Color Tutorial Flashcards
Color matching experiments
and trichromatic theory
people with normal color vision had to combine three different wavelengths of light to match a test color that was made of a single wavelength of light
results: the experiments showed that colors that are perceptually similar (metamers) can be caused by different physical wavelengths
metamer
when different distal stimuli produce the same color experience
produced by different stimuli because they have indistinguishable firing patterns from color receptors
physiological evidence for trichromatic theory
proved that 3 different cones existed and that each cone had its own visual pigment, and those pigments responded maximally to 3 different wavelengths of light
Problems trichromatic theory can’t solve
3
1) why color blindness occurs in pairs of colors
2) individuals have difficulty in visualizing combinations of red and green or blue and yellow
3) why color afterimages have characteristic pairs: red after green adaptation, and blue after yellow adaption
What explains afterimages?
opponent process theory:
- fatiguing cells selectively responsive to a particular color (red) and then seeing a desaturated (adding whtie light that contains all frequencies) opponent color (turquoise greenish thing)
- opponent cells responding to white light will fire roughly equally, when exposed to (example) red light R+G- will be very excited, (Y+B- will also be excited because they’re close in the color spectrum), G+R- are strongly inhibited (almost no firing), when you finish adapting to red and go back to white background, the R+G- and Y+B- will be fatigued and fire less. The G+R- will rebound from inhibition and fire more (B+Y- will fire a bit more as well)
opponent process theory of color vision
(Ewald Hering)
(1800s)
accounts for afterimages and colorblindness
3 different units in the retina that are either excited or inhibited by different stimuli: excited by white light (+) and inhibited by blackness (-), red (+)/green(-), blue (-)/yellow(+)
- these pairs respond in an OPPOSING FASHION - these responses were believed to be the result of chemical reactions in the retina (at the time they didn't understand how cones work)
what explains colorblindness?
in the opponent process theory
if you didn’t have one of these units of color pairing you would be blind to both colors (e.g. blue/yellow)
single-cell recordings found opponent neurons in the 1950s
located in?
respond in what kind of manner?
- are located in the retina, LGN, and later in cortex,
- respond in an excitatory manner to one range of wavelengths and in an inhibitory manner to other specific wavelengths
How does the brain implement opponent cells?
simple neural circuits can be constructed from receptors in the retina to form the opponent neurons
convergence from cones to create simple receptive fields
why when doing the color afterimages with one eye closed does that eye not see the afterimage?
simple cells NOT in the brain
but in the retina (or LGN) (before convergence happens)
trichromatic and opponent theories are fairly compatible
they explain diff phenomena and work at diff stages in visual processing
tri: tells us what diff cones to expect in vision (explains the responses of the cones in the retina)
opp: tells us how that info is gunna be combined
where does color perception take place in the brain?
color is manufactured in different stages over several parts of the visual system
V4
seems esp important to color vision
if it’s damaged then people can lose their color vision
cerebral achromatopsia
damage to V4
inability to perceive color
cortical cells that respond to color may also respond to….
…..WHITE
