Scents Sense: Olfaction, Memory and the Capabilities of the Brain
The human nervous system is made up of three overall types of neuronal connections. These connections link sensory neurons to the rest of the nervous system, the nervous system to motor neurons, or neurons within the nervous system to other neurons in the nervous system. Inter-neuronal connections are by far the most numerous of all connections in the nervous system, while sensory neuron connections are relatively sparse. Because of this disproportionate number of connection types, it is essential that the human brain be able to derive complex reactions from very few sensory inputs. The link between olfaction and memory provides a truly remarkable example of this ability. Olfactory memory is made up of two parts: the brain’s ability to recognize and classify scents, and the ability of scents to influence memory and evoke emotion without additional context. Whenever a wine taster is able to identify the scents of blackberry or chocolate in a wine made only of grapes and water, she is using olfactory memory. When the smell of pine trees and cinnamon makes you think of Christmas regardless of the season, you are also using scent memory.
Our sense of smell, though not as simple as our sense of taste, is very simple compared to the rest of the processes of our nervous system. Olfactory receptor proteins are able to recognize physiochemical molecules called odorants and relay information to the brain. Approximately one thousand of these proteins have been characterized. Each one is only able to recognize one odorant, yet we are able to experience and characterize many more than one thousand unique scents (1). Once an odorant binds to a receptor protein, the massage is sent via olfactory neurons in the olfactory epithelia to the olfactory cortex in the brain. These olfactory neurons only survive for approximately 60 days before they are replaced with new neurons (2). Olfactory neurons are also unmyelinated, making scent the slowest of the five senses and allowing the sensation of odor to persist for a relatively long period of time (1). It is perhaps the proximity of the olfactory bulb to the limbic system that gives scent such a strong influence over memory and emotion. The limbic system, specifically the amygdala and the hippocampus, are associated with processing emotion and memory (1).
Olfaction, either paired with other senses or on its own can be a very powerful accessory to the formation of many kinds of memory.
Autobiographical memory is enhanced more by scent than by any of the other senses, beginning even before birth. People with mothers who smoked while pregnant tend to be less repulsed by the smell of cigarettes than the children of non-smokers. This is also true of many scents present during gestation or early childhood (3). It is thought that this link may have been advantageous in helping our ancestors recognize their kin or identify which foods were and were not good to eat, among other necessary benefits (4).
Producers are using the link between olfaction and memory to their advantage. In one study, a group of students were given scented pencils along with a bulleted list of the pencil’s selling points. Another group was given unscented pencils and a similar list. Two weeks later, the group given scented pencils could remember on average more than three of the selling points, while the group given unscented pencils could barely remember one (5).
Olfactory memory enhancement may even help with skill and information learning. Studies have shown that associating scents with the material a person is studying and then exposing that person to the same scent while they processed the material in their sleep increased their recall test scores (6).
Most of the research I’ve come across deals with structured tests of the way olfaction works or the way it can be used to enhance memory and recall. However, I began researching this topic with more of an interest in the ability of scents to evoke powerful, even visceral emotions and memories without any of the other sensory context associated with the emotion or memory. It appears that, while researchers have been able to document the brain activity associated with this phenomenon, they have yet to satisfactorily explain how or why it happens. I find it very interesting that the process of experiencing the way something smells is essentially so simple and yet the response that experience can cause in the brain can be so complex. It makes me wonder whether the other senses are as necessary as we believe them to be. Scents can conjure up “phantom stimuli” from the other senses in order to paint a very vivid picture in our minds. I think it must be necessary for the other senses to contribute to the input that forms a memory, but it seems that those senses are not necessary for accessing that memory once it’s been made. For instance, if a family member passes away, the immediately accessible memory of their face or of the sound of their voice begins to fade with time. However, catching even a hint of the scent we associate with that person can cause immediate, involuntary, highly detailed recollection of many aspects of that person. How is it possible that this one sense is capable of that kind of phenomenon when the others don’t seem to be?
I am also curious as to why scent seems to be the most weakly connected to language if they play such an essential role in the way we perceive and learn about our world. That is, why is it that we can distinguish between an impressive number of scents, but we are woefully inadequate at identifying them or even describing them? Is it because scent is hard-wired to a portion of our brains that evolved before our use of language did? Or might it have something to do with the face that scent automatically calls up a great deal of information, so much so that it is unnecessary to try and communicate that information with words?
While it would probably take me years of research to answer these and other questions I have on this topic, I think it is fair to say that the power of the human sense of smell is not to be underestimated. It is just one of the many hints as to the true power and complexity of the human brain, one that no doubt will continue to provide insight into the way we behave.
1. Wilson, DA. (2003) The fundamental role of memory in olfactory perception. Trends in Neurosciences, 26(5), P 244.
2. Pines, Maya. “The Mystery of Smells: The Memory of Smells.” Seeing, Hearing and Smelling the World: A Report from the Howard Hughes Medical Institute 2008. Howard Hughes Medical Institute. Available at <http://www.hhmi.org/senses/d140.html>
3. Dowdey, Sarah. "How Smell Works." 29 October 2007. HowStuffWorks.com. <http://health.howstuffworks.com/smell.htm>
4. Dill, Amanda James. “A Look at the Link Between Olfaction and Memory.” associatedcontent.com 6 July, 2007. Available at <http://www.associatedcontent.com/article/237908/a_look_at_the_link_between_olfaction.html>
5. Mindlin, Alex. “It’s the Scent that Tickles the Memory.” New York Times 15 Nov, 2009. Available at <http://www.nytimes.com/2009/11/16/business/16drill.html>
6. Carey, Benedict. “Study Uncovers Memory Aid: A Scent During Sleep.” New York Times 9 March, 2007. Available at <http://www.nytimes.com/2007/03/09/science/09sleep.html>