What flavor is this shape?S

No ideas? What about a passing train — has listening to one ever evoked a specific color in your mind?

The questions I'm asking you have to deal with something known as synesthesia — a rare condition wherein a person's senses become entangled, causing them to associate different senses with one another. A person with synesthesia might perceive a certain color, for example, when reading specific numbers or letters.

But Vera Ludwig — a cognitive neuroscientist at Charité Medical University in Berlin, Germany — thinks that low-grade synesthesia may actually be very common, and that these so-called "cross-modal correspondences" could actually find their origins deep in our evolutionary history.

SciAm's Ewen Callaway writes:

Nearly all humans tend to link high-pitched sounds with lighter, brighter hues and bass-filled sounds with dark shades. People judge high vowels, such as 'mil', as white, for example, and consider lower-toned syllables, such as 'mol', as black. Ludwig thinks such connections represent a mild form of synaesthesia, with both emerging from neural cross-wiring between nearby brain regions involved in processing senses.

To determine whether humans learn to associate sounds and colours from others, or whether they are innate and do not require language, Ludwig searched for the associations in captive chimpanzees.

She and colleagues at Kyoto University in Japan showed six chimps aged 8 to 32 a small black or white box, and then trained them to to select a square of the same colour on a screen to receive a fruit reward. The apes also heard a high or low tone when making their choice.

When high tones accompanied white squares and low tones were matched with black, the animals picked the correct colour 93% of the time, on average. When the colours and sounds were reversed, their success rate fell to about 90%.

In the same game, 33 humans made too few mistakes to detect any perceptual bias. But humans did make correct decisions more quickly when sounds and colours matched.

Because chimps and humans showed similar biases to paired tones and colours, language is not needed to perceive the links, Ludwig says. She suggests instead that these biases were present in the common ancestor of both species, which lived approximately six million years ago, and may have influenced human language.

Ludwig's findings are published in the latest issue of Proceedings of the National Academy of Sciences. You can read more about the study over on Scientific American.

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