most important sense for humans and other highly mobile creatures. Think of the eye as the brain’s camera. It gathers light, focuses it, converts it to a neural signal, and sends these signals on for further processing. The eye transduces the characteristics of light into neural signals that the brain can process. This transduction happens in the retina, the light sensitive layer of cells at the back of the eye. In the visual cortex, the brain begins working by transforming neural impulses into visual sensations of color, motion, form, and depth. This process is called parallel processing -- the simultaneous processing of several aspects of a problem

Visual Information Processing Different parts of the visual cortex are used to identify different images Afterimages – Sensations that linger after the stimulus is removed Transduction -- The purpose of the visual system is transforming light energy into an electro-chemical neural response (i.e. action potential and synaptic transmission) represented in the characteristics of objects in our environment.

Color Vision Despite the way the world appears, color does not exist outside the brain, because color is a perception the brain creates based on the wavelength of light striking our eyes. Color is created when the wavelength in a beam of light is recorded by the photoreceptors in the form of neural impulses. It is then sent to specific regions of the brain for processing. Color brightness depends on the intensity of the light waves. Color blindness is a visual disorder that prevents an individual from discriminating certain colors. Opponent-process theory --Sensory receptors come in pairs (red-green/ blue-yellow/black-white) --If one color is stimulated, the other color is inhibited --Explains afterimages Young-Helmholtz trichromatic (three color) theory -- There are three cones (red, green, blue) that form millions of combinations of colors -- Monochromatic and dichromatic vision

Ishihara Test (Color Blindness) People who suffer red-green blindness have trouble perceiving the number within the design They lack functioning red- or green- sensitive cones, or sometimes both

Sensory Deprivation And Restored Vision Critical period for normal sensory and perceptual development Subjects blind from birth whose sight is later restored have trouble making perceptions Facial recognition, perceptual constancy is lacking

Visual Problems Farsightedness--misshapen eye focuses light rays from near objects past the retina. See far! Nearsightedness--misshapen eye focuses light rays from a distant object in front of the retina. See near! Misshapen eye focuses light rays from a distant object in front of the retina. Can see near but not far. Presbyopia--form of farsightedness caused when lens becomes brittle and inflexible Astigmatism--uneven curvature of the cornea causes multiple focus points/images on the retina, resulting in blurry vision

How Do We Correct Vision? Glasses, contact lenses, or LASIK surgery reshape the cornea (which is also involved in bending light to provide focus).

Parts Of The Eye

Parts Of The Eye Pupil --> adjustable opening in the center of the eye that controls amount of light (bright conditions, iris expands, pupil smaller; dark conditions, iris contracts, pupil larger) Iris --> ring of muscle that forms the colored portion of the eye around the pupil and controls the size of the pupil opening Cornea --> near the center of the retina that begins to focus the light by bending it; protects the eye Lens --> transparent structure behind pupil that changes shape through accommodation to focus images on the retina Retina --> light-sensitive inner surface of the eye, containing receptor rods and cones plus layers of neurons that begin the processing of visual information Optic Nerve --> carries neural impulses from the eye to the brain Blind Spot --> point at which the optic nerve leaves the eye, creating a “blind spot” because there are no receptor cells located there Fovea --> central point in the retina; highest concentration of rods and cones; area of sharpest vision (visual acuity)

Retina’s Reaction To Light Rods are visual receptors that detect black and white and respond to less light. Cones are visual receptors that detect sharp images and color and respond to more light. Ganglion cells are neurons that pass information from the bipolar cells; their axons form the optic nerve.

Retina’s Reaction To Light Photoreceptors are light-sensitive cells (neurons) in the retina that convert light energy into neural energy. That is, light energy strikes the rods and cones to produce chemical changes that general neural signals. Optic Nerve: The bundle of neurons that carries the visual information from the retina to the thalamus to the occipital lobe of the brain. --Where the stimulus, once changed into a neural impulse, gets passed onto the brain. Blind Spot: The point where the optic nerve exits the eye and where there are no photo-receptors. --Any stimulus that falls on this area cannot be seen. --We do not notice it because each eye compensates for the other and your brain “fills in” for the missing information. (top-down process & Gestalt)

Retina’s Reaction To Light Cover your right eye and stare at the can as you move closer to the screen. Notice the spider disappear in your peripheral vision?

The Cosmic Flower Does this picture seem to pulsate? Because the lens of your eye is not perfectly round some parts of what you look at are blurry. Your eyes make micro movements to try to put this entire picture into focus, which creates the pulsation.

The Hermann Grid Are there gray dots between the squares? Rods in the periphery are responsible. When you look at an area directly there is no dot because you are using your cones but the periphery has dots because the rods are trying to do two things, show you there is a dark area and a light area.