The Fraser spiral

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Do you see a spiral?

Start at the red dot and try to follow the spiral to the centre. 

Most people see this as a spiral although they are in fact a series of circles.

Cells in your brain become active when they see lines at different tilts.

Some respond to lines like this / others to this \ and still others to | or  _.

The activity from all these brain cells is put together, allowing us to see the huge variety of shapes and forms in the natural world.

When you look at this image, your eyes send electrical messages to your brain and activate the appropriate brain cells.

Because of the way this image is drawn, the segments that make up the circles appear to tilt more than they actually do.

Your brain makes a best guess of what is there and decides it is a spiral.