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I can understand that colours are just manipulation of our brain to light rays of different wavelength and energy. We perceive patterns better at higher contrast. Do we perceive patterns better at high contrast because of adaptive processes in our brain, i.e., do we habituate to the colours?

For example, consider a black board; we find texts written by white chalk easier to see than if it was written by blue chalk. Is white chalk on black board a form of adaptation of our brain, or it is always preferred by the brain in comparison to blue? In terms of wave mechanics, the white chalk may cause greater disturbance in the light waves travelling from the black board to our eyes. Please don't consider the case of any eye defects.

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  • $\begingroup$ Hi. As far as i know this has to do with expertise. In humans there is the fusiform face area, I.e. an expertise area. In humans it responds to faces, whereas in zebras it responds to stripes. It is THE part that is expert in making distinctions between difficult to recognize patterns. Over the years symbols (letters) may have evolved into a human expertise. $\endgroup$ – Robin Kramer May 24 '16 at 20:23
  • $\begingroup$ I'm not talking about symbols and letters itself, I'm talking about perception of contrast colours. Letters or symbol are information generated by shape, the information of a shape can be same but why it is preferred when it comes in contrast with the background? $\endgroup$ – user56396 May 24 '16 at 23:29
  • $\begingroup$ Consider a white board for instance, make two dots of equal dimension but of different colours. One by black colour marker and one by white similar colour like cream colour, which would be easier to get? Obviously Black. The information both dots contain is same- mainly information of shape and dimension. But what makes us catch the black dot easier than another one is my question. $\endgroup$ – user56396 May 24 '16 at 23:36
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Short answer
Contrast is hardwired in the visual system and can be explained by retinal and brain connectivities without the need for adaptive processes. My answer pertains to adaptation at the neurophysiological level. In other words, short-term neural adaptation in the retina or the visual cortex are not necessary components for color-contrast coding in the visual system.

Background
According to the model of Hering, color contrast in trichromatic species (such as most humans) is basically established between three sets of opponent systems: yellow-blue, red-green and the achromatic channel (dark-bright), and depicted in Fig. 1. The opponent system is established in the brain by cells in the visual cortex responding to both colors of a pair, but one color (e.g. blue) excites and the other (yellow) inhibits the cell, or vice versa. These systems are basically established by funneling the color information of opponent colors from the primary sensory neurons (the photoreceptive rods and cones) to color sensitive cells in the visual cortex (Gouras, 2009). In effect, we cannot perceive a yellowish blue, or a reddish green, because these colors are opponent colors in a pair.

![Hering
Fig. 1. Hering model of color opponency. source: Mark Green

Because of this, the primary color-pairs yellow-blue and red-green yield excellent contrast, because in the brain the opponent response of cells will cause the edge to be sharply defined. Likewise, the achromatic channel yields high-contrast acuity. Mixed colors will impinge on this system not in an ON/OFF way, but in a gradient. For example, in your example, the blue chalk will be visualized through the yellow-blue opponent channel. On a black background, color contrast will be established against the achromatic channel. The highest possible contrast would be delivered with the blue chalk on a yellow background such that the blue-yellow opponent system could be used (e.g. blue chalk on a yellow 'white' board :-). However, blue chalk on a blackboard had to be contrasted using the yellow-blue against the achromatic channel, which are phsyiologically not opponent systems and therefore yield less sharp contrast.

Another reason is foveal tritanopia: the fovea has the highest resolution for perceiving fine detail. There are no short wave cones (blue cones) in the very center, the area of maximum resolution (Williams et al., 1981), so presumably it is impossible to see blue with the area of the retina with the highest contrast acuity.

Note that while the aforementioned Hering model does not depend on adaptation, many illusions do depend on adaptive processes in the color system, most notably after-images.

References
- Gouras, Color Vision. In: Webvision. The Organization of the Retina and Visual System (2009)
- Williams et al., Vis Res (1981) 21(9): 1341–56

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@AliceD: All of this is very accurate. However, I would like to add that the idea of adaptation in colour perception is not entirely off the mark. What is being described in @AliceD 's reply pertains to the processes in the visual cortex, but of course the colour signal needs further processing before it can be perceived consciously. What is certainly universal is the contrast perceived between the primary colour-pairs you mentioned (because the cones in the retina block the cones of the respective opposite colour), but there is evidence that finer contrasts - for instance between different shades of blue - are acquired through culture. In Russian, there are two words for blue - a darker and a lighter shade, but they lack a common word to denote a general concept of "blue" as in English. Studies (although not entirely consistently) have shown that, therefore, Russian speakers are better at distinguishing finer contrasts among shades of blue (see Paramei, 2005). So there appears to be some (cultural) learning to how we perceive colour contrasts.

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