This is not a spiral

A fun optical brain teaser.

http://www.neatorama.com/wp-content/uploads/2010/04/not-a-spiral1.jpg

Why does it appear to be a spiral?

I thought perhaps it had something to do with depth perception - but closing 1 eye doesn't trick it in anyway.

I think it has to do with the losangles in every concentric circle.

Looks to me it's because the "radial" lines aren't actually radial. So locally, it looks like the "spiral" arms are moving closer to the centre.

Originally posted by joneschr
A fun optical brain teaser.

http://www.neatorama.com/wp-content/uploads/2010/04/not-a-spiral1.jpg

Why does it appear to be a spiral?

It appears to be a spiral because it IS a spiral. I think you are saying it should be a TURNING spiral or something.

Originally posted by sonhouse
It appears to be a spiral because it IS a spiral. I think you are saying it should be a TURNING spiral or something.

It's not a spiral by any definition I've heard before. What do you mean by "spiral"? And "turning spiral" for that matter?

I'm not sure if sonhouse didn't catch the figure, or he's a step ahead of all of us

Originally posted by sonhouse
It appears to be a spiral because it IS a spiral. I think you are saying it should be a TURNING spiral or something.

I think a spiral has to revolve around a point and get larger at the same time.

This is not a spiral, it is a set of circles inside other circles.

There are two basic effects at work.
1. There are essentially sections of spiral arranged in a circle.
2. The design confuses our eyes so that we cannot see a complete circle at once.
The combination of the two effects is that we believe it to be a spiral based on the hints (sections of spiral).

Originally posted by sonhouse
It appears to be a spiral because it IS a spiral. I think you are saying it should be a TURNING spiral or something.

If you dont believe it is not a spiral, take your finger and place it on the "spiral" Then follow it down to the center...You will realize it is just concentric circles.

Originally posted by joneschr
I'm not sure if sonhouse didn't catch the figure, or he's a step ahead of all of us

Well my bad for sure! I followed the curve around and saw circles second time round. Great illusion.

The effects that I see that make it look like a spiral.

1) Alternating black/white and light/dark blue squares from one concentric circle to the next are alternating, so the squares themselves (e.g. just the black squares) DO form a spiral toward the center.

2) the diamond pattern at the corners of the black/white squares make it appear that each square is "slanted" in the direction of the apparent spiral. Therefore, the entire circle appears to be nearing the center as you follow it around.

Originally posted by joneschr
I thought perhaps it had something to do with depth perception - but closing 1 eye doesn't trick it in anyway.

It may still have something to do with depth perception. Many of the clues for depth are not binocular (.e.g, occlusion); they can still operate with just one eye open. That's why the world does not go flat when you close one eye.