I came across this animation years ago and most people I shared it with could only perceive one direction of motion - including folk I'd put money on having a h

I came across this animation years ago and most people I shared it with could only perceive one direction of motion - including folk I'd put money on having a higher IQ than myself (I don't know my IQ, though). I suppose another variable is how long you get to look at it - I seem to remember being similarly frustrated at first.