chapter 9 part 2 Flashcards
opponent process theory of color vision
idea that there are pairs of colors that have opponent, or opposite responses
hering’s opponent process theory states that there are two pairs of chromatic colors:
- red green
- blue yellow
two types of behavioural evidence for opponent process theory:
phenomenological and psychophysical
phenomenological evidence
based on people’s experience when they look at colors and central to Hering’s proposal of opponent process theory
based on describing the appearance of colors
Hue scaling
procedure in which ps are given colors form around the hue circle and told to indicate the proportions of red, yellow, blue, and green that they perceive in each colors
used to determine the primary colors - because each of the primaries were pure
hering’s primary colors
red, yellow, green, and blue - hering proposed that each of the other colors are made up of combinations of these primary colors
unique colors
name given by ewald hering to what he proposed were the primary colors: red, yellow, green, and blue because they are not made up of any other colors
psychophysical evidence
quantitative measurements of the strengths of the blue-yellow and red-green components of the opponent mechanisms
hue cancellation experiments
hue cancellation experiments
procedure in which a subject is shown a monochromatic reference light and asked to cancel it out with another color
- ex. how much yellow needs to be added to blue in order to cancel out all of the blueness
these experiments provided support for opponent process theory
physiological evidence
discovery of opponent neurons
opponent neurons
neurons that have an excitatory response to wavelengths in one part of the spectrum and an inhibitory response to wavelengths in the other part of the spectrum
provided physiological evidence for the opponency of color vision
circular single opponent cortical neuron
centre surround - firing increases in centre (medium wavelength) and decreases in surround (long)
responds to large areas of color
circular double opponent neuron
firing increases w medium wavelength in centre and long wavelength in surround - however, firing decreases when wavelengths are presented in opposite spaces
responds to color patterns and borders
side by side double opponent cortical neurons
increases firing when a vertical medium wavelength bar is presented to the left side and when a vertical long wavelength bar is presented to the right side and decreases firing when the wavelengths are presented to the opposite sides
respond to large areas of color
criticisms of unique hue theory
- the wavelengths that cause maximum excitation and inhibition don’t match the wavelengths associated with the unique hues
- hue scaling experiments also work with different primary colors
alternative functions of opponent neurons
indicate the difference in responding of pairs of cones
can compare the excitatory and inhbitory responses in each cone to determine the overall response of the pair
ex. +L -M neuron has a negative response to 500nm light because the M receptor has a larger inhbitory response than L’s excitatory response
how is color represented in the cortex
color processing is distributed across a number of cortical areas rather than confined to one area
interestingly, 72 percent of patients with achromatopsia (color blindness) also have prosopagnosia
color constancy
we perceive the colors of objects as being relatively constant even under changing illumination
partial color constancy occurs when our perception of hue changes a little when the illumination changes, though not as much as we might expect from the change in the wavelengths of light reaching the eye
color constancy
we perceive the colors of objects as being relatively constant even under changing illumination
partial color constancy occurs when our perception of hue changes a little when the illumination changes, though not as much as we might expect from the change in the wavelengths of light reaching the eye
chromatic adaptation
prolonged exposure to chromatic color - This adaptation can cause a decrease in sensitivity to light from the area of the spectrum that was presented during adaptation
means that different types of light only have slight effects on our perception of color
responsible for color constancy
memory color
the effect on perception of prior knowledge of the typical colors of objects
because people know the colors of familiar objects, they judge these familiar objects as having richer, more saturated colors than unfamiliar objects that reflect the same wavelengths
when does color constancy work best
when an object is surrounded by objects of many different colors
and when objects are viewed with two eyes (better depth perception)
and when objects are viewed in a three dimensional scene
lightness constancy
the constancy of our perception of an object’s lightness under different intensities of illumination
what does the intensity of light reaching the eye from an object depend on?
- illumination - total amount of light that is striking the object’s surface
- reflectance - the proportion of light that the object reflects into our eyes
what happens to illumination and reflectance when lightness constancy occurs
the perception of lightness is determined by the object’s reflectance instead of the intensity of illumination
related not to the amount of light reflected, but the percentage of light that is reflected
ratio principle
when the illumination is the same over an entire object, lightness of the object is determined by the ratio of reflectance of the object to the reflectance of surrounding objects - as long as this ratio stays the same, the perceived lightness will stay the same
works well for flat, evenly illuminated objects but gets more complicated in three dimensional scenes
reflectance edge
an edge where the reflectance of two surfaces changes
contributes to uneven illumination and poses a problem to the perceptual system since it needs to correctly perceive it in order to accuractely perceive the actual properties of a scene
illumination edge
an edge where the lighting changes - like shadows
the perceptual system needs to distinguish this from reflectance edges in order to be able to accurately perceive the scene in situations of uneven lighting
