For one person, chocolate smells pinky and stripy, for another it was a hazy mist. These were not smell
associations – the images are how some people actually experience the smell of chocolate. The people drawing the images have a rare form of synesthesia, in which a smell elicits a color. And in a new study on this form of synesthesia, researchers found that it’s not just about color but also patterns and shapes.
Alex Russell of Southern Cross University in Australia says his team set out to investigate the nature of odor-induced (or olfactory-visual) synesthesia. They did this by asking synesthetes to smell 20 different odors; some were fruity, some were not very pleasant, some were more familiar than others (e.g. vinegar). The participants had to sniff the odor, try to name it, and then draw whatever image they saw when they smelled the odor using a computer program like Paint.
Some days later, they came back and did the same thing again. The researchers looked at their answers from the two sessions to understand how the images changed over time, based on whether participants were consistent in their naming of the odor. The researchers also did follow-up tests to better understand what happened when the smells hit their olfactory receptors via the mouth instead of the nose.
The results of the study, published online last month in Cognitive Neuroscience, show that olfactory-visual synesthesia goes beyond color, that the naming of the odors is important, and that the experience involves cognition. Russell spoke with CNS about synesthesia, how he got started in the topic, and the implications of the findings.
Russell: Synesthesia (or synaesthesia) is a condition where stimulation in one sense induces an involuntary, unusual experience in either the same or a different sense. For example, many synesthetes report that letters, words or numbers are inherently colored – that is, when they see the letter A, a particular color may come to mind. Synesthesia is not just deliberately making an association between the senses – for example, if you smell cut grass, you might think of the color green – although it might build on mechanisms we all share linking our senses.
It seems that just about any combination of the senses is possible, although some are more common than others. There are reports of people hearing sounds and experiencing smells, or of musical sounds having spatial coordinates.
It’s important to note that it’s not a disorder or anything like that. It’s just a thing that happens for some people.
CNS: How did you become personally interested in synesthesia?
Russell: I was studying odor and wine perception for my Ph.D. I asked my participants to smell some odors and try to describe them in a way that meant something to them. One of the participants started describing the odors using musical terms (notes, instruments, etc).
The week before this had happened, one of my collaborators, Anina Rich, had given a colloquium at my uni about her fascinating studies into synesthesia, so I contacted her and we planned a project, along with Dick Stevenson, who is an expert in smell, taste and flavor. We organized a project and put together a grant application, and it all went from there. See, it pays to attend talks!
CNS: Your latest study looked at olfactory-visual synesthetes – can you define that please? How common is that type of synesthesia compared to others?
Russell: In olfactory-visual synesthesia, when the synesthete smells an odor, they perceive it as inherently colored. So, for one synesthete, caramel was a kind of purply color with blue and brown blobs, while for another, caramel was a blue pentagram with yellow dots in the corners. For most of us, if we smelled caramel and were ask to think of a color, we might think of that as a weird request. But when pressed, we might describe a light brown color. For these synesthetes, there often isn’t an obvious rhyme or reason why these colors are related to the odors, but the request to “draw what they see” makes perfect sense to them. We initially called this “odor-color” synesthesia until we realized that it wasn’t just one color (for most of the synesthetes), but a few colors. Thus we called it olfactory-visual synesthesia instead.
In terms of how common it is, well it’s quite difficult to tell. I often find that when I give a talk on our synesthesia research, someone in the audience says either “Oh yay! Someone knows what’s going on with me! I just thought I was weird” or “Doesn’t this happen for everyone? I thought it was really common.” So because many synesthetes don’t disclose their synesthesia (for whatever reason), we don’t really know how prevalent it is. But of those who have been studied, forms of synesthesia where odors are the actual eliciting stimulus are quite rare – one estimate is that about 6.5% of all synesthetes have olfactory-visual synesthesia. Compare this to about 62% having colored letters.
CNS: What percentage of the population has synesthesia?
Russell: Again, we don’t know. Estimates range from about 1% to about 4% (and I’ve seen an estimate of 25%, but that’s probably too high). Again, it’s hard to tell because many don’t disclose it.
