Geometrical illusions are simple optical illusions where background patterns make shapes and lines appear to bend, break, or warp in bizarre ways. While scientists are still figuring out exactly why they occur, some of these illusions might arise from glitches in our brains as we process two-dimensional visual information to create three-dimensional perceptions.
These illusions have fascinated people for generations. For instance, in the mid-1800s three brand-new geometrical illusions were spotted back to back over the span of just a couple years, starting with the Zöllner illusion. In it, a bunch of short hatch marks cross long parallel lines at an angle. That makes the longer lines look like they're tilting toward and away from each other — even though they're actually parallel.
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When astrophysicist Karl Friedrich Zöllner first published his illusion, the cause of the distortion was a mystery. But now, some experts think that our brains somehow expand the acute angles formed where the hatch marks meet the long parallel lines.
This could be because of a phenomenon in the visual processing center of the brain, called lateral inhibition. The idea is that some neurons in this part of the brain specifically respond to lines oriented in different directions — and when one neuron is turned on, it turns off the activity of its neighbors.
Imagine a clock. If a neuron that's stimulated by lines oriented toward the 12 blocks the activity of a neighboring neuron that responds to lines angled toward 2, that might make the lines appear to skew more toward, say, the 11 and the 3. That makes acute angles look wider than they actually are, which in turn makes the long parallel lines seem to be tilting toward and away from each other.
Lateral inhibition is a pretty satisfying explanation for how the Zöllner illusion could be occurring. But it doesn't work as an overarching explanation for other geometrical illusions like the Poggendorf illusion — discovered within Zollner's original manuscript by the editor of the scientific journal. To learn more, check out the video above.