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(Mark Lazenby for The Globe and Mail)
(Mark Lazenby for The Globe and Mail)

Smelling a sound? Hearing a sight? The co-mingling of senses is not rare Add to ...

When you hear words containing the letter P, is the taste more like coffee or peaches? When you think about the month of November, do you see bright red or a deeper shade of blue? And what sort of personality traits do you experience for different numbers? Is the No. 3 a jerk for you, too?

If questions such as these seem absurd, then you are probably not one of the million or more Canadians who have synesthesia. For these people, the perceptual experience produced by stimulation of one sense – just hearing a sound or seeing a sight – is accompanied by others. Sounds can be smelled. Sights can be heard. Tastes can be felt as textures on the skin.

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Scientists have been interested in these vivid co-minglings of the senses for centuries, with several studies published in the late 1800s by scientists such as Francis Galton, Charles Darwin’s cousin. But a historical shift toward studying observable behaviour rather than subjective reports of one’s mental experiences meant that synesthesia was largely ignored until it again became a topic of serious scientific investigation over the last few decades.

Developments in brain-scanning technology have allowed neuroscientists to assess the subtle differences in the brains of those with synesthesia. These are thought to include extra connections between neural regions responsible for processing different types of sensations and other perceptual information.

Consider someone with grapheme-colour synesthesia. Seeing the letter R might induce the perceptual experience of green for them, because of extra connections between a region of visual cortex that helps identify individual letters and a nearby region devoted to colour perception. The additional connections apparently cause the colour-intensive region to become cross-activated by the letter-processing region, even when the letter itself is printed in black ink.

The presence of extra connectivity throughout the brains of those with synesthesia may also give them an advantage in recognizing associations between ideas and experiences that might otherwise be viewed as unrelated. Indeed this notion may explain the link between synesthesia and creativity, and why artists and musicians are much more likely to have synesthesia than less creative types. It also runs in families, reflecting a potential genetic component.

The development of new ways to study synesthesia has also fuelled research into the various ways that the different senses are linked in all of us. Some of these discoveries seem intuitive: Almost everyone, for example, associates high-pitched sounds with small visual objects and links brighter objects with louder sounds. Or changes in the colour of food can dramatically impact our perception of its flavour. Perhaps that explains the infamous failure of purple ketchup.

What about the connection between sounds and smells? Recent studies by a British team of researchers suggest consistent links here too. They found that fruity smells, for example, are consistently linked with high-pitched notes. The smell of blackberries is most often matched by participants with the sound of a piano; whereas a musky odour leads people to choose a brass instrument as the best-matching sound.

The same group found similar links between sound and taste, with sweet and sour flavours being consistently associated with high-pitched and piano sounds, and bitter flavours linked with low-pitched and brass sounds. Furthermore, participants who listened to low-pitched brass music reported that a dessert they were given tasted significantly more bitter than individuals who ate the same desert while listening to high-pitched piano music.

So don’t be too surprised if the near future includes restaurants that worry as much about the colour of your plate or your salad’s soundtrack as they do in pairing the right wine with your food. And take note as stores begin piping in distinct smells or odd ambient noises in their bid to make certain products more visually appealing. It’s all aimed at squeezing the most out of our multi-sensory world.

Mark Fenske, co-author of The Winner's Brain: 8 Strategies Great Minds Use to Achieve Success, is an associate professor in neuroscience at the University of Guelph.

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