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Eye Seeks Contrast


Explanations > Perception > Visual Perception > Eye Seeks Contrast

Description | Example | Discussion | So what?



When looking out at the world, the eye is fed with a huge amount of visual information that it translates into points of colour, much like a computer screen. So what does it do first when faced with this data? It first of all looks for contrast, which can be seen as adjacent changes in brightness or colour. In other words, the eye looks first for differences.


Look at this rectangle. Where does your eye go?












With amazing speed, your eye will find the little 'x' near the bottom left corner.


The eye is overall looking for things that the mind can name and assess. We need to identify the objects in our field of vision so we can decide how to handle them. They may be dangerous. They may be useful. We may just need to avoid them. Or we can simply ignore them.

Contrast is at the root of all perception. If we do not detect change then we perceive nothing. There is an apocryphal tale of a frog, placed in a gently warmed pan of water that eventually gets boiled alive. Yet if the frog is dropped in hot water it will leap out. When the frog cannot contrast the temperature increments, it does not realize it is getting hotter until it is too late. This principle of contrast, of differences, also applies to other senses. It lets us sort out different sounds, smells and so on. It lets us name what we sense and so decide what to do about this named thing.

When the contrast is sharp, we can detect edges and hence things. When it is fuzzy and gradual, we may miss anything there. Yet suddenness can shock and a little gradient, a blur around the edges, can be softer and more pleasant.

Without contrast our senses wander, seeking anything of contrast to latch onto. Without contrast, such as in a dark night, we may hallucinate, creating our own perceptual contrasts. Put a person in sensory deprivation tank and they will soon start seeing and hearing things (and the longer they are there, the more real these imaginings become). Perhaps this related to imagination and dreaming. In the dark of nights or daytime musing, our mind needs to perceive something, so it just makes things up.

Visually, contrast is detected in a static image by the differing signals from the optical nerves in our eyes. We will detect a tiny white light in a dark room (though the overload on our optic nerves of all white makes a black dot white harder to see).

Contrast is also detected over time as we compare succeeding mental images. In nature, this is a critical ability for predators who look for movement before static image contrast. The natural camouflage that their prey have evolved weakens the contrast between them and their backgrounds. Yet this advantage is lost when they move, which is why many have learned to freeze all motion.

So what?

See also

How the Eye Detects Light,


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