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Comment by egypturnash | Hacker News Reader

There is a line on the inside and outside of the circles where the colors go in and out of sync with the circle as the arrows change directions - it snaps between trailing by half the length of the color segments, going being ahead by the same amount, or in sync.

When the outer edge of a part of the circle is trailing and the inner edge is ahead, the circle appears to move outwards; reverse this and it appears to move inwards. The parts where the circle appears to be moving up/down/left/right have one side set to the 'outwards' motion, the opposite side set to the 'inwards' motion, and the other sides in sync.

The human visual cortex has so, so many edge cases like this. It's wonderful!

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This shows the circles without the inner circles: https://twitter.com/i/status/1331542934056001536

And this with stabilized rotation: https://twitter.com/spacecatmeowart/status/13316956817451499...

Super cool effect! This is the type of magic trick that becomes cooler as you learn more about it.

This makes much more sense. I thought that my brain was unconsciously interpreting the meaning of the arrows, which was completely unbelievable to me.

I was thinking the same, covered the arrow with my thumb... didn't help at all.

what I found fascinating is that the effect is maintained even when you're not focused (like lens focus, not attention focus) on the circles. If you let your eyes relax their focus to a point beyond the screen, the blurry circles maintain the illusion.

And bonus points if you find the focal point where a third circle is in the middle which is a composite of both the left and right circles, and... it doesn't move. I don't know how to better explain the effects of letting your eyes wander out of focus and the artifacts it creates in your vision, but I'm sure someone will get it.

Took me a while to get an overlapping 3rd circle.

Feels almost normal on the left/right moving ones, but when the grow/shrink stage comes my brain has no idea what to do with the arrows. The arrows looked like they were both pointing in and out, sometimes flipping between them really fast.

Somehow the right side is slightly out of focus, but the left is sharp. I think I had each eye at a different focal point. Is that possible? lol

It sure is. It’s also quite interesting that i have to get very close to the screen to create that third circle, but then can get quite far without loosing it, even when looking at the right or left circle.

Yeah, my first guess is the movement of that faint inner line triggers the Parasol cells or some other movement detection cells in our retina (https://en.wikipedia.org/wiki/Parasol_cell).

Possible underlying circuit: https://www.cell.com/neuron/pdf/S0896-627318%2930105-3.pdf

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