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Visual System I Spy With My Little Eye… Electromagnetic energy enters the eye in the form of light waves http://faculty.washington.edu/chudler/eyetr.html The Eye http://webvision.umh.es/webvision/sretina.html Enter Light The amount of light entering the eye through the cornea is controlled by the pupil. Light then passes to the lens and is focused through the movement of extraocular muscles. Light lands on the retina at the back of the eye http://contactlensdocs.com/ContactLensInformationCenter/BasicEyeAnatomy/tabid/122/Default.aspx The Retina Images are projected upside-down onto the retina. Photoreceptors on the retina convert the light to electrical signals that the brain can process. Neural processing then interprets the objects in their correct right-side-up position. Photoreceptors 2 Types of Photoreceptors: Rods work well in dim light. Cones work well in bright light for color vision. Fovea: Center of retina which only contains cones for acute vision Signal with several photosensitive chemicals Rhodopsin Retinal Transducin Opsin http://health.howstuffworks.com/human-body/systems/eye/eye2.htm From Retina to Brain Axons merge at the optic disk to leave the eye and travel to the brain via the optic nerve. The optic nerves cross at the optic chiasm en route to the brain http://thebrain.mcgill.ca/flash/d/d_02/d_02_cr/d_02_cr_vis/d_02_cr_vis.html Creates a “blind spot”: no photoreceptors! Thus, information from left eye goes to right side of the brain, and vice versa. Visual Processing in the Brain Lateral Geniculate Nucleus (LGN) of the Thalamus Consists of 6 layers, each of which receives input from only one eye Magnocellular: Depth vision (inner) Parvocellular: Color & detail vision (outer) Information continues to the Primary Visual Cortex (V1) http://mcb.berkeley.edu/courses/mcb64/cortex.html Information Coding Visual cells are specialized to handle a specific type of information: Light & Dark (Retina) Color (Retina) Orientation (Visual Cortex) Movement (Visual Cortex) Form or Shape (Visual Cortex) The specialized areas that process these types of information are often referred to as “columns” or “blobs”. These can be charted in a process called topographical mapping. http://faculty.washington.edu/chudler/eyecol.html Hubel & Wiesel Created topographical maps of the cat visual cortex by placing an electrode in the visual cortex By flashing light & dark patterns, lines of various orientations, etc to the cat, they were able to map which neurons responded to which stimuli. Hubel & Wiesel Cow Eyeball Dissection Video http://www.exploratorium.edu/learning_studio/cow_eye/step01.html Stroop Test Afterimages & Complementary Colors Dizzying Dots Do you see dots that appear at the corners of the squares? What happens if you stare at one dot? How about now? Afterimages Stare at the yellow + in the middle of the blue field for 1530 seconds. Now quickly stare at a blank white page. What do you see? http://faculty.washington.edu/chudler/chvision.html Can you put the fish in the fishbowl? Stare at the yellow stripe in the middle of the fish for 15-30 seconds. Move your gaze to the fishbowl—it may help to blink once or twice. http://faculty.washington.edu/chudler/chvision.html Seeing in the Dark Experiment Photoreceptors & Color Recall Photoreceptors in the Retina: Rods: Dim light Cones: Bright light, color & detail vision So how do Cones process Color? Both Rods and Cones use a pigment molecule Opsin (a large protein) Chromophore (a form of Vitamin A that couples to opsin) When light hits the chromophore, it changes shape This change activates opsin Ultimately, an electrical signal is transmitted http://www.pdn.cam.ac.uk/staff/harris/cell.jpg 3 Cones for Color http://www.bio.miami.edu/dana/dox/photosynthesis.html http://faculty.washington.edu/chudler/eyecol.html Combined response patterns of these 3 cone types are responsible for our perception of color. Further Color Processing Additional specialized retinal cells called ganglion cells enhance the cone response patterns to adjust for differences in light levels. Information then continues through the LGN to V1. http://faculty.washington.edu/chudler/eyecol.html Something to Ponder… We can see yellow-green. We can see blue-green. Why can’t we