The world presents itself to us scant-haired apes through our senses. Our attention alights on the warm, flaking brown tactility of a sunlit bench or the gut-tugging stench of rainwet, festered trash, and these details stand out from the remaining mess of things unattended to. Later, a string of sensations can be woven together to give memory the sensible fabric of an afternoon, a first meeting, a childhood. For most people, these sensations are of a distinct qualitative sort: touch, smell, taste, etc. But a very few, synesthetes, perceive one channel of sense as entangled with another. And, if The Atlantic is to be believed, everyone may be able to develop some form of this doubled sense perception.

But we'll get to that. First, there's the problem of sense itself: before it is possible to argue whether a person is a synesthete, you have to know which senses are overlapping. Most of us follow Aristotle's lead and break the sources of this sensuous world into five facets: those revealed by sight, smell, taste, touch, and hearing. But nowadays scientists tend to agree that we must include the sense of balance in our account of the senses, bringing the total to six. And then they argue about whether we need to keep counting. Remember those pre-umami days, when your middleschool biology teacher told you that the tongue had different regions for tasting sweet, salty, sour, and bitter? Like that.

So even the most foundational aspects of the way the world presents itself to our primate organs is not fully understood. But not being able to pin down just how many channels there are to our sensory world is just the beginning. Add synesthesia to the mix—and the possibility that anyone can experience it—and the matrix of possible sensuous experience begins to show its true complexity.

Consider the synesthete's perceptual world: the sight of given letter will summon the sense of a certain shape, or a given tone will bring to mind a certain hue. In the jargon of neuroscience, they experience "synesthetic percepts": colors without light, tones without waves of sound. Instead, there is another object that sets off this sensory pas de deux.

Synesthesia makes the unsimple tangle of our six or twelve senses even more interesting. And, because it may be the case that more people could develop synesthetic associations, things as banal as the dark letters forming this sentence could excite an accompanying flurry of sensations. Of course, for these hypothetically trained synesthetes, this conjoined perception would be very different from the spontaneously knotted senses of the true synesthetes. But it would be something. People with flat senses—people who think their sense of the hardness of this rock or the savor of that cheese is composed of discreet elements—would potentially have access to the echoing richness of a world that resonates across senses.

Which would add depth to memory. Because the world's sensory richness determines its texture, be it silky or coarse. And, unlike quintuple-rainbow synesthete Solomon Shereshevsky, trained synesthetes wouldn't have to worry about someone's harsh tone recalling the sensuous qualities of smoking coals.

Image: Matthew Brandt