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Survival: ancient senses in a modern world

Who's in control?

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A huge amount of processing is carried out on the raw information gathered by our eyes and ears, before we consciously try to interpret what we're seeing or hearing.

One way to understand what happens during such processing is to study people who have suffered damage to the brain, through disease or accident. The altered way in which such people perceive the world gives us clues as to the functions of those parts of the brain that have suffered damage.

It's also possible, however – for vision and hearing – to use images and sounds that are designed to highlight certain quirks of our sense processing.

Images such as the ones included on this page (and on other pages on this site, and many other sites) give us clues as to how the complex task of vision is actually carried out.

Light or Dark...

It's easy to tell just how dark or light an object is: you just look at it.... or do you?

Consider the image below created by Edward H. Adelson – in particular the checkerboard pattern, beside the green cylinder.

Which of the squares, A or B, is darker? It seems straightforward, but if you run your mouse over the image, a mask is placed in front of the picture, leaving only squares A and B showing. (This isn't a trick: the mask doesn't change the two squares in any way.)

Try it now: were you correct? (Click on the image for a larger version.)

Square B is, in fact, slightly darker than A...

If you take away the mask, can you see this? Or does A look darker, once again? Why?

If you click here, or on the image above, a new page will open with a larger (and higher-quality) version of the illusion.

An explanation of the "illusion"

Taken from Prof. Adelson's explanation page:

"The visual system needs to determine the colour of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

"The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighbouring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbours. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

"A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the colour of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

"As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view."

Click here to see other illusions at Prof. Adelson's website.

The Cube Illusion

The human visual system has evolved over many tens of millions of years to operate in a world that is mostly lit by white light. Systems in the brain, working entirely unconsciously, compensate for changes in illumination. We usually don't notice that anything is happening, and it's only when another optical system such as a camera is used, that we understand just how sophisticated our visual senses are.

Another example of colour adaptation is shown in the extraordinary cube illusion, below. Developed by (and available at his lottolab studio website), this illusion demonstrates the way in which entirely unconsciously (and seemingly instantaneously) our brain makes assumptions about the actual colour of objects, taking into account their perceived colour, and local lighting conditions.

Here is a small version of the illusion. It shows two large Rubik-style cubes, which appear to be lit by yellow light, on the left, and blue light on the right.

Move your cursor over the image, and you should see a black mask appear, which is covering everything except the "blue" squares on the left, and the "yellow" squares on the right. (There's no trick here, incidentally: the squares do not change colour when the mask is superimposed.)

If you click here, or on the image itself, you'll be taken to a larger version of the illusion (which may take a little time to load, but which is of much better quality).

The Cube Illusion

The squares are actually grey! But your brain is working perfectly: it isn’t being fooled.

When you look at the original scene, your visual system “recognises” that the cubes are illuminated by yellow and blue light, respectively, and compensates by adjusting its idea of what a “blue” or “yellow” square would look like: blue squares in a yellow light are actually grey, as are yellow squares in a blue light. (The effect is enhanced by the comparison with surrounding colours.)

All of this happens in the merest fraction of a second, and is an entirely unconscious process.

Red, Green or Blue: saying colours

Sometimes our conscious brain has to deal with confusing inputs, or conflicting information.

Communication is at the core of what it means to be human. Language is what defines us: it forms our modern culture. Children are wired to learn languages, and a large part of the brain is associated with our processing of language, both spoken and written.

The dominance of language can occasionally cause a problem, however, as we can easily demonstrate with the simple test below.

As quickly as you can, read out loud the COLOURS of each of the words below, NOT the words themselves. For example, for the first word you should say "BLUE", because the word is coloured blue.

red
yellow
blue
green
red
yellow
blue
green
red
blue

Most people find it very difficult, and stumble through the list. Your brain recognises the shapes of the words (and the sounds associated with them), and the overwhelming urge is to call out those names. This clashes with the apparently simpler task of just recognising and then saying out loud the colour of each word.

Here's an equivalent list, but this time the "words" are simply collections of symbols. You should have no problem in quickly reading out the colours, this time:

••••••
••••••
••••••
••••••
••••••
••••••
••••••
••••••
••••••
••••••

Go back to the previous list and try again. With practice (as with everything), you can try to ignore the symbols, and just see the colours. (There's some evidence to suggest that people who are artistic – who have learned to ignore distracting symbols, and to paint or draw simply what is in front of them – can read through the list more quickly.)

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