Changing Light Waves to Neural Impulses Vision Changing Light Waves to Neural Impulses

Parts of the Visual System Cornea protective layer of the eye Iris controls the amount of light allowed in the eye Pupil - opening in the eye Lens clear, fibrous muscle that focuses light on the back of the eye accommodation – changing of the shape of the lens Focuses light on the back of the eye

Parts of the Visual System Retina black covering on the back of the eye that contains photoreceptors rods – photoreceptors that react to light and dark stimulus cones – photoreceptors that react to color and provide visual acuity Fovea depression in the center of the visual field contains only cones best detailed vision  Optic Nerve – ganglion cell axons bundled together optic disc – point where optic nerve leaves the eye blind spot – no photoreceptors carries visual info. to the brain

Visual Acuity Acuity – sharpness of vision Good vision vs. poor vision Normal vision = image comes into focus on the retina near-sighted = see objects up close OK, but far objects poorly Objects come into focus in front of retina far-sighted = objects far away OK, but close objects poor Objects focus behind retina

Parts of the Eye Review

Light Input/Stimulus Wavelength (frequency) Intensity (brightness) determines the color (hue) red = long wavelength (low frequency) blue = short wavelength (high frequency) Intensity (brightness) determined by the amplitude (height) of the wave high amplitude = bright colors low amplitude = dull colors

Light energy begins as white light Light Stimulus Light energy begins as white light

Psychophysics Frequency (length) determines color (hue) Amplitude (height) determines intensity (brightness)

Colors are reflected White light strikes an object The color we label the object as is REFLECTED to the eye All other colors are absorbed at the object.

Transduction in the Retina light waves strike the retina & cause chemical reactions in rods & cones causes proteins to breakdown based on wavelength and intensity  rods & cones activate bipolar cells rods are grouped to 1 bipolar cell some cones attach to their own bipolar cell bipolar cells activate ganglion cells ganglion cells bundle to form optic nerve send millions of messages @ the same time

Visual Transduction Review

Visual Information Processing Optic nerves connect to the thalamus in the middle of the brain, and the thalamus connects to the visual cortex. OBJECTIVE 7| Discuss the different levels of processing that occur as information travels from the retina to the brain’s cortex.

Feature Detection Nerve cells in the visual cortex respond to specific features, such as edges, angles, and movement. Ross Kinnaird/ Allsport/ Getty Images

Shape Detection Specific combinations of temporal lobe activity occur as people look at shoes, faces, chairs and houses. Ishai, Ungerleider, Martin and Haxby/ NIMH

Perception in Brain Our perceptions are a combination of sensory (bottom-up) and cognitive (top-down) processes.

Visual Information Processing Processing of several aspects of the stimulus simultaneously is called parallel processing. The brain divides a visual scene into subdivisions such as color, depth, form and movement etc. OBJECTIVE 8| Discuss parallel processing and discuss its role in visual processing.

Theories of Color Vision Trichromatic theory: Based on behavioral experiments, Helmholtz suggested that the retina should contain three receptors that are sensitive to red, blue and green colors. Standard stimulus OBJECTIVE 9| Explain how the Young-Helmholtz and opponent-process theories help us understand color vision. Comparison stimulus Max Medium Low Blue Green Red

Subtraction of Colors If three primary colors (pigments) are mixed, subtraction of all wavelengths occurs and the color black is the result.

Addition of Colors If three primary colors (lights) are mixed, the wavelengths are added and the color white is the result. Fritz Goro, LIFE magazine, © 1971 Time Warner, Inc.

Photoreceptors Blue Cones Green Cones Red Cones MacNichol, Wald and Brown (1967) measured directly the absorption spectra of visual pigments of single cones obtained from the retinas of humans. Short wave Medium wave Long wave

Color Blindness Genetic disorder in which people are blind to green or red colors. This supports the Trichromatic theory. Ishihara Test

Opponent Colors Gaze at the middle of the flag for about 30 Seconds. When it disappears, stare at the dot and report whether or not you see Britain's flag.

Opponent Process Theory Hering proposed that we process four primary colors combined in pairs of red-green, blue- yellow, and black-white. Cones Retinal Ganglion Cells

Color Constancy Color of an object remains the same under different illuminations. However, when context changes the color of an object may look different. OBJECTIVE 10| Explain the importance of color constancy. R. Beau Lotto at University College, London