Color sense (visual perception)

It is the ability of the eye to discriminate between different colours excited by light of different wavelengths. Colour vision is a function of the cones and thus better appreciated in photopic vision. In dim light (scotopic vision), all colours are seen grey and this phenomenon is called Purkinje shift.

Theories of colour vision
The process of colour analysis begins in the retina and is not entirely a function of brain. Many theories have been put forward to explain the colour perception, but two have been particularly influential:
1. Trichromatic theory. The trichromacy of colour vision was originally suggested by Young and
subsequently modified by Helmholtz. Hence it is called Young-Helmholtz theory. It postulates the existence of three kinds of cones, each containing a different photopigment which is maximally sensitive to one of the three primary colours viz. red, green and blue. The sensation of any given colour is determined by the relative frequency of the impulse from each of the three cone systems. In other words, a given colour consists of admixture of the three primary colours in different proportion. The correctness of the Young- Helmholtz’s trichromacy theory of colour vision has now been demonstrated by the identification and chemical characterization of each of the three pigments by recombinant DNA technique, each having different absorption spectrum as below:

• Red sensitive cone pigment, also known aserythrolabe or long wave length sensitive(LWS) cone pigment, absorbs maximally in a yellow portion with a peak at 565 mm. But its spectrum extends far enough into the long wavelength to sense red.
  
  

• Green sensitive cone pigment, also known aschlorolabe or medium wavelength sensitive(MWS) cone pigment, absorbs maximally in the green portion with a peak at 535 nm.

• Blue sensitive cone pigment, also known ascyanolabe or short wavelength sensitive (SWS)cone pigment, absorbs maximally in the blue-violet portion of the spectrum with a peak at 440 nm.

  

  Absorption spectrum of three cone pigments.

  
  
  Thus, the Young-Helmholtz theory concludes that blue, green and red are primary colours, but the cones with their maximal sensitivity in the yellow portion of the spectrum are light at a lower threshold than green. It has been studied that the gene for human rhodopsin is located on chromosome 3, and the gene for the blue-sensitive cone is located on chromosome 7. The genes for the red and green sensitive cones are arranged in tandem array on the q arm of the X chromosomes.

2. Opponent colour theory of Hering. The opponent colour theory of Hering points out that some colours appear to be ‘mutually exclusive’. There is no such colour as ‘reddish-green’, and such phenomenon can be difficult to explain on the basis of trichromatic theory alone. In fact, it seems that both theories are useful in that:

• The colour vision is trichromatic at the level of photoreceptors, and

• Colour apponency occurs at ganglion cell onward.According to apponent colour theory, there are two main types of colour opponent ganglion cells:

1. Red-green opponent colour cells use signalsfrom red and green cones to detect red/green contrast within their receptive field.
2. Blue-yellow opponent colour cells obtain a yellow signal from the summed output of red and
   green cones, which is contrasted with the output from blue cones within the receptive field.