Humans have two nostrils, which you’d think would allow us to determine the direction of smells, in the same way that two ears let us determine the direction of sounds. But that’s not how it works, sadly—humans, in general, are not stereo smellers. We can track down a smell by moving our head and body while sniffing, searching for increasing smell strength, but that’s much different from stereo smelling, which would allow us to localize smells based on different intensities wafting into each nostril.
Branching off from earlier work accessing alternative physiological smelling systems, researchers at the Human Computer Integration lab at the University of Chicago have developed a way to augment our sense of smell with a small piece of nose-worn hardware that uses tiny electrical impulses to give us the power of directional smell.
When you smell a smell, your olfactory bulb gets most of the credit for what that smell smells like, but there’s also a complex facial nerve system called the trigeminal nerve that adds some smell sensations, and your brain fuses them together into one distinct smell. The trigeminal nerve and olfactory bulb react to different smells in different ways, and with some particular kinds of smells, like mint, that “cold” smell is coming from your trigeminal nerve. The trigeminal nerve is also responsible for the “hot” smell of peppers, and the “sharp” smell of vinegar.
While research has shown that humans cannot consciously determine the directionality of smells using our olfactory bulb, we can determine direction with very high accuracy if the smell triggers our trigeminal nerve, meaning that you can localize the smell of mint chocolate chip ice cream, but not regular chocolate chip ice cream.
One way of triggering the trigeminal nerve is through smells, but you can also do it with direct electrical stimulation. This works on the olfactory bulb as well, but to do that, you have to stick electrodes up your nose. Way up your nose, way up there, way waaaaay up there, since your olfactory bulb is back behind your eyeballs (!). Meanwhile, your trigeminal nerve extends all around your face and into your nasal septum, meaning that you can interface with it pretty easily using just a small bit of electronic kit:
Images: Human Computer Integration lab/University of Chicago
You’d think that the way to mimic a stereo smell with this kind of device would be to stimulate one side of your nose differently than the other side, but remarkably, it turns out that you can generate stereo smell sensations (as well as smell intensity sensations) using only electrical waveform variation. The wireless, battery-powered device uses magnets to keep itself attached to the inside of your nose; it can detect when you inhale, and then uses electrodes to stimulate your septum. The current implementation communicates with external sensors, and this system works so well that completely untrained people can use the device to localize virtual smells, following electrically-induced virtual odors around a room.
Image: Human Computer Integration lab/University of Chicago
The real question, of course, is what does it actually feel like to have this thing in your schnoz? Apparently, it doesn't feel like electricity in the nose or anything like that—it’s actually smell-like, somehow. We asked Jas Brooks, first author on a paper being presented at CHI this week, to try and explain:
Describing this sensation with words is a tricky endeavor. The sensation our device produces can feel like a "tickling" or "sting," not far from that of wasabi or the smell of white vinegar, except it is clearly directional. Meaning, participants were able to understand if this sensation meant the "smell" source is to your left/right/front even without any training or explanations of any sorts. The actual "quality" of the sensation varied across the range of what the trigeminal nerve usually senses (e.g., some stimulations felt tingly like the wasabi, while others felt like pressure or even warmth in a nostril), which again suggests we are really hitting the trigeminal nerve with our electrical stimulation. Moreover, we're actually really excited about the fact that we seem to be able to trigger the quality of those trigeminal sensations too, e.g., perhaps we can create different qualities for these "smells" by only changing the waveforms.
This device could be of immediate use as an assistive device for people experiencing anosmia, or loss of olfactory function, since in many cases (including anosmia due to Covid-19), damage to the olfactory bulb does not extend to the trigeminal nerve. In the future, the researchers suggest that it may be possible to map between trigeminal stimulation and olfactory stimulation, meaning that a wider range of smell sensations could be electrically induced. It may also be possible to use this device to make people into super-smellers, not only localizing odors but also leveraging external sensors to smell things that they’d never be able to on their own. Like, imagine being able to smell carbon monoxide, or perhaps something more exotic, like radioactivity. Or, as the researchers suggest, a mapping application—smell your way home.