For many people, moving nearer to, or further away from, this design triggers an optical illusion: its components appear to rotate.
The image is known as a Pinna-Brelstaff figure, and why it seems to move in a clockwise, or anti-clockwise, direction has long been a matter of conjecture.
Now, researchers led by Yong Gu from the Chinese Academy of Sciences have at least answered part of the question.
They determined that the image triggered reactions in the area of the brain responsible for detecting motion – but that the relevant neurons seemed to realise their mistake after about 15 milliseconds and switch off again.
Their work, the researchers say, provides fresh insight into how the brain distinguishes between perception and reality. It is published in the journal JNeurosci.
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