Ch. 5-6 Flashcards
three steps to perceiving color
1) detection –> detecting wavelengths; photoreceptors convert light into signals in the nervous system
2) discrimination –> neurons compare inputs from different kinds of photoreceptors
3) appearance –> assign perceived colors to lights and surfaces in the world
three types of cone receptors
S-cone, M-cone, L-cone
S-cone
a cone that is preferentially sensitive to short wavelengths; blue cone
M-cone
a cone that is preferentially sensitive to middle wavelengths; green cone
L-cone
a cone that is preferentially sensitive to long wavelengths; red cone
spectral sensitivity
the sensitivity of a cell or a device to different wavelengths on the electromagnetic spectrum
photopic
referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors, that is, drive them to their maximum responses
scotopic
referring to light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors
principle of univariance
the fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor; one photoreceptor type cannot make color discriminations based on wavelength
trichromacy/trichromatic theory of color vision
the theory that the color of any light is defined in our visual system by the relationships of three numbers— the outputs of the three cones
metamers
different mixtures of wavelengths that look identical, or more generally, any pair of stimuli that are perceived as identical in spite of physical diffferences
example of metamers
single wavelength that produces equal M- and L-cone activity will look yellow and the correct mixture of longer- and shorter-wavelength lights will also look yellow
additive color mixture
a mixture of lights; if light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together
subtractive color mixture
a mixture of pigments; if pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B; only the remainder will contribute to the perception of color
to tell the difference between lights…
the nervous system will look at differences in the activities of the three cone types
convert the three cone signals into three new signals
- L-M
- (L+M)-S
- L+M
cone-opponent cell
a cell type— found in the retinal, LGN, and visual cortex— that, in effect, subtracts one type of cone input from another
equiliminant
referring to stimuli that vary in color but not in luminance
mesopic
referring to the middle range of light intensities
color space
the three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors
because we have exactly three different types of cone photoreceptors…
the light reaching any part of the retina will be translated into three responses, one for each local population of cones
nonspectral hues
hues that can arise only from mixtures of wavelengths
opponent color theory
the theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white
hue cancellation
cancelling a color through their opposite; ex. finding out how much blue or yellow we need to cancel the yellow or blue in a light of that color
unique hue
in the context of opponent color theory, any of four colors that can be described with only a single color term: red, yellow, green, blue; other colors can also be described as compounds
achromatopsia
an inability to perceive colors that is caused by damage to the central nervous system
transformations that produce perceived color take place in…
visual cortex
basic color terms
color words that are single words are used with high frequency and have
cultural relativism
the idea that basic perceptual experiences may be determined in part by the cultural environment
number of basic color terms in English
11
tetrachromatic
referring to the rare situation where the color of any light is defined by the relationships of four numbers— the outputs of those four receptor types
determining factors of color blindness
- type of cone affected
- type of defect –> either the photopigment for that cone type is anomalous (different from the norm) or the cone type is missing altogether
most common defects in color blindness
M- and L-cone defects
deuteranope
an individual who suffers from color blindness that is due to the absence of M-cones
protanope
an individual who suffers from color blindness that is due to the absence of L-cones
tritanope
an individual who suffers from color blindness that is due to the absence of S-cones
color anomalous
most color-blind individuals can still make discriminations based on wavelength; those discriminations are different from the norm
cone monochromat
an individual with only one cone type; truly color-blind
rod monochromat
an individual with no cones of any type; in addition to being truly color-blind, they are badly visually impaired in bright light
agnosia
a failure to recognize objects in spite of the ability to see them; typically due to brain damage
anomia
an inability to name objects in spite of the ability to see and recognize them; typically due to brain damage
synesthesia
the perceptual experience (e.g., a color) elicited by a stimulus (e.g., a music note) that does not typically produce that experience while the stimulus (e.g., wavelength information) that does normally produce the experience is absent
color contrast
a color perception effect in which the color of one region induces the opponent color in a neighboring region
color assimilation
a color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other
unrelated color
a color than can be experienced in isolation
related color
a color, such as brown or gray, that is seen only in relation to other colors
negative afterimage
an afterimage whose polarity is the opposite of the original stimulus; light stimuli produce dark negative afterimages; colors are complementary
adapting stimulus
a stimulus whose removal produces a change in visual perception or sensitivity
neutral point
- the point at which an opponent color mechanism is generating no signal
- if red-green and blue-yellow mechanisms are at their neutral points, a stimulus will appear achromatic
- the black-white process has no neutral point
color constancy
the tendency of a surface to appear the same color under a fairly wide range of illuminants
illuminant
the light that illuminates a surface
spectral reflectance function
the percentage of a particular wavelength that is reflected from a surface
spectral power distribution
the physical energy in a light as a function of wavelength; relative amount of light at different visible wavelengths
reflectance
the percentage of light hitting a surface that is reflected and not absorbed into the surface; typically given as a function of wavelength
What is color vision good for?
- makes it easier to find candidate foods and to discriminate good food from bad food
- color can have a significant impact on your experience of the flavor of a food
- central role in searching for and assessing potential mates
realism
a philosophical position arguing that there is a real world to sense
positivists
a philosophical position arguing that all we really have to go on is the evidence of the senses, so the world might be nothing more than an elaborate hallucination
problem that the visual system needs to solve
how to construct a three-dimensional world based on the inverted images on the retina of each eye
Why have two eyes?
