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CIS 140 Week 2

by: Alexis Mitchnick

CIS 140 Week 2 CIS 140

Alexis Mitchnick

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About this Document

Perception, color vision
Intro to Cognitive Science
David Hoyt Brainard, Lyle H Ungar
Class Notes
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This 5 page Class Notes was uploaded by Alexis Mitchnick on Sunday September 11, 2016. The Class Notes belongs to CIS 140 at University of Pennsylvania taught by David Hoyt Brainard, Lyle H Ungar in Fall 2016. Since its upload, it has received 37 views. For similar materials see Intro to Cognitive Science in Cognitive Science at University of Pennsylvania.


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Date Created: 09/11/16
Tuesday, September 6, 2016 CIS 140 Week 2 Perception - Human Eye: Aqueous (fluid in between cornea and lens): purpose - can’t have blood in between • because it would block the light; therefore, must have some other fluid to bring oxygen/nutrients to cornea • light is focused by the lens and the cornea onto the retina • pupil is hole where the light goes through (iris can open and close to adjust for more or less light) • send signals through optic nerve - Rods and cones (two different photoreceptors) transduce light - Rods and cones found in retina — light must go through cells before hitting rods and cones (seems backwards) - Vision has evolved to provide accurate representation of our environment: How good is the eye as optical instrument? - Basic Facts of vision: - Strong sensitivity: receptors can respond to single photon - operates over full (~10 ^11) variation of light intensity - high spacial resolution (can see 60 black and white stripes on thumb at arm’s length) - fairly high temporal resolution - Why isn’t vision understood by understanding the eye? — need to understand connection to brain, how the brain perceives the data (perception is done in the brain not the eye) - Ex. Kanisza Triangle, visual illusion (completion of the white triangle is done in the brain) - Ex. Two X’s on rectangles, look like different colors, but are the same (simultaneous color contrast) 1 Tuesday, September 6, 2016 - Ex. Rabbit vs. Duck (ambiguous figure) - * Perception is not only what appears on retina, it is a construction of the brain/mind - Why don’t we just see the retinal image? - logical difficulty: with only retinal image, who is looking at the image? - practical difficulty: retinal image provides only ambiguous information about physical arrangement of surroundings - Additional Ex. One “monster” looks bigger than the other, why? — perspective of vanishing point (with the walls) & strategic placement of monsters, scaling off of bricks — falls under representational Marr level: how we represent size/scaling in our brain, procedural/algorithmic theory (instead of actual neurons [hardware]) - Size and distance of objects are confounded - size of objects in world = distal size - size of image of object on retina = proximal size - As object gets further away, visual angle decreases, proximal size decreases, but distal size stays the same - Monster example from before: have same distal and proximal size - Formula to figure out visual angle: theta = 2arctan(s/2d), where s = size of object, d = distance away - Context can resolve ambiguity: context provides information about distance, influences how retinal size is mapped to perceive size… brain works to see objects at consistent size - Size, distance, shape, and pose: Turning Tables example: two tables are the same size, but they look different because they’re turned different ways, affecting our perception of them - Holway and Boring’s experiment (1941) measures size constancy: adjusted size of comparison circle to match size of test circle; kept proximal size of test circle constant - Results: for perfect size constancy — Distance from observer vs. Size of comparison circle is a direct relationship - As they reduced the depth cues, the size constancy got worse and worse, and the relationship line became more and more flat (closer to zero) 2 Tuesday, September 6, 2016 - Ames Window: rotates fully around, but appears to oscillate/stretch, with stick added: stick appears to be rotating through the window - A non-rigid oscillating rectangle produces same retinal image as rigid rotating trapezoid - Ambiguity about motion: ball on surface, looks like it’s floating the second time because of placement of the shadow Studying Color - Why is color vision important? — seeing objects of interest, identity or state of some object (green vs red apple, underripe vs ripe banana), signaling (ex. coloration signals poisonous frogs, stop lights) - Path of color vision: Illuminant (lights in a room, sun outside) —> Object (any color object, reflects color) —> Incident Light —> Observer (then goes through transduction, then perception) - Transduction: connection btwn behavior and biology - Transduction takes place in rods and cones: - rods: operate in dim light, do not provide color (just black and white) - cones: operate in brighter light, use them to see color - Light determined by its wavelength and intensity - Any light can be thought of as a mixture or combination of photons at different wavelengths - Lights with different spectra (different lines on a Wavelength vs. Power graph) cannot necessarily be told apart by normal human color vision - Color reproduction works by mixing light from 3 primary light sources (red, green, and blue) - Color Matching Experiment: create spectra with a test light on one side, compare this to a RGB primary mixture subject makes on the other side… try to match the RGB mixture to the test light… the final “matched” RGB mixture creates a different spectra than the original light, but human with normal color vision can’t tell the difference! 3 Tuesday, September 6, 2016 - “Behavioral Trichromacy” — for humans with normal color vision, a test light can be matched by a mixture of three primaries; if there are only two, some lights can’t be matched; if four are available, matches won’t be unique - Implication: our perception of color is not equivalent with physical correlate (we can approximate distance, etc. but we can’t approximate “blueness” and draw a spectra solely based on our perception) - Cone Spectral Sensitivities: 3 different types (L, M, S) — have different ‘quantal efficiency’ (sensitivity) and peak at different wavelengths - S cones: lower peak at shorter wavelength - M cones: medium peak at middle wavelength - L cones: highest peak at longest wavelength - 3 and only 3 classes of cones in retina, each with different spectral sensitivity - Cone response is number of isomerizations - Hypothesis to understand behavior: Two lights match when they produce the same triplet of cone responses - Ex. from color matching experiment: both different spectra cause same ‘isomerization rate’ in each type of cone; therefore, produce the same cone response, so humans see them as the same color - To find isomerization rate: sum of (# of photons in light at each wavelength) x (corresponding sensitivity for the cone type) for each wavelength - Ex. 200 quanta at 460 nm, 50 at 540 nm, and 100 at 560 nm - —> (200)(0.06) + (50)(0.88) + (100)(0.98) = 154 - Hypothesis links biology and behavior* - Color Blindness: - A typical RG color blind person is either missing the M or the L cones (They are dichromatic) - Color informs us about object properties - Retinal image is ambiguous with object reflectance (ex. Uluru rock appears to change color depending on the light hitting it throughout the day) 4 Tuesday, September 6, 2016 - C(lamba) = E(lamba)S(lamba)… by changing function of light source or function of fractional light coming off surface, we can change the color we see: - C’(lamba) = E’(lamba)S(lamba) - Color Constancy: similar to size constancy, keeps colors consistent regardless of the lighting (ex. Demo — white looked blue with small blue square over it, but stayed white when blue covered entire slide) - visual system takes illuminant into account when transforming cone responses of light reflected from an object to perceived color - the dress: depending on our personal color constancy (warm illumination on a blue/black dress & cool illumination on a white/gold dress would produce same color) 5


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