Why do some foods taste better than others, and why do certain aromas appeal to us, while others make us hold our noses? Learn the answers to these questions and more at the event, “How Your Brain Distinguishes Taste and Aromas,” next Tuesday, May 6, in Washington, DC. The event, co-hosted by AAAS and the Dana Foundation as part of the Neuroscience and Society series, is free and open to the public, but you must register.
We traditionally refer to five senses: sight, hearing, taste, smell, and touch. But it’s not that simple. Our new primer, on the senses, delves into the complex systems that enable us to connect to the world.
It’s a dynamic process. The brain is not simply a receiving station for sensory signals, and what we see, hear, and feel are constantly shaped by emotions, memories, moods, and beliefs. Our sense of the world is a creation of the brain, and the same physical sensation may be experienced quite differently at different times of life, and even from day to day.
Read Part I of the primer now; Part II will post on the Dana Foundation homepage on Monday, August 26.
– Ann L. Whitman
It’s lunchtime. You heat up those Chinese takeout leftovers in the microwave and the aroma makes you hungry. Then you take a bite—delicious.
Clearly, you process the smell from the microwave through your nose. But did you know that processing flavor also comes from your sense of smell?
In his new book Neurogastronomy: How the Brain Creates Flavor and Why It Matters, Dana Alliance Member Gordon M. Shepherd explains that we actually have two senses of smell: orthonasal and retronasal.
Orthonasal smell is what happens when we sniff—we breathe in through our noses to sense environmental odors, like the smell from the microwave.
Retronasal smell, on the other hand, is where flavor comes from. Try holding your nose while eating some of those leftovers—you won’t taste much. Breathing out while eating is the doorway to flavor. While we are born recognizing sweet, sour, salty, bitter, and umami—which are tastes, not flavors, writes Shephard, “retronasal smells are learned and thus open to individual differences. They account, therefore, for the vast variety of cuisines in the world.”
I had my first colorblindness test in seventh grade. Well, it wasn’t really a formal colorblindness test so much as a side effect of a science class in which we were discussing vision.
“Look at the picture in your textbook,” the teacher requested. “What number do you see?”
“17,” we answered in near-unison.
“No…” started one boy hesitantly. “It’s a two.”
You can imagine what happened next. As middle schoolers are all too wont to do, we each turned simultaneously to stare at him, our mouths agape. After further clarification that he really did see a 2 despite trying his hardest to see the 17, our teacher confirmed what we already suspected.
“Erik, I think you’re colorblind.”
He looked as if he were about to cry, and I can imagine why. He had just discovered (in front of all his peers, no less) that he was markedly different—that there was something wrong. Something wrong with him. But in reality, nothing was wrong with him, per se. Most likely, a gene on the X chromosome of his ancestral line had mutated decades ago and had been passed down through generations until it manifested itself in him as anomalous trichromacy, a mild color-blindness disorder that primarily affects males. But despite the innocuous nature of color blindness, it has serious implications for his perception of reality. After all, he saw an image that nobody else in that room could perceive. And what is reality, really, if not our perception of sensory information?
On Wednesday night, Dr. Richard Axel of Columbia University began a talk, part of a lecture series presented by the Columbia University Mind Brain Behavior Initiative and sponsored by the Dana Foundation, by arguing just that. He opened with an idea that is, at first, a hard pill to swallow. “Your reality does not exist outside of your brain.” It may seem simple on the surface, but thinking deeply about this question makes me very uncomfortable. How is my world different from everyone else’s? What am I incapable of perceiving? What am I perceiving incorrectly? Am I living a lie?! (OK, perhaps a bit dramatic.)
Axel explained that information from our environment enters our brain through our five senses. Our brain synthesizes this information to create a reflection of the outside world—this synthesis is our reality. And if our brain is constructed based on instructions issued by our genes, then it follows that we are trapped in terms of the type of reality we can perceive. We don’t choose our genes, and because of genetics, Erik could perceive something I could not and vice-versa. Pythons, Axel pointed out, have an extra sensory system—infrared—that allows them to perceive things in the environment that we cannot.
Axel studies one particular sense, olfaction, to attempt to address the question of how the brain uses sensory information to form our subjective realities. Humans sense odors through olfactory receptors located in our noses, just below the spot where a pair of sunglasses would sit. We have approximately 400 functional genes that encode around 1,000 different types of olfactory receptors (note that 1,000 represents the number of variations of receptors; we have approximately five million receptors in total). And here’s the thing: Those 1,000 types of receptors are identical from person to person. We all have the same receptors that are stimulated by the same odors with the same molecular structures. Furthermore, those odors activate certain parts of the brain in exactly the same way from person to person.
So why can one person hate one smell while another loves it? Take, for example, vanillin. Vanillin is a phenolic aldehyde that is composed of eight carbon atoms, eight hydrogen atoms, and three oxygen atoms, but it is more commonly recognized as a smell of scented candles or the flavor of vanilla ice cream. Its structure is easily recognized and therefore easily quantifiable. When I smell vanillin, it activates my olfactory system in precisely the same way that it activates yours. I love the smell, but I know people who can’t stand it.
Axel said he will spend the rest of his life trying to find out how the brain blurs and distorts objective sensory input (like the shape of an odor molecule) into a subjective sensory experience (“Whoa, that smells terrible.”). Unfortunately he had no answers for us at last night’s lecture, but some interesting questions lingered. While loving one smell and hating another might seem bland, there are other aspects of our environment to consider—what sensory abilities do we lack? Why can animals sense an oncoming natural catastrophe? What is there that we can’t see? How different is my reality from yours?
On the other hand, you can simply ignore these questions and continue living comfortably. After all, it’s your world—I just live in it.
Thanks to a stuffy nose, I’m not smelling much of anything right now (and based on the sneezes I hear on the subway, I’m not the only one). But when I’m not congested, the odors I like and don’t like are somewhat off-kilter: Gas stations smell really good to me, while lilies smell like death.
An article from Scientific American explains a possible reason for my skewed aroma preferences: The many genetic combinations that code our olfactory receptors differ markedly from one person to the next. One person may not even be able to detect a scent, while another may find it particularly intense.
Experience may play a large role, as well. In 2009, Andreas Keller, one of the scientists quoted in the article, reported on research into the variability of odor memories for Current Biology. Scientists from the Weizmann Institute of Science determined that the hippocampus, a memory center of the brain, likely plays a role in the formation of strong associations between scent and memory. Early associations—even those formed in utero and while breastfeeding—will persist, while later ones will not be as strong. This may be one reason for strongly held cultural scent and food preferences
Dr. Keller is one of several researchers now studying the inability of people with schizophrenia to identify some common scents; this dysfunction is likely due to the brain regions impacted by the disorder, which are also connected to the ability to process smells. Like those with schizophrenia, people with bipolar disorder also appear to have difficulty identifying odors, rating scents as being more pleasant that people without the disorder.
Want to know more about how well people smell? My colleague, Ann Whitman, wrote about the topic for this blog last year. You can also check out a 2001 Cerebrum story on olfaction, which explains why some people like the smell of skunks.