CNS: Why did you want to study olfaction in particular?
Russell: Studying olfactory-induced synesthesia offered a few unique opportunities. When you look at a letter, you pretty much process it automatically – so you get the perceptual elements (e.g. sharp corners, intersections, rounded parts, etc.) at about the same time you get the conceptual information (e.g. this is the letter B, which is related to these things). With odor, quite often you’ll get the perceptual elements (how irritating it is, how strong, etc.), but not be able to name it straight away – “Oh I know I’ve smelled this before, but I can’t quite put a name to it! It’s on the tip of my tongue…” Without the name, you don’t get all of the conceptual information, so studying odor-induced synesthesia allowed us to separate out the conceptual and perceptual elements of the stimulus.
But odor can also be perceived via two routes, one of which isn’t commonly associated with odor. We can sniff an odor, and we know that’s a smell. But when we put something in our mouth, odor is once again involved, as the odors enter the nasal cavity via the throat – a thing we call retronasal olfaction. A lot of people don’t realize that smell is involved there, so again that allowed us to work with that in the design.
Finally, it’s pretty rare! There were two studies from about 100 years ago, and another brief one in the 1980s, so it was time for an update in the form of a systematic study to characterize it. We put together a really nice design and off we went.
CNS: What were you most excited to find?
Russell: A few things! First up, when synesthetes describe letters as having colors, they just talk about colors. For example, their letter H might be a precise shade of orange, but just orange – typically no other colors. For these odors, all but one of the synesthetes drew things that involved patterns, shapes, and often multiple colors. So odor-induced synesthesia appears to be different in some ways to at least some other forms of synesthesia.
Secondly, we found that there appears to be both a perceptual and conceptual element to the nature of their pictures. Their images were the most similar between the first and second smell session when they identified the odor as the same thing in both sessions. But if they identified the same odor as, say, vanilla in one session and strawberry in the second session, then the images were a bit more different.
Finally, when we compared the images from two completely different smells (which were identified differently), those images were even more different. So, the smell itself plays some role in how the image looks, but their identification [the naming]is also important.
Some researchers suggest that synesthesia is purely about sensation and not cognition, so this finding suggest that, at least for this form, cognition is important.
CNS: The title of the paper is quite evocative – can you break down what it means in terms of your study’s results? Can you give an example of another interesting image that came out of the study?
Russell: For one synesthete, when she smelled chocolate, she drew a pink and stripy shape. For another, chocolate was completely different. Some drew actual physical objects (walls, logs), others described things like hazes or mists. Just talking to them was fascinating, and for them, it was interesting to talk to us because we actually had an insight into their experiences.
CNS: What is the significance of your findings for the general population? Does the work with synesthetes inform our thinking about the senses, in this case smell, for non-synesthetes?
Russell: This is a good question! Our study aimed to help us understand more about this particular type of synesthesia, but whether or not our findings extend to other forms of synesthesia remains to be seen. While we found some unique aspects to this form, there are also some similarities with other forms. We hope that this study sheds new light on studies on these other forms.
What does it mean for non-synesthetes? We won’t know until we have a better understanding of what exactly synesthesia is. However, we think that synesthesia as a whole builds on subtle mappings we all have across the senses, such as brighter lights matching higher pitched tones. There are some unanswered questions about olfaction and it is possible that this form of synesthesia could give us insights into how olfactory perception links with our other senses.
Hopefully this study contributes to our understanding of synesthesia and to perception in general.
CNS: What’s next for this work? What do you ultimately hope to accomplish?
Russell: We have completed this particular study, but we have future studies in mind. Let us know if you experience olfactory-visual synesthesia and you would like to take part! Anina Rich continues her work into all forms of synesthesia, and I hope that we’ll all have the opportunity to collaborate again as it was a fascinating project. In the meantime, Dick and I continue on other olfaction studies.
If you are interested in synaesthesia, Anina Rich heads up the Synaesthesia Research Group. They have an online questionnaire and research participation page. I would also suggest looking at Sean Day’s excellent page.
-Lisa M.P. Munoz