Most of us take our senses for granted, at least until one of them stops working. But despite the usefulness of smell, sight, touch and the other senses, they took millions of years to work themselves out. Indeed, many creatures in the tree of life have evolved just fine without eyes or ears, while others have senses we can only imagine. Just the fact that we can see and smell is something of a miracle, and scientists are still learning how they work — and how they work together.
“Natural environments require humans and other species to constantly integrate visual and olfactory sensory cues,” the authors of an October study in Nature describe how odors are represented in the human brain at the most basic level, that of individual neurons involved in olfaction, or smell. Until now, little has been known on how this actually happens in the human brain.
That’s because investigating how olfaction works at the level of individual neurons required a delicate process allowed for recording of single neuron activity. Such recording is often done in animal models but rarely, for ethical reasons, in humans. In this case, epilepsy patients in need of invasive diagnostic surgery offered the opportunity to carry out the procedure without adding to subjects’ existing risk.
The researchers inserted a bundle of fine micro wires, capable of recording the action potential of a single neuron, through the hollow inner canal of depth electrodes implanted as part of the epilepsy procedure. The patients, awake during the procedure, were given an odor rating (like or dislike) and identification task. They would identify the smell of bananas, garlic, licorice, fish and so on, while the researchers took recordings of the activity of individual neurons in their piriform cortex and medial temporal lobe.
Florian Mormann, lead author on the study, told Salon in a video interview from Bonn that “the surprising finding to us was that we found cells that responded both to an odor and to the picture … of an object that is associated with the odor. For example, the smell of a banana, the picture of a banana and the written word ‘banana.’”
The latter response, Mormann noted, took place in a neuron of the amygdala, which is already known to contain semantic concept cells and to be involved in olfaction. These are neurons that humans, but seemingly not other animals, possess that flexibly encode elements of experience. A concept cell might be invariably linked to a specific person, for example, and will fire on seeing a photo of the person, or reading their name, or hearing their voice.
These concept cells have to date only been found in the medial temporal lobe, where the amygdala is located and where many structures related to cognition and emotion are found. But in the piriform cortex, a brain region associated with olfaction, they also found a neuron that increases firing in response to both the smell of licorice and the images of a piece of licorice candy and the written word licorice — and even to the smell of anise, which is similar in flavor and often used in licorice candy.
The fact that what Mormann calls olfactory concept cells found in the piriform cortex respond to visuals is surprising. But this wasn’t just a single phenomenon: Of 1,856 neurons, 66 responded to both images and odors.
“They reliably encode odor-related images,” Mormann noted. “It’s supposed to be the primary olfactory cortex.”
Precisely. You wouldn’t expect your vision centers to respond to odors or sounds. But we do know that when you imagine something without actually receiving any visual stimulus from your retina, your primary visual cortex does get activated.
“My best explanation for that is that this reflects olfactory imagery,” Mormann said. “This has to be brought about by some top-down process in which some conscious part of the brain decides ‘I want to think of something now and then.’”
Mormann is suggesting that olfactory concept cells allow you to smell something you merely imagine. “So I see the banana, and I think of the smell that I just smelled, and thereby reactivate these representations in the olfactory cortex. It’s the best explanation. It’s purely speculative,” he noted. Which is why he and his team are now looking into how quickly the neural response to visual stimuli occurs in the olfactory regions.
“You would expect that first of all, you need to recognize the picture of a banana, or you need to read the word ‘banana.’ And by the time that the conscious recognition of that has happened, which is around 300 milliseconds … Then the onset latency should be at least that amount of time, if not even more.”
That would suggest that Mormann’s speculation is correct, and the banana visual is triggering our olfactory concept cells to imagine the rich, ripe banana odor that results, when there’s actually a banana there, from a compound called isoamyl acetate. Presumably reading the word isoamyl acetate would not trigger the smell of a banana though — except perhaps in biochemists, science nerds or perfumers for whom it’s become part of their internal concept of the fruit.
How does a color smell?
Olfactory concept cells, important for memory, thus seem to bring together the senses of sight and smell. But this isn’t the only way in which these two senses overlap. What about less obvious mental connections between the senses? For example, what color is the smell of a banana? What shape is its smell? Sure, as Shakespeare’s Juliet said, “That which we call a rose by any other name would smell as sweet–but would a rose smell as sweet if it looked more like the “world’s ugliest orchid”?
Perhaps not, if olfaction and sight are closely linked. Crossmodal correspondences are “the tendency for a sensory feature / attribute in one sensory modality (either physically present or merely imagined), to be associated with a sensory feature in another sensory modality”, according to University of Oxford experimental psychologist Charles Spence. The physical co-location found between olfaction and sight that Mormann’s team found in the olfactory cortex may be part of the physical apparatus behind such crossmodality. These can be far more imaginative than associating the smell of a thing with its visual representation (banana to banana smell.)
There is evidence that humans associate certain smells or tastes with certain visual attributes, suggesting a crossover between the olfactory and the visual senses. In fact, often what we think is taste is actually olfaction, Spence notes: “If I make a loud noise, suddenly the light seems brighter. That’s a crossmodal effect. It made the light seem bright — I didn’t put [the senses] together into a ‘sounding light.’ Whereas multisensory would be like when you hear my voice, you see my lips moving, we actually perceive it as one audiovisual [object].”
“And then synesthesia would be … these rare individuals who experience additional sensations that sometimes happen to be across the senses, but most commonly sight,” Spence told Salon in a video interview.
Crossmodal correspondences, which are less idiosyncratic and unlike synesthesia (where a common example is the synesthete seeing letters in color) always involve more than one of the senses, are of greater interest to Spence, who sees them as a kind of universal synesthesia.
Bouba is round; kiki is pointy
“Why do people think that sweet is round … that bitter is angular and sour is high-pitched?” Spence asked rhetorically. And most people would agree. Crossmodal correspondences like these are fundamentally different from synesthesia, where a flavor/sight example might be the case of the British man who tastes words and has identified not just the flavors of every stop on the London Tube, but also every subway station in far-away Toronto.
“These cross modal correspondences are shared across people. Is it across everybody? Is it universal? Sometimes, perhaps yes,” Spence said. “But it’s much more consensual, much more agreed. I know that 95% of the people in the world, no matter what language they speak, will think that [the nonsensical word] ‘bouba’ is round, ‘kiki’ is angular.”
In fact, there is significant academic literature on the bouba-kiki effect. And it will come as no surprise to most (unless you’re in the benighted 5%) when I tell you that most people who are told the nonsense or pseudoword “maluma” know for a fact that it’s sweet — despite not having the slightest idea what a maluma is.
It’s not clear, though, whether these reliable associations between different sensory modes, including sight and olfaction, result from shared early experiences, like an almost-universal cultural influence, or from co-location of concept neurons like those identified by Mormann and his team in their single neuron study, and to what extent the visual appearance of the word (like seeing the written word banana can trigger the smell) determines the associations.
A rose by any other name might smell as sweet, but if you changed its color? All bets are off. Perhaps if you rename it a Maluma Rose.
