When the chevron pattern in the movie is in perspective, so that the bars get thinner and closer together with distance, the bars and the fan of bright bands on them appear to stream past us, as if we were travelling along a tunnel. When the bars are all the same size and equally spaced, so that they don’t show perspective depth cues, the fan of bright bands appears to be expanding. The outer bright bands even look as if they are sliding along the bars.

Thanks to Priscilla Heard for the suggestion that the key to the expansion effect is in the absence of perspective cues. If you’d like more on that ….

I guess what’s happening is that the fan of bright bands is interpreted by the brain as a perspective, depth cue whether the bars suggest perspective or not. That leaves the brain with a puzzle to solve when the underlying bars don’t also appear in perspective. The brain resolves the mystery by interpreting bright bands that would be more distant as expanding laterally.  The expansion certainly seems strongest with those bands.

Maybe it’s a bit like the way that the brain also interprets absence of perspective in Shepard’s tables and related, static size-constancy illusions as implying expansion, but static expansion with distance in space, rather than active expansion as in our movie.

I think the effect in my movie helps explain the expansion effect we see in the beautiful spiral ring illusion described in an earlier post and also shown (at the time of writing) in a Youtube movie. The bars without perspective in my movie play the role of the repeating spiral patterns presented by the ring. The highlights that fan out across the ring with rotation play the role of the bright bands in our movie. However, the stimulus presented by the ring is very complex, and I’m not sure that’s all there is to it. The way the ring itself appears vividly to expand along with the fan of bright highlights suggests that something else may be going on as well.