Smell Stimulates Early Visual Processing in Women But Not in Men

Smells are undeniably powerful, able to transport us to different places and times in our memories. Think of how you feel when you smell cookies baking in the oven. But can they also change how we see things? New research shows that smells can enhance visual processing – but only in women, not men.

“The odors in our study might have enhanced attention toward the matching visual objects, or made them easier to discriminate from other objects, suggesting a possible role for the sense of smell in identifying visual objects,” says Amanda Robinson of the University of Queensland, who led the study with Judith Reinhard and Jason B. Mattingley.

In the experiment, participants briefly saw pictures of familiar objects and either inhaled a matching odor, non-matching odor, or plain air. The odors and matching objects corresponded to oranges, mint leaves, and roses. The non-matching pictures were a box, cap, and folder. The researchers recorded their ERP (event related potential) brain responses using electroencephalography (EEG). “Our study was designed to determine how odors influence the processing of matching and non-matching visual objects, and whether or not the color of an object modulates any such effect.”

“Importantly, participants never responded to the odor or indeed to the object image,” Robinson explains. “Instead, we had them perform an irrelevant shape-discrimination task, so that any observed effect in the EEG signals could not be attributed to cognitive factors.”

As published in the Journal of Cognitive Neuroscience, when participants viewed an object that matched the odor they were inhaling – for example seeing an orange and inhaling the smell of an orange – brain activity enhanced a mere 150 milliseconds after seeing the object. The enhanced activity was the N1 ERP component, which is associated with spatial attention and visual discrimination processes.

But surprisingly, this N1 spike happened in women and not men – indicating that the influence of odors on visual processing is gender-biased. Robinson spoke with CNS about this gender difference and the work’s significance for understanding how our various senses interact.

Very few studies to date have focused on whether olfaction and vision interact in the way that vision, audition and touch do. Indeed, this is the first study to show that smells can influence an early stage of visual perception.

CNS: How did you personally become interested in studying this topic?

Robinson: The sense of smell is fascinating, yet it has received relatively little attention in human research compared with the senses of vision, audition, and touch. When we started this project, we were interested in the ways our sense of smell might influence cognition and behavior. Odors have very special connections with emotion and memory, and are important for flavor perception, but olfaction seems to operate mostly below consciousness. The interactions between olfaction and the other senses have been largely ignored. We were intrigued by the idea that the sense of smell might influence the dominant visual sense, and embarked upon a number of studies to investigate this topic.

CNS: People often do not think of smells and sights going together; they think of them separately… Prior to your study, what did researchers know about those processes in the brain?

Robinson: The senses of smell and vision are often considered independent, and indeed they have anatomically distinct brain pathways. Yet, we are constantly smelling and seeing concurrently; when eating or walking outside, for example, we are exposed to a varied landscape of sights and smells. It is therefore not surprising that these two senses interact in the brain. It has long been known that vision can influence odor perception. For example, in one famous study by Morrot and colleagues, wine experts were asked to describe the aroma of a white wine that had been colored red. All used traditional red-wine descriptors to describe the aroma (e.g., “raspberry”, “cherry”), suggesting that their perception had been influenced by the color of the wine. Whether smells can also influence what we see, however, has not received much attention to date. In recent years, studies have shown that odors can influence vision in behavioral tasks, but until now it was not known just how early these processes interact in the brain.

CNS: What were you most excited to find in your results?

Robinson: We expected, based on previous research, that odors would influence an early stage of brain activity in visual areas when odors and images matched. Indeed, we found enhanced responses to matching images (e.g., an image of an orange paired with orange odor) just 150 milliseconds after the image appeared.

Surprisingly, however, the color of the image did not seem to influence the interaction between smell and sight. An odor had no effect on processing of a non-matching object with matching color (e.g., rose odor and red box). Furthermore, an odor enhanced brain activity in response to matching objects regardless of whether they were in a matching color or in black and white. This color-independent effect is particularly interesting because the link between colors and odors is quite robust, and colors certainly influence odor perception. Clearly, although smell-sight interactions are bidirectional, odor-driven visual enhancement is based on complex object representations rather than specific odor-color associations.

A second somewhat unexpected finding was the large disparity in the effect between males and females. We anticipated that females would exhibit a larger multisensory effect than males due to gender differences in the sense of smell. Indeed, we found that females demonstrated marked odor-image interactions. Instead of a smaller effect in males, however, we observed no effect whatsoever of odors on visual ERPs. Evidently, gender differences in olfaction can lead to differences in multisensory interactions even for vision, a sense for which there are few documented gender differences.

CNS: Why do you think there was a greater effect in women than men?

Robinson: In general, women are more sensitive to odors than men, and perform better on most tests of olfactory perception. Recent evidence also suggests that there are gender differences in the anatomy of olfactory regions of the brain. Until now, however, there has been little focus on whether gender differences in olfaction also have differential effects on other processes.

Our results show that multisensory interactions between olfaction and vision are markedly different in males and females, to the point that odors exert effects on visual processing in females that seem to be absent in males. It could be that women allocate more attention to odors or that males are more distracted by odors, but this remains to be tested. One intriguing possibility is that females might utilize different brain regions for olfactory identification. Nevertheless, the gender differences in our results are not due to differences in general olfactory sensitivity.

CNS: How does your work fit in with past work on multisensory integration?

Robinson: A large body of work has focused on multisensory interactions for stimuli that are semantically related. In this context, our research is a continuation of previous work looking at how learnt multisensory features of objects – e.g., shape, touch, smell, sound – are integrated in the brain and influence perception. There is growing evidence that multisensory integration occurs at a relatively early stage of perceptual processing, sometimes even within primary sensory areas of the brain. Very few studies to date, however, have focused on whether olfaction and vision interact in the way that vision, audition and touch do. Indeed, this is the first study to show that smells can influence an early stage of visual perception.

CNS: What is the significance of your findings for how we think about our senses and their interactions?

Robinson: We are constantly integrating information from the different senses to get a feel for the world, but our sense of smell is often overlooked. This research shows that odors can influence basic visual processes, and thereby indicates that what we smell has a large influence on perception in general. Such a result highlights the involuntary and ubiquitous role of multisensory integration in perception.

CNS: What’s next for this work? What do you ultimately hope to accomplish?

Robinson: In the future, we hope to elucidate the brain areas responsible for integration between smell and sight. Our research to date suggests that olfaction and vision interact at an early stage of perception, possibly in an involuntary manner. Whether or not this takes place in sensory areas of the brain – such as the primary visual cortex or olfactory cortex – is still a mystery. Ultimately, we would like to determine the process by which different sensory features of objects – for example, smell and shape – come to be associated, where this happens in the brain, and how such associations function to facilitate object identification.

-Lisa M.P. Munoz