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Visual System

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.

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Visual System

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  1. Visual System

  2. I Spy With My Little Eye… • Electromagnetic energy enters the eye in the form of light waves http://faculty.washington.edu/chudler/eyetr.html

  3. The Eye http://webvision.umh.es/webvision/sretina.html

  4. 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

  5. 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.

  6. 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

  7. From Retina to Brain • Axons merge at the optic disk to leave the eye and travel to the brain via the optic nerve. • Creates a “blind spot”: no photoreceptors! • The optic nerves cross at the optic chiasm en route to the brain • Thus, information from left eye goes to right side of the brain, and vice versa. http://thebrain.mcgill.ca/flash/d/d_02/d_02_cr/d_02_cr_vis/d_02_cr_vis.html

  8. 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

  9. 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

  10. 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.

  11. Hubel & Wiesel

  12. Cow Eyeball Dissection Video http://www.exploratorium.edu/learning_studio/cow_eye/step01.html

  13. Stroop Test

  14. Afterimages & Complementary Colors

  15. Dizzying Dots • Do you see dots that appear at the corners of the squares? • What happens if you stare at one dot?

  16. How about now?

  17. Afterimages • Stare at the yellow + in the middle of the blue field for 15-30 seconds. • Now quickly stare at a blank white page. • What do you see? http://faculty.washington.edu/chudler/chvision.html

  18. 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

  19. Seeing in the Dark Experiment

  20. 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

  21. 3 Cones for Color • Combined response patterns of these 3 cone types are responsible for our perception of color. http://www.bio.miami.edu/dana/dox/photosynthesis.html http://faculty.washington.edu/chudler/eyecol.html

  22. 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

  23. Something to Ponder… • We can see yellow-green. We can see blue-green. • Why can’t we see red-green or blue-yellow? Opponent Process Theory

  24. Opponent Process Theory • Color information is sorted into 3 different channels from the retina to V1. • Red-Green: increased firing for red decreased firing for green • Yellow-Blue • Intensity

  25. 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

  26. Optical Illusions

  27. Muller-Lyer Illusion Which line is longer? http://faculty.washington.edu/chudler/chvision.html

  28. 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

  29. 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

  30. Titchner Illusion Which center circle is bigger? http://faculty.washington.edu/chudler/chvision.html

  31. Filling In Do you see a shape—a cube, a triangle, a square? http://faculty.washington.edu/chudler/chvision.html

  32. What do you see? http://faculty.washington.edu/chudler/chvision.html

  33. Is it moving? Try staring at the center circle… http://www.artlex.com/ArtLex/o/opticalillusion.html

  34. Visual Techniques & Brain Tricks

  35. 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

  36. Ocular Motor Cues • Convergence • Eyes move inward as an object moves nearer • Also a Binocular Cue • Divergence • Eyes move outward as object moves farther away • Accommodation • Lens & cornea adjust shape to focus an object • Closer image: Lens thickens

  37. 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 Monocular Cues

  38. Kinetic Depth Effect

  39. Motion Parallax

  40. 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

  41. Relative Height • Objects located higher on the y-axis are seen as higher on visual field & thus further awayThe Gilded Cageby Evelyn Pickering De Morganhttp://www.1st-art-gallery.com/Evelyn-Pickering-De-Morgan/Evelyn-Pickering-De-Morgan-oil-paintings-2.html

  42. Shadowing and Shading • Natural assumption is that there must be a light source and a third dimension in order to cast a shadowEntering the Studio by Raphael Soyer http://www.flickr.com/photos/artimageslibrary/5378298822/

  43. 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 Biddlehttp://www.columbiamuseum.org/exhibitions/artistseye/sneakpeek.php

  44. 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

  45. 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 Bolognaby Frans Koppelaar http://en.wikipedia.org/wiki/File:Frans_Koppelaar_-_Landscape_near_Bologna.jpg

  46. 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 byAntonio Canalettohttp://www.lyons.co.uk/Canaletto/bightm/molo.htm

  47. Texture Gradient • Close objects have distinct texture; far away objects appear smootherParis Street: A Rainy Day by Gustave Caillebotte http://psych.hanover.edu/krantz/art/texture.html

  48. Binocular Vision Experiment

  49. Visual Review

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