Illusions are for more than just fun - they also provide insight into the inner workings of our brains (Source:

The last time someone told you to look at an optical illusion, they probably described it as playing a cool trick on your eyes. But these quirks of perception – as well as most other illusions – have more to do with tricking your brain than anything else.

Rather than thinking about illusions as being something you fall for, scientists have realized that these mis-perceptions are actually powerful glimpses into the work that the brain typically seems to do so easily. In fact, some well-known illusions linked to sights and sounds have such a strong foundation in our brains that they can still affect people who have been blind or deaf since birth.

The obvious question is why we are even susceptible to illusions in the first place. If our brains and senses have developed specifically to help us interpret and navigate the world around us, why is there this room for error? The answer, at least in part, is efficiency.

First we need to make a distinction between sensation – the information sent to our brains from our senses – and perception – the meaning that the brain assigns to this input.

If we only interacted with the world around us using the information that is sent from our senses into our brains, we would be very inefficient creatures. We could not learn that a purple sweater is having shadows cast on it rather than changing color, or that a cat standing behind a fence is a whole cat instead of six, separate cat-like pieces perfectly blocked by fence posts. Every piece of input would have to be interpreted separately.

When you look at the picture, do you see one cat behind a fence? How does your brain know? (Source: Herrmann C and Murray M (2013) )

Instead, our brains apply a lot of information and filtering in what are called top-down processes. This is what tells you that it is much more likely a single, connected cat rather than separate cat pieces (though the latter could technically be possible, experience tells us and our brains that this is highly unlikely).

This top-down information covers everything from the color of a shirt remaining the same over time to knowing where the edges of objects are. It makes us more efficient at processing the world around us and is so important that much of our brain is made up of specialized regions just for taking the information from each of our senses and turning it into a world we understand. Specialized top-down input is also what leaves us susceptible to illusions.

So what happens to those highly specialized brain areas when one of your senses is lost, such as for people who are deaf or blind? It turns out that the brain can rewire itself to use those areas for other things, better maintaining its efficiency. This process of adaptation in the brain – called neuroplasticity – is what allows our brains to change with our needs as we develop from newborns to infants to kids to teens and eventually into adults.

Researchers like to remind us that we are cognitive misers. We want to use the limited processing power that the brain has as effectively and efficiently as possible – even if that means using part of the brain that has developed over many thousands of years for visual processing to start processing sound.

One of the interesting quirks of this kind of rewiring, and one important insight for scientists, is that illusions typically linked with one sense can end up spilling over into another sense in people whose brains have repurposed those specialized areas for new types of information.

The size of an object, not just its weight, can affect how heavy it feels (Credit: Amanda Baker)

One group of researchers was studying the well-known size-weight illusion, which has been recognized in some form since the 1890’s. The basic premise is that when a person is presented with two objects of different sizes that weigh the same, the larger object will feel lighter. This illusion is so powerful that even showing someone the relative sizes of two boxes, blocking their view of the boxes, and then asking them to lift the boxes with a pulley and string will still result in the larger box seeming lighter.

Scientists had already found that blind subjects who were able to touch the boxes experienced the same illusion, but it was unclear whether there was a role being played by the visual center of the brain or the illusion was a result of touching the boxes directly.

To study this, the researchers needed a very special set of people: human echolocators. These individuals, many blind since birth, have learned to navigate their surroundings and estimate the sizes of objects using the feedback of sound from clicks, claps, and hums. When determining how humans could develop this bat-like ability, researchers found that the echolocation process activated areas of what would be the main visual center of the brain in these blind individuals.

For this study, the researchers asked three groups of people to perform the same task – lift differently-sized boxes using a pulley system and determine their relative weight.

The first group was made up of people with normal vision, who fell victim to the size-weight illusion as expected. The second group, blind individuals who could not use echo-based judgements to estimate size, correctly identified the objects as the same weight. The third group, the blind individuals who could first use echo-based judgements to determine the size of the boxes, experienced the illusion and rated the large objects as lighter even though they were the same weight.

How many flashes of light? The answer has to do with more than just what your eyes see. (Credit: Amanda Baker)

Similarly, another group of researchers found that a typically audio-based illusion affected deaf individuals whose brains now use part of their sound centers to process somatosensory (touch) information. This illusion, known as the double-flash illusion, typically occurs when beeps of sound are paired with a flash of light. When humans with normal vision and hearing abilities are exposed to two beeps at the same time as a single flash of light, they often perceive two distinct flashes of light.

The groups in this study included both hearing and deaf individuals, but instead of pairing the flash of light with beeps, they paired it with two somatosensory inputs – puffs of air on the individuals’ cheeks.

The hearing individuals did not experience the double-flash illusion. But the deaf individuals - who had likely repurposed some of this auditory part of their brain for somatosensory information - did! With two puffs of air and a single flash, they experienced the double-flash illusion one would typically experience with sound.

So the next time you start entertaining yourself by playing with illusions, remember that it isn’t your eyes or ears that are being tricked. And instead of being disappointed that you could be fooled, remember that these illusions are so strongly linked to your brain’s mission to interpret the world efficiently that you may not even need to be able to see them in the first place to be affected.


Herrmann C and Murray M (2013) Seeing Things That are Not There: Illusions Reveal How Our Brain Constructs What We See. Front Young Minds. 1:6. doi: 10.3389/frym.2013.00006

Bhanoo SN (2015, January 5) Illusions Fool Even the Blind. The New York Times. Retrieved from:

O’Connor A (6 August 2012) Really? The Brain Gets Rewired if One of the Senses Is Lost. The New York Times. Retrieved from:

Karns CM, Dow MW, Neville HJ (2012) Altered cross-modal processing in the primary auditory cortex of congenitally deaf adults: a visual-somatosensory fMRI study with a double-flash illusion. Journal of Neuroscience 32:28 doi: 10.1523/JNEUROSCI.6488-11.2012.

Buckingham G, Milne JL, Byrne CM, Goodale MA (2014 December 19) The Size-Weight Illusion Induced Through Human Echolocation. Psychological Science OnlineFirst doi: 10.1177/0956797614561267


Herrmann C and Murray M (2013) Seeing Things That are Not There: Illusions Reveal How Our Brain Constructs What We See. Front Young Minds. 1:6. doi: 10.3389/frym.2013.00006