Theories of Colour Vision

Theories of Colour Vision

      There are two major theories that explain and guide research on colour vision: the trichromatic theory also known as the Young-Helmholtz theory, and the opponent-process theory.  These two theories are complementary and explain processes that operate at different levels of the visual system.

 

Trichromatic Theory

      Evidence for the trichromatic theory comes from colour matching and colour mixing studies. Young and Helmholtz carried out experiments in which individuals adjusted the relative intensity of 1,2, or 3 light sources of different wavelengths so that the resulting mixture field matched an adjacent test field composed of a single wavelength.  Individuals with normal colour vision needed three different wavelengths (i.e., primaries) to match any other wavelength in the visible spectrum.  This finding led to the hypothesis that normal colour vision is based on the activity of three types of receptors, each with a different peak sensitivity.  Consistent with the trichromatic theory, we now know that the overall balance of activity in S (short wavelength), M (medium wavelength), and L (long wavelength) cones determines our perception of colour as shown in the figure below.

      Several colour perception phenomenon cannot be explained by the trichromatic theory alone, however.  For example, it cannot account for the complementary afterimages in which the extended inspection of one colour will lead to the subsequent perception of its complementary colour (see demonstration below).  Complementary afterimages are better explained by the opponent-process theory.

 

Opponent-Process Theory

      Developed by Ewald Hering(1920/1964), the opponent-process theory states that the cone photoreceptors are linked together to form three opposing colour pairs: blue/yellow, red/green, and black/white.  Activation of one member of the pair inhibits activity in the other.  Consistent with this theory, no two members of a pair can be seen at the same location, which explains why we don't experience such colours as "bluish yellow" or "reddish green".  This theory also helps to explain some types of colour vision deficiency.  For example, people with dichromatic deficiencies are able to match a test field using only two primaries.  Depending on the deficiency they will confuse either red and green or blue and yellow.

     The opponent-process theory explains how we see yellow though there is no yellow cone.  It results from the excitatory and inhibitory connections between the three cone types.  Specifically, the simultaneous stimulation of red ( L cones) and green (M cones) is summed and in turn inhibits B+Y-, which results in the perception of yellow.  However, when blue light is present, the S cone is activated, the B+Y- cell receives excitatory input and blue is perceived.

     You can see the opponent relationships between red and green, and blue and yellow.  View the four-colour patch afterimage stimuli below for 30 seconds.  Then remove the colour stimuli by moving your cursor mouse over the image causing it to become a blank white field.  When you fixate at the dot in the center of the field you should notice that the original colours are all reversed - where you saw red it is now green and vice versa.  Likewise for blue and yellow.

 

    

   Complementary Afterimage

 

      How are colour afterimages explained by the opponent-process theory?  When one member of the colour pair is "fatigued" by extended inspection, inhibition of its corresponding pair member is reduced.  This increases the relative activity level of the unfatigued pair member and results in its colour being perceived.

              Trichromatic Theory or Opponent-Process Theory?

      In fact, as you have seen, both theories are needed to explain what is known about colour vision.  The trichromatic theory explains colour vision phenomena at the photoreceptor level; the opponent-process theory explains colour vision phenomena that result from the way in which photoreceptors are interconnected neurally.

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