Do you see raised bumps or dents?

A cream, orange and yellow exhibit table and backboard, wit the words 'Do you see bumps or dents?'. On the table sits two round displays that have grey bumps and dents on the surface.

Your visual system interprets the position of shadows to judge whether something is a 'dent' or a 'bump'.

How it works

Look at pictures on a disc, then turn the disc upside down to check whether the pictures change in appearance from being 'bumps' or 'dents'.

Things to try or ask around the exhibit

  • What happens if we leave the disc alone, but we turn out heads upside down and look at the pictures?
  • Do the bumps and dents change when you turn the disc upside down?

Background

Our brain does a lot of analysing in the background which we often don’t notice until we see a test like this. Your brain is hard-wired to assume that light comes from the top of your visual field. This doesn’t mean that it calculates where the Sun is positioned in the sky, instead, your brain simply assumes that light should always fall from above.

Even when your head is turned upside down and you look at the picture in its normal upright position, your brain assumes that light falls from the top of your visual field rather than from the sky above. (Your visual system doesn’t pay attention to signals from your vestibular system when you’re trying to work out the shape of something from its shading.) Anything that looked like a bump when your head was upright continues to look like a bump when your head is upside down (while the dents still look like dents).

However, when your head is the right way up and you turn the picture upside down, the shadows change position, but your brain still thinks that light is shining from the top of your visual field. So according to your brain, bumps turn into dents and dents turn into bumps.

Finding the science in your world

Even simple clues like the placement and angle of shadows are automatically processed by your brain so you know the depth of a hole, or the height of a bump, and you manage to avoid tripping and falling.