- allow you to see more of the world
- if you lose one eye you can still see with the other
binocular
referring to two eyes
probability summation
the increased detection probability based on the statistical advantage of having two (or more) detectors rather than one
binocular summation
the combination of signals from both eyes in ways that make performance on many tasks better than with either eye alone
binocular disparity
the differences between the two retinal images of the same scene; disparity is the basis for stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision
monocular
referring to one eye
stereopsis
the ability to use binocular disparity as a cue to depth
monocular depth cue
a depth cue that is available even when the world is viewed with one eye alone
binocular depth cue
a depth cue that relies on information from both eyes
pictorial depth cue
a cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures
occlusion
a cue to relative depth order in which, for example, one object obstructs the view of part of another object
nonmetrical depth cue
a depth cue that provides information about the depth order (relative depth) but not depth magnitude (e.g., his nose is in front of his face)
metrical depth cue
a depth cue that provides quantitative information about distance in the third dimension
projective geometry
for purposes of studying perception of the three-dimensional world, the geometry that describes the transformations that occur when the three-dimensional world is projected onto a two-dimensional surface
texture gradient
a depth cue based on the geometric fact that items of the same size form smaller images when they are farther away; an array of items that change in size smoothly across the image will appear to form a surface titled in depth
relative height
as a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image; objects farther away will be seen as higher in the image
relative size
a comparison of size between items without knowing the absolute size of either one
familiar size
a depth cue based on knowledge of the typical sizes of objects
occlusion is a…
nonmetrical cue
relative metric depth cue
a depth cue that could specify, for example, that object A is twice as far away as object B without providing information about the absolute distance to either A or B
absolute metrical depth cue
a depth cue that provide quantifiable information about distance in the third dimension
haze/aerial perspective
a depth cue based on the implicit understanding that light is scattered by the atmosphere; more light is scattered when we look through atmosphere; thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct
linear perspective
a depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image
vanishing point
the apparent point at which parallel lines receding in depth converge
anamorphosis/anamorphic projection
use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion
triangulation
in vision, this refers to the triangle formed by the two eyes and the point on which they fixate in the 3D world; the angles of that triangle are related to the location of the fixated point in depth
motion parallax
depth cue based on head movement; the geometric information obtained from an eye in two different positions at two different times in similar to the information from two eyes in different positions in the head at the same time
optic flow
the pattern of apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene
accommodation
the process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects)
convergence
the ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye); reduces the disparity of that feature to zero (or nearly zero)
divergence
the ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye); reduces the disparity of that feature to zero (or nearly zero)
vergence angle
the angle formed by lines from each eye to the current object of fixation; a larger vergence angle implies a closer object
absolute disparity
the difference in the angular distance of the images of an object from the foveas of the two eyes
relative disparity
the difference in the absolute disparities of two objects
corresponding retinal points
two monocular images of an object in the world are said to fall on corresponding points are the same distance from the fovea in both eyes; the two foveas are also corresponding points
Vieth-Müller circle
the location of objects whose images fall on geometrically corresponding points in the two retinas
zero binocular disparity
if the two eyes are looking at one spot then there will be a surface of zero disparity running through that spot
horopter
the location of objects whose images lie on corresponding points; the surface of zero disparity
diplopia
double vision; if visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic
Panum’s fusional area
the region of space, in front of and behind the horopter, within which binocular single vision is possible
crossed disparity
the sign of disparity created by objects in front of the plane of fixation (the horopter); the term crossed is used because images of objects located in front of the horopter appear to be displaced to the left in the right eye and to the right in the left eye
uncrossed disparity
the sign of disparity created by objects behind the plane of fixation (the horopter); the term uncrossed is used because images of objects located behind the horopter will appear to be displaced to the right in the right eye and to the left in the left eye
stereoscope
a device for simultaneously presenting one image to one eye and another image to the other eye; can be used to present dichoptic stimuli for stereopsis and binocular rivalry
free fusion
the technique of converging or diverging eyes in order to view a stereogram without a stereoscope
stereoblindness
an inability to make use of binocular disparity as a depth cue; typically used to describe individuals with vision in both eyes
random dot stereograms (RDSs)
a stereogram made of a large number of randomly placed dots; contain no monocular cues to depth
cyclopean
referring to stimuli that are defined by binocular disparity alone
correspondence problem
in reference to binocular vision, the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye
uniqueness constraint
in reference to stereopsis, the observation that a feature in the world is represented exactly once in each retinal image; simplifies the correspondence problem
continuity constraint
in reference to stereopsis, the observation that, except at the edges of objects, neighboring points in the world lie at similar distances from the viewer; helpful in solving the correspondence problem
Bayesian approach
a way of formalizing the idea that our perception is a combination of the current stimulus and our knowledge about the conditions of the world
binocular rivalry
the competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes
binocular rivalry chooses…
the more interesting stimuli
development of vision
- infants essentially blind to disparity until about 3-4 months
- infants 6 months and older are sensitive to depth based on pictorial cues
- infants as young as 4 months are sensitive to relative height
- infants are essentially stereoblind before 3 months, with most infants showing a sudden onset of stereopsis between 3 and 5 months
stereoacuity
a measure of the smallest binocular disparity that can generate a sensation of depth
once an infant develops stereopsis…
stereoacuity increases rapidly to near adult levels
dichoptic
referring to the presentation of two different stimuli, one to each eye
critical period
a period of time during development when the organism is particularly susceptible to developmental change
strabismus
a misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and on a nonfoveal area of the other (turned) eye
estropia
strabismus in which one eye deviates inward
exotropia
strabismus in which one eye deviates outward
suppression
in reference to vision, the inhibition of an unwanted image
three types of dichromatic vision
protanopia (red-blindness), deuteranopia (green-blindness), tritanopia (blue-blindness)
five principles of middle vision
- Bring together that which should be brought together
- Split asunder that which should be split asunder
- Use what you know
- Avoid accidents
- Seek consensus and avoid ambiguity
