Can you see zig-zags? You're suffering from 'curvature blindness'

Can you see the zig-zags? Photo / Dr Kohske Takahashi

It is the optical illusion which might just send you round the bend.

Dr Kohske Takahashi, a psychologist at Chukyo University in Japan, has discovered that, in certain circumstances, human brains exhibit "curvature blindness" and automatically swap undulating waves for corners.

To demonstrate the new phenomenon, Dr Takahashi designed an optical illusion which appears to consist of pairs of dashed wavy lines and zig-zags, lain over a white, grey and black background.

But look closely, and the lines are all curvy, with no sharp points at all. It is simply a quirk of the brain which is adding in the peaks.

In a new report, published in the journal i-Perception, Dr Takahashi explains that "curvature blindness" occurs on the lines where the light and dark grey dashes end at the peak or valley of the curve. In contrast where the dashes cross the peak or trough, the brain continues to see the lines as wavy.

The effect is caused because when the brain spots the sudden shift from light to dark grey at the bottom and top of the curve, it mistakes it for a sharp angle.

Dr Takahashi speculates that humans may have evolved to spot corners ahead of curves, and when there is confusion - compounded in this case by the changing background colours - opts for the former. Although he is puzzled as to why that might be.

"I'd say that our eyes and brain may have been evolutionarily adapted to detect corners more efficiently than curves," he told The Telegraph.

"We are surrounded by artificial products, which have much more corners than natural environment does, and hence our visual.

"This visual phenomenon doesn't cause the problem in our everyday life, otherwise someone should have found this illusion earlier."

However scientists discussing the new paper on Discover magazine's Neuroskeptic blog have offered explanations as to why the phenomenon might occur. Most speculate that the grey and white dashes trick the brain into thinking it is looking at shadows.

Michael Mullins said: "When we see dark on one side of a curve, and light on the other, our brain wants to infer a shadow formed by a peak.

"A curved hill has an oblong shadow further down the sine wave. I bet if you moved the dark area to simulate that, it would look like rolling hills. The original sine waves look like snow covered peaks and shaded valleys. - the shadow perception bias translates the image into landscape."

Isaac Maxwell added: "To me this seems like a defensive mechanism of assuming a pattern is human-made. The presence of right angles indicates the presence of humans, and the presence of humans would indicate a higher threat level than whatever natural objects would present the wavy lines. Most likely vines."