Whenever a beautiful sunset rolls in, our Instagram stories tend to fill up with the same horizon. For a moment, it feels like proof that we’re all seeing the world in the same way.

But are we?

It’s easy to assume that perception is universal. Yet neuroscience tells a different story. For some people with a neurological condition called synaesthesia, colours have sounds and flavours have shapes.

Contact caught up with Professor Derek Arnold to explore how synaesthesia works – and what it reveals about our perception of the world.

What is synaesthesia and how common is it?

People with synaesthesia have unusual additional perceptual experiences when they encounter specific inputs. Seeing black text spelling the word ‘Wednesday’, for instance, can make a synesthete have a sensation of the colour yellow; or in an unfortunate example for myself, hearing my name ‘Derek’ caused one synesthete to experience the taste of earwax.

Between 2% and 4% of people have synaesthesia.

Are there different types of synaesthesia?

There are many types of synaesthesia. One of the most common types is Grapheme-Colour, where seeing a colourless letter or numeral can cause a synesthete to experience a specific colour.

Seeing the letter ‘A’ often triggers perception of the colour red. But there are many other types. For instance, hearing a musical note can trigger sensations of colour or a sense of touch on a specific part of the body. Researchers have documented more than 80 types of synaesthesia.

What is happening in the brain that causes senses to overlap?

Synaesthesia is caused by a complex and automated set of associative processes, that culminate in an unusual sensory experience.

Adults are so used to being able to read that we have forgotten how remarkable it is that we have learnt to associate a cluster of oriented black lines, grouped to spell a word, with the meaning of that word. We don’t just see the word ‘beach’. Reading that word automatically makes us think of a beach and its related concepts, of sunshine, sand and salt water.

If you can visualise, you might even picture a beach in your mind’s eye. Synaesthesia is similar, but seeing a colourless letter ‘A’ can make a synesthete have a sensation of red – presumably because they learnt this association in early childhood. The origin of other automated associations can be more mysterious.

When synesthetes have unusual triggered sensations, we can detect activity spread across a network of brain regions – not just the area linked to the original sense. These areas of brain encode related semantic information.