Painters create the illusion of depth by playing with lots of the cues that your visual system uses to figure out how far away things are in the world. Some of those cues seem obvious. Objects that are in front of others block your view of what is behind them. In Caillebotte's painting, the umbrella of a man at the very front of the painting partially blocks the view of the umbrella of the couple walking out of the painting, and so that man must be nearer to the viewer than the couple.

Another cue is size. If two objects are actually the same size, then the one that is further away makes a smaller image on our eyes than the one that is nearby. The people who are far away on the street are painted as smaller images than those that are close.

If you have taken any classes in drawing, then you have also learned about vanishing points. As objects move away from us, they shrink to a point into the distance. That is why the tracks of a railroad appear to join together in the distance, even though the tracks are actually parallel.

All of these tricks have one thing in common. They can all be seen with one eye. That is, even if you were to cover one eye, you could still use information about the relative size of objects, whether one obstructs the other, and vanishing points.

There are also cues to depth that require both eyes. One of them is called stereoscopic vision. Put an object on a table in front of you. Now, cover one eye and look at the object. You get a particular view of it. Now cover the other eye. Notice that the image you get from the other eye is similar, but there are a few parts of the object that you can see with one eye but not the other. The closer that an object is to you, the more of a difference there is in the images that go to each eye.

Vision scientists have known for a long time that the brain takes the images from each eye and matches up the corresponding parts. It uses information about differences between the images to decide which objects are close by and which are far away.

An interesting paper in the March, 2011 issue of Psychological Science by Margaret Livingstone, Rosa Lafer-Sousa, and Bevil Conway explored the ability of artists to use stereoscopic cues for depth. They had artists and non-artists come into the lab and look at lots of random dot images like the ones I just described. In this case, many of the images had squares that were shifted so that you might see a square either in front of or behind the rest of the dots. The artists had a much harder time seeing the squares floating in depth than the non-artists.

In a follow-up study, the authors found photographs of artists and of age-matched members of Congress. They measured the alignment of people's eyes in the pictures. If a person's eyes are not aligned well (so that both are looking forward), then the two eyes will not get images that allow the brain to develop the stereoscopic cues to depth. The artists were much more likely to have eyes that were misaligned than the members of Congress.

These findings are interesting, because they suggest that people who become successful artists may be those who routinely have to use the single-eye cues for depth to see the distance of objects in the world. Because they rely so strongly on these cues, they may be better able to exploit them to make drawings and paintings that convey depth than people who also use the two-eyed cues well.

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