Why do we see illusions?

It was a simple question that ignited the Internet: was this dress blue and black or white and gold? Simple as it was, it divided people, with some claiming that they saw one pair of colours and others claiming that they saw the other pair. It was what we call an illusion.

The debate was eventually settled when it was revealed by the original Tumblr poster that the dress was indeed blue and black. Nonetheless, this controversial dress is a timely reminder of how our perception is far from fool proof.

Whilst I will not go into the science behind this particular illusion as I feel that it has been explained (a detailed account can be found here), there are numerous other examples of illusions that have had us scratching our heads for decades.

Rabbit or Duck?

What animal do you see? A rabbit or a duck? This classic illusion is known as a “reversible figure” – an image that is compatible with two or more interpretations. These types of optical illusions are perhaps the most well known category. There a bunch of examples just like this one on the internet.

How is that possible?!

Look closely at the image above. Although it may seem normal at first glance, the object cannot conceivably exist in three-dimensional space. It is what’s known as an “impossible figure”. It is the result of people’s tendency to process parts of an image first before examining it as a whole (i.e. bottom-up processing). As a result, our minds are tricked into thinking that an image is normal when it is not.

Which monster is bigger?

Which monster is bigger – the one on the left or right? Although the monster on the right looks bigger, they are in fact the same size. This illusion is based on the “Ponzo illusion” that exploits the mind’s tendency to judge an object’s size by the background. Once you remove the background, it becomes obvious that they are indeed, the same size.