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Have you ever tasted a word, or seen colours while listening to music?

If you have, you may be among the 1% to 4% of people who have a fascinating trait known as synaesthesia.

Synaesthesia is a neurological phenomenon where the activation of one sense, such as hearing, triggers the activation of another usually unrelated sense, such as sight. This means people with synaesthesia often experience additional sensations compared to the rest of us.

We’ve devoted a lot of time to understanding this rare phenomenon. While there’s much more to unpack, what we do know shows we don’t all perceive the world in the same way.

What is synaesthesia?

People with synaesthesia are known as synaesthetes. Research suggests synaesthesia may be more common among women, although this could reflect sampling biases, and may be influenced by genetics.

There are many different types of synaesthesia. Some people have auditory–visual synaesthesia, meaning they see colours when they hear sounds. Others see colours when they read, hear or think about letters or numbers. This is known as grapheme-colour synaesthesia. Another example is mirror–touch synaesthesia, where a person feels sensations on their own body when they see another person being touched.

All of us naturally combine information from different senses. For instance, when you watch someone speak, your brain blends what you see and hear to understand them better. In synaesthesia these links are a bit different – a sound might, for example, trigger a visual experience – but may still depend on the same mechanisms.

People with synaesthesia don’t have any control over how their senses collide. Instead, these are spontaneous, vivid experiences that usually stay the same over time. For example, today a person with grapheme-colour synaesthesia may perceive the letter “A” as being red. And they’ll most likely see it as being the same shade even years later.

It’s worth noting synaesthesia is not an illness or disorder. And it doesn’t cause harm or impairment, although some people may find their synaesthesia overwhelming at times. For example, if they feel pain every time they see someone else in pain, going to the movies can be quite disturbing. However, on the whole it does not seem to interfere with daily life. In fact, many people don’t realise they have synaesthesia because it’s simply how they perceive the world.

What causes it?

We don’t yet know exactly what causes synaesthesia. But scientists have come up with two main theories.

1. Synaesthetes have more connections in their brain

According to this view, known as the cross-activation theory, people with synaesthesia have more connections between different parts of their brain. This could happen because their brain hasn’t gotten rid of unused connections between brain cells. This process, known as synaptic pruning, helps the brain work more efficiently and is part of normal development.

Under this theory, a person with grapheme-colour synaesthesia for example, would have the region that recognises letters directly linked to the part that processes colour. So when they see a letter, they perceive it with a colour.

2. Synaesthetes have slightly different activity in their brain

The other main theory is that people with synaesthesia have the same neural connections as non-synaesthetes, but certain pathways might be stronger or more active. Synaesthesia does seem to build on mechanisms we all have. For example, when you see a picture of a grey banana, you know bananas are usually yellow. We even see patterns of brain activity that reflect this. Grapheme-colour synaesthetes might also do this with letters so that when they see black letters, their brain activates specific colours.

Simply put, the debate about what causes synaesthesia comes down to whether synaesthetes have a different brain structure or just use their brains in an alternative way.

Does it make you more creative?

You might’ve heard artists such as Kandinsky or musicians such as Lorde describe their synaesthesia-like experiences. And there is some evidence to suggest synaesthesia is more common among people in creative fields.

One large survey of Australian synaesthetes found roughly 24% had creative occupations, such as being an artist, musician, architect or graphic designer. This is compared to the less than 2% of people in the general population who have these jobs. This gap is striking, even though we don’t understand what’s behind it. One reason may be synaesthetes link ideas and sensations in unusual ways, helping them think more creatively. Research suggests people with certain kinds of synaesthesia may form stronger memories or have more vivid imaginations, but only to a limited extent.

Synaesthesia is a powerful window into how our brains make sense of the world. It reminds us perception is not a fixed, one–size–fits–all process. Rather, it’s something the brain actively builds in ways that are often more varied, and far richer, than we might expect.

Anina Rich receives funding from the Australian Research Council.

Sophie Smit does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.