I'm getting e-mails about this lovely new illusion that appears to show cognitive penetration of perception! Could it really be that these little arrows completely change our perception of motion and size? 1/ https://twitter.com/jagarikin/status/1331409504953540613

That would be cool! But (un)fortunately that's not what's going on. As @_OliSharpe, @wouterkool and others show in some follow-up demos, it's not about the arrows at all! 2/ https://twitter.com/_OliSharpe/status/1331588353859399683

So what is it? Just the good ol' reverse phi illusion! The key is the contrast between the color/brightness of the wheel and the color/brightness of the small lines at its edge. Here's a different variation where you can see this more clearly. 3/ https://twitter.com/sina_lana/status/1331049253280497670

And so whereas removing the arrows doesn't eliminate the illusion, removing that contrasting edge does. 4/ https://twitter.com/_OliSharpe/status/1331588739634765826

So it's basically a magic trick: The illusion is achieved one way, but some extra presentational flair makes it seem like it's being achieved a different way. 5/

You can read about and play with the reverse phi illusion yourself here ( https://michaelbach.de/ot/mot-reversePhi/), courtesy of @MichaelBach99 (who may well be the Internet's most helpful vision scientist! his fuller page is amazing). 6/6

Of course, even if the presence of an arrow *did* change perception (and as @ceptional notes, there are cases like this), there would be further questions about the mechanism. (Does the arrow just make you attend differently? Etc…) But all great stuff to think about in any case!