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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. - PowerPoint PPT Presentation
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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. 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
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
Do you see dots that appear at the corners of the squares?
What happens if you stare at one dot?
Dizzying Dots
How about now?
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
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.
Can you put the fish in the fishbowl?
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
Combined response patterns of these 3 cone types are responsible for our perception of color.
http://www.bio.miami.edu/dana/dox/photosynthesis.htmlhttp://faculty.washington.edu/chudler/eyecol.html
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 blue-yellow?
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
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 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
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/artimageslibrary/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 byAntonio 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