Our visual systems are equipped with special cells in the visual cortex whose job is to detect vertically aligned edges, bars, and lines.
There are other simple cells that detect horizontal edges. And there's a third group of cells that detect diagonal ones.
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| Ivan Shishkin |
Our relative deficiency at judging diagonal angles compared to verticals and horizontals is known as the oblique effect. According to Wikipedia, "People are very good at detecting whether a picture is hung vertically, but are two- to fourfold worse for a 45-degree oblique contour, even when a comparison is available."
Another remarkable fact is that the relative number of these cells depends on the visual environment that we grow up in. People raised in forests have more vertical cells than people raised on the plains.
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Quotes from Cognitive Psychology For Dummies
Wikipedia on The Oblique Effect.
Interesting to learn that there is a physiological basis for something that I think most artists have intuitively known all along. As for judging 45-degree angles, I often tell my students to question whether any diagonal would bisect (diagonally) an imaginary square. If so, it must be 45 degrees. I suppose this works because it goes back to the ability to more accurately judge verticals and horizontals, and to visualize them in the form of a square.
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