The Spiral Ladder

Climb over the Wall

Seeing is believing?


In order to set the scene for what’s to come, the first few posts will concentrate on some of the idiosyncrasies of human perception, beginning with Seeing is believing? The Implications of Optical Illusions reposted from Chris Madden’s blog, You are here…

When you try to understand your position in the universe you’d think that a very good way to start would be simply to look around. To use the evidence of your eyes.

Unfortunately this approach is hampered by the strange fact that our eyes don’t actually see what’s there. This may come as a surprise, because as a rule we tend to take our sense of vision for granted, and we assume that the image that we perceive is an accurate representation of the world ‘out there’.

Nothing could be further from the truth.

I’ll pursue the subject of the nature of our sense of vision, in all of its weirdness, later, but for now, by way of a taster, I’ll just show you an entertaining example of how your sense of vision can give you false information.

Look at the photograph below. This image is just an ordinary photo of an ordinary apple on an ordinary checked surface. I’d like to draw your attention to the light square and the dark square with the numbers on them, because there’s something very interesting about them. Something that you probably could never guess at.

When you look at these two squares you see a light square and a dark square, but in fact both squares are exactly the same shade of gray.

This is almost impossible to believe, and no amount of staring at the image will convince you of its truth. You can only prove it to yourself by doing something such as cutting the image up and moving the two squares next to each other, or as I’ve done below, by placing a strip that’s the same shade of gray across both.

This illusion applies to the real apple on the real checked surface by the way, not just to the photo – it’s not an optical illusion of the type that only works on two-dimensional images (in the way that, say, Escher drawings of endless staircases do).

Why do your eyes deceive you when you look at this image?

This illusion seems to show a gaping deficiency in our sense of vision. You may be relieved (and perplexed) to hear that in truth it’s showing the exact opposite: that your eyes are actually working extremely well and in a very sophisticated way.

This is how it works.

In reality the squares on the checked surface are either white or uniformly gray, of course. The ‘white’ square with the number two on it is only gray in this photo because it’s in the shadow created by the apple. (It’s the same shade of gray as the gray square with the number one in it purely because of the light levels involved – with different brightnesses of light the ‘white’ square may be lighter than, or even darker than, the gray squares that are not in the shadow. Bear in mind that a white square that has no light shining on it, such as at night, can actually appear to be black! I deliberately chose to use the light level with which the values of gray happened to be the same purely for dramatic effect). Your brain takes into account the fact that the white square is darkened by being in the shadow and adjusts what it sees accordingly. It effectively lightens the square so that it’s closer to its true value. Your brain doesn’t only do this with apples on checked surfaces. It does it all of the time, constantly compensating for the variations in the amount of light that’s falling on objects around you. Your brain knows that the lighting of an object is an incidental quality, secondary to the actual colour or tone of the object itself, and it therefore compensates by filtering some of these variations out. If you think of your brain as acting like a computer you may say that it was using a ’shadow-compensation algorithm’.

So it is that your brain doesn’t show you what’s actually in front of your eyes (which in the photo above is two similarly gray squares), but shows you a version of things that in some ways is more accurate than the raw visual information (that is, it shows you a light square stripped of some of the unnecessarily complicating effects of the shadow).

Studying visual effects such as the one in this photograph is very entertaining in itself, but the phenomenon also has interesting philosophical implications.

The fact that we don’t interpret the visual world as it actually is but modify it for our own ends has huge philosophical implications in itself of course – but on top of this it also implies that it’s quite possible that we do the same with our other faculties, including our mental faculties. As a result, the way that we ’see’ the world intellectually and emotionally may be equally open to modification for practical ends. And again, as in the photo above, even when we’re concentrating on the subject with all of our attention, we can’t actually see what’s there. Only what we think is there.

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