Musician Kaitlyn Hova sees the music she plays in colors. To show the world what she sees, she built a glowing violin that lights up in different colors when different notes are played. People with this perception associate different senses (like sight and sound, taste and touch) in their heads. This is called synesthesia. Four percent of people are 'synesthetes'. How do they perceive the world?

Caroline Beier is a doctor in Hamburg, Germany. Her synesthesia helped her to remember things better in school because she sees letters and numbers in different colors and in 3D: for example, eight is always a cucumber-green, three is grass-green, four is yellow like the sun and number two creamy-white.

To remember 18 x 18, she sees two dark green cucumbers in a light-green vegetable stand with a yellow and white-striped awning.

Sounds can also be associated with colors. This helps Alexandra Kirschner in her profession as a voice trainer for a boys' choir near Stuttgart. She hears different voices in different colors.

For Alexandra, higher voices are often yellow, and lower voices are dark blue. When someone sings, she sees the tones as circles in front of her. If one tone is off, the color of the circle is grey, blurry and out of line.

If one has a form of synesthesia, the chance of experiencing a connection of more sensations is higher. Not only sounds trigger colors for Alexandra Kirschner, but also touch. If she pinches her arm briefly, for example, a pinkish light-blue ellipse appears in her head as if drawn by a pencil. Colors can also trigger tastes. When she sees a pink wool sweater she tastes strawberry ice cream.

It isn't just the case that people see the color of sound — colors can also evoke sounds. Musician Katja Krüger automatically hears certain tones when she sees certain colours. As with almost all synesthetes, the connections developed when she was a child — before she even played an instrument.

When Katja sees a red balloon in front of her, she hears the note "A" played by an oboe.

Once Krüger even wrote down the sound of a Nefertiti statue in music notes. If she looks at the colours of Nefertiti's head from bottom to top, she can transform them into notes. From this she has written a whole piece of music.

Some people, like Martin Schmiederer, have many forms of synesthesia at once. So there can be quite a lot going on in his head. The scientist and philosopher can remember passages of text well with his color and sound codes. Despite his 21 forms of synesthesia, he rarely experiences a sensory overload. He is able to concentrate on several perceptions, but can also ignore them.

For him, different sensory experiences are like blocks building up on top of each other. He sees colors in different places. For example, when hearing the lowercase 'k', Martin sees it in the upper left corner of the room in a blackish-green in 3D. When he reads it, he tastes something that reminds him of the dentist. If he speaks it, the tone produces a color that appears at the bottom right.

Synesthesia is not a disease, but simply another way of perceiving things. Most people find their additional sensations pleasant, but they can also push them into the background. Alexandra Kirschner and Martin Schmiederer, for example, both say that they don't like to wear scratchy woollen sweaters because this feeling evokes an unpleasant color.

Synesthesia isn't just an illusion. MRI scans have shown that the brain areas are active for different senses. There are also genetic connections. But many people don't know they have synesthesia because their impressions are completely normal. Famous synesthetes include Lady Gaga, Pharell Williams, as well as artist Wassily Kandinsky and philosopher Ludwig Wittgenstein.