Despite the tendency of humans to underestimate the role of smell in our every day lives, for most mammals, smell is the most important sense. Dogs are probably the most obvious example of this, it is through the use of the olfactory system that animals are able to find food, reproduce, and even communicate. While being one of the oldest and important parts of the brain, our failure to fully realize the importance of the olfactory system resulted in it being surrounded by numerous questions (2). How does it work? How do we identify smells? While these are only a few questions out of a whole list, research has progressed in recent years that we know much more about the olfactory system than before, but the fact remains that much remains to be found.
Through research conducted on mice, it is approximated that humans have 1000 different sensors in their nose (3). While this might seem like a large amount of sensors, it is not enough considering mice and humans can identify about 10,000 odors. The mystery surrounding this ratio can be explained through the unique features of the olfactory system. Odors are molecular so the method used is different from light or sound that come in waves (4).
Inside your nose about the level of your eyes, is a small patch of tissue containing millions of nerve cells. The odor receptors (sensors) lie on these nerve cells. Each of the receptors recognizes several odors, and likewise a single odor could be recognized by several receptors. Thus similar to codes, what happens is that different combinations of the 1,000 receptors result in our ability to identify 10,000 different odors. Linda Buck, an associate professor at Harvard, makes an analogy of this quite efficient system to letters being used in different combinations to make individual words. She goes on to say that this system 'greatly reduces the number of sensors needed to code for the smells" (3).
The process that takes place is quite complex. After an odor molecule enters the nose and are recognized by the olfactory sensors, the signals are eventually sent to the olfactory bulb that is located right above the eyes (3). The signals only go to two areas in the olfactory bulb, and signals from different sensors are targeted to different spots that then form a sensory map. From there the signals reach the olfactory area of the cortex (smell sensory cortex) (5).
An important quality of the olfactory system is that information travels both to the limbic system and cortex. The limbic system is the primitive part of the brain that include areas that control emotions, memory and behavior. In comparison the cortex is the outer part of the brain that has to do with conscious thought. In addition to these two areas, information also travels to the taste sensory cortex to create the sense of flavor (2). Because olfactory information goes to both the primitive and complex part of the brain it effects our actions in more ways than we think.
Many wonder how certain smells able to trigger memories of events taking place several years ago despite the fact that sensory neurons in the epithelium survive for about only 60 days (1). The answer is that the neurons in the epithelium actually have successors. As the olfactory neurons die, new olfactory neurons generated by the layer of stem cells beneath them, which eventually takes the role of the old neuron as it dies. Linda Buck points out that the key point to the answer is that "memories survive because the axons of neurons that express the same receptor always go to the same place" (1). The memories are stored in the hippocampus, and through relational memory certain smells trigger memories.
Another popular question is the reason behind smell having such a strong role in instantaneously recalling memory. Despite our belief that sight and hearing are the two most important senses to our survival, from an evolutionary perspective smell is one of the most important senses. To recognize food or to detect poison, smell is the sense that almost all other mammals use. Because of this basic feature yet vital role, smell is one of the oldest parts of our brain. Trygg Engen, a psychology professor at Brown University notes that smells serve as "index keys" to quickly retrieve certain memories in our brain. This primitive yet essential role is probably why smells trigger memory more than does seeing or hearing.
Professor Engen goes on in attempting to further explain the relation of odor and memory. His controversial views basically states that the way we sense odors are all results of "nurture" and not "nature" (6). He says that initially all smells are neutral, and that whether a odor is pleasant or unpleasant has to do with the initial condition in which the smell is perceived. It follows from this that when we smell odors, it triggers a certain memory that has to do with that particular odor and thus is decided whether it is pleasant or unpleasant. Engen's views are controversial because of the lack of convincing data to back his views up. Although certain points about Engen seem to make sense, such as how odor serve to trigger memories like index keys, his views about the "nurture" vs "nature" are a little harder to understand. If odors are decided by "nurture", it leaves the question of how so many people have a similar view towards many odors. There is probably nobody who would say that the smell of rotten food is good. Yet Engen's views are definitely worth considering because for some odors like gasoline, some people say it is good while others detest it.
It is said that people can identify about 10,000 different smells, but have many smells can you name off the top of your head (3)? In comparison, look at how many colors there are in a crayon box, or the many varieties of music existing. This lack of understanding and appreciation of odors is a result of our over reliance on our eyes and ears, even to the extent that we suppress our awareness of what our nose tells us. Our underestimation of the role of smell results in our lack of extensive knowledge concerning many aspects of the olfactory system. But as Proust stated, smell has such a strong power to vividly bring back memories, it is definitely more important than we realize. To a large extent smell is more personal than other senses so it brings back memories of people, not just places, or things.