see red-green or blueyellow? Opponent Process Theory Color information is sorted into 3 different channels from the retina to V1. Red-Green: Yellow-Blue Intensity increased firing for red decreased firing for green Opponent Processes Because information about red & green is traveling in the same pathway through opponent firing patterns, it is physiologically impossible to signal for both colors at the same time. But, yellow and green are on different channels, allowing them to be processed simultaneously to express yellow-green. http://faculty.washington.edu/chudler/eyecol.html Optical Illusions Muller-Lyer Illusion Which line is longer? http://faculty.washington.edu/chudler/chvision.html I See, You See…What? What do you see? Does your neighbor see the same thing as you? http://faculty.washington.edu/chudler/chvision.html Poggendorf Illusion Is the line behind the rectangles connected? Or do you see 3 separate lines? http://epsych.msstate.edu/descriptive/Vision/DepthValley/Poggendorf/pog02.html Titchner Illusion Which center circle is bigger? http://faculty.washington.edu/chudler/chvision.html Filling In Do you see a shape—a cube, a triangle, a square? http://faculty.washington.edu/chudler/chvision.html What do you see? http://faculty.washington.edu/chudler/chvision.html Is it moving? Try staring at the center circle… http://www.artlex.com/ArtLex/o/opticalillusion.html Visual Techniques & Brain Tricks Perceiving Depth If the retina is flat, how do we see in 3-D? Brain uses 3 types of cues: Ocular Motor Cues Monocular Cues Binocular Cues Fusing of two slightly different images from your retinas Ocular Motor Cues Convergence Divergence Eyes move inward as an object moves nearer Also a Binocular Cue Eyes move outward as object moves farther away Accommodation Lens & cornea adjust shape to focus an object Closer image: Lens thickens Monocular Cues Kinetic Depth Effect Timing of changes in lights appears as movement Motion Parallax In a train, the movement of other trains distorts your sense of motion Immobile stimuli appear to be moving Pictorial Cues Occlusion Relative Height Shadowing & Shading Relative Size Familiar Size Atmospheric Perspective Linear Perspective Texture Gradient Kinetic Depth Effect Motion Parallax Occlusion If one object is occluding a second object, it is assumed that the occluding object is “closer” Imaginary Landscape by Bernardo Bellotto http://www.topartprint.com/artists/ Bernardo_Bellotto/art_prints_posters /7024/Imaginary_Landscape.php Relative Height Objects located higher on the y-axis are seen as higher on visual field & thus further away The Gilded Cage by Evelyn Pickering De Morgan http://www.1st-art-gallery.com/Evelyn-Pickering -De-Morgan/Evelyn-Pickering-De-Morgan-oil -paintings-2.html Shadowing and Shading Natural assumption is that there must be a light source and a third dimension in order to cast a shadow Entering the Studio by Raphael Soyer http://www.flickr.com/photos/artimageslibr ary/5378298822/ Relative Size Object that is farther away will take up less retinal space, and thus appear smaller. Which building is closer to you? South Carolina Landscape by George Biddle http://www.columbiamuseum.org/ exhibitions/artistseye/sneakpeek.php Familiar Size If an object whose size is familiar to you is re-sized, you assume the object has moved. http://johndollin.blogspot.com/2010/08/why-do-objects-still-appear-in-3d-with.html Atmospheric Perspective When outside, things that are far away have an increased amount of atmosphere between us and them, making them appear blurred and bluish. Landscape near Bologna by Frans Koppelaar http://en.wikipedia.org/wiki/File:Frans_Koppelaar _-_Landscape_near_Bologna.jpg Linear Perspective “Parallel” lines are drawn as converging at a point as they go off into the distance (farther away = closer together) View of the Molo by Antonio Canaletto http://www.lyons.co.uk/Canaletto/bightm/molo.htm Texture Gradient Close objects have distinct texture; far away objects appear smoother Paris Street: A Rainy Day by Gustave Caillebotte http://psych.hanover.edu/krantz /art/texture.html Binocular Vision Experiment Visual Review A & M United Methodist Church Stained Glass Windows Window A Window B Window C Window D Window E Window F Window G Window H Window H Window I Window J Window K