Try and cross your eyes to make the top gridded circles overlap. Give up? Try doing it on the bottom ones that are surrounded by the border.
A lot easier, right? The square surrounding the circles helps your eyes stabilize as you cross them.
From Blake 1989
Also notice how your eyes process the diagonal lines themselves. Your brain is receiving drastically different signals from each eye creating a state of binocular rivalry, where your perception often alternates between the signals received from one eye and the other.
If you’re interested in this phenomena and learning a theory about it’s neural underpinnings check out the paper where I came across it. http://www.psy.gla.ac.uk/~martinl/Assets/MCMPS/Blake_89.pdf
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Blake, R. A neural theory of binocular rivalry. Psychol. Rev. 96, 145-167 (1989).
