How can shadows shift shapes?
The placement of shadows can affect how we interpret a picture.
What do you think?
Our brain does a lot of analysing in the background which we often don’t notice until we see an illusion or a test like this.
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.
In fact, shadows are one of the first things our visual system processes when we’re trying to work out the shape of something.
Your brain is hard-wired to guess that light comes from the top of your visual field which doesn’t mean that it calculates where the Sun is positioned in the sky.
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. So 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).
This also indicates that 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.
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.
Our ancestors generally kept their heads upright, so our brain can get away with this simplified shortcut that light could be analysed ‘top down’ in your visual system.