trichromatic theory
Summary
Trichromatic theory (Young-Helmholtz) says color vision comes from three cone types, most sensitive to short (blue), medium (green), and long (red) wavelengths, with each color coded by their relative activity. It explains the receptor stage; opponent-process theory explains later processing, and both are accepted.
Detail
Meaning: The retina has three types of cones, each with a photopigment that responds best to a different part of the spectrum: short (S, bluish), medium (M, greenish), and long (L, reddish). Each cone responds to a range of wavelengths, not one color. A given wavelength activates the three types to different degrees, and the brain reads the ratio of their activity as a hue. Roughly equal activation of all three is perceived as white or gray. Rods handle dim light and do not support color vision.
On the MCAT: Expect a passage on color vision deficiency. A person missing or having a defective cone type (red-green deficiency is the most common form, and the genes are on the X chromosome, so it is more frequent in males) confuses colors that depend on that cone. You may also be asked which theory explains an observation. Trichromatic theory fits the three cone types and color-matching. Opponent-process theory (red-green, blue-yellow, black-white pairs) fits afterimages: staring at red fatigues the red side, so a green afterimage follows.
Common confusion: Students treat the two theories as rivals, or think each cone fires for only one color. Both hold at different stages: trichromatic coding in the cones of the retina, opponent processing in later cells such as retinal ganglion cells and the thalamus.
Sources
- Psychology (Myers)
- Sensation and Perception (Wolfe et al.)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.