There are a handful of people in this world who possess a special gift, something that goes far beyond a great sense of color or a technical prowess: the ability to experience multiple senses at once. Imagine how much richer your world would be if you could experience the sound of D minor in periwinkle or feel the geometry of a flavor. I think I would give my right arm to design an interface and hear its rhythms as Beethoven's Fifth, or my own unique song, richly layered with color and expression.

Synesthesia, Greek for "union of the senses," is a neurological condition in which two or more sensory experiences are inextricably linked in an individual. Experienced disproportionately by left-handed women, the phenomenon manifests itself differently in various individuals. Some see letters in color – "A" might be pistachio green and "B" peacock blue – while others hear chords in a variety of hues - and, interestingly enough, tend to have perfect pitch. There are fewer still who taste shapes, assigning food a "pointy" or "round" flavor in relation to how long it's been cooked.

Two synesthetes' view of an alphabet from MIT's website on synesthesia

There is some belief that synesthesia is a residual imprint from childhood, that the rest of us have merely outgrown this union of the senses. Because a child uses all sensibilities to learn about the world, an important cognitive milestone is the ability to distinguish between those inputs. For synesthetes, this moment of differentiation never comes.

The world of synesthetes has been shared, at least linguistically, with those of us whose senses remain autonomous. We describe jealousy as green or anger as black. We assign our wines a smooth finish or a sharp bite. We struggle to capture the rich complexity our experiences deserve. Not only are we multi-layered beings, but our world and our reactions to it are also multi-layered. If we were all infant synesthetes at one point, then we still have a gift, however small, for experiencing and describing those layers.

Because our way of meeting the world incorporates these many, interconnected sensory experiences, our way of understanding another individual's experience must be multifaceted, as well. This is especially crucial in design, where a user's reaction to a product or service is the leading factor in its refinement. I believe that this measurement of emotional response in a user can be approached from a synesthete's point of view – with individual metrics that are discretely layered, but also conjoined, because emotion cannot often be separated from our sensory experience. After all, emotion is hard to capture, and even harder to define. It's transitory, it's visceral, and it's extremely personal. Traditional measurements fail to capture the complexity. How do you see a rainbow and taste the colors? It's not with a Likert scale.

Before we can begin to describe this approach, one must first understand the relationship between emotion and design.