Color Harmony and the Opponent-Process Channel Theory Christina Lewis Psych 159

TRICHROMATIC THEORY Thomas Young and Hermann von Helmholtz, 18’th-19’th century

Opponent-Process Theory 1878 Ewald Hering Certain color combinations don’t exist (we never see them), such as reddish-green or yellowish-blue Three receptor types, each with opposing pairs: red green, blue yellow, black white

Opponent Neurons Excitatory response to some wavelengths and inhibitory response to others Red-Green receptors cannot send information about both colors at the same time Responses to one color of an opponent channel are antagonistic to those to the other color. **More efficient, given that for the cones, responses to certain wavelengths overlap. Differences are more important.

CONES  BIPOLAR CELLS  GANGLION CELLS PARVOCELLULAR MAGNOCELLULAR Processes differences between L & M Cones, Red – Green differences Processes difference between S cones, blue-yellow differences Intensity of light

How it Works Red-Green Channel: The difference between long-wavelength and middle-wavelength cone signals. Yellow-Blue Channel: The difference between short wavelength cones and the sum of the other two cones. *Luminance Channel*: Based on inputs from all the colors. Detects the difference in brightness of color information.

ISOLUMINANCE DIFFERENT COLORS - SAME BRIGHTNESS

Youhellohavepsychnoclassproblemthisreadingisonly athereadinggreentestwords. This is a very bad words-on-background color-pair, because there is very little difference between the luminance of the color dark-blue and the luminance of the color black.

BAD BAD BAD DON’T DO THIS IN YOUR FUTURE POWERPOINTS

Implications Color-opponent channels: Color is good for SEPARATING OBJECTS Separating regions Luminance Channel: Contrast transmits SHAPE INFORMATION (**edges**) Fine detail

Other Important Properties of Opponent Channels Luminance > Purely Chromatic Information: (for many aspects of vision including): Stereoscopic depth: Cannot detect differences in depth based purely on color channel information

Other Important Properties of Color Channels Motion Perception: Luminance > Purely Chromatic Information If gratings of different colors but equal luminance are moving, we detect the speed much slower (or for some humans, completely immobile) as compared to a grating of very large contrast difference (for example a black and white or black and yellow grating).

After-Images -Fatigue of one color receptive causes stimulation of its opponent color in the pair