01 October 2001
How smells may influence your brain and your behavior, whether you know it or not
There are many ways our brains gather information about the world around us. Some of those ways are through the things we call senses: sight, sound, touch, taste, smell. If you asked most people which sense they’d least like to lose, they’d probably say sight. Many people would also say they wouldn’t be too upset about losing their sense of smell. In fact, judging by the amount of money people spend on deodorants, you might sometimes think people would prefer never to smell anything at all.
One reason people think their sense of smell is unimportant is that a lot of its work goes on without their being aware of it. The systems in the brain that handle intellectual skills that humans have but other animals don’t, such as language, evolved very recently. The brain’s smell centers evolved much longer ago. So did the brain’s memory and emotion centers. In the brain, your smell system has a more direct connection to memory and emotion than any other sense. So smells can affect your emotions or trigger memories that you might have thought were long forgotten. Connections between the brain’s smell and language centers are weak and indirect. That’s one reason it’s so hard to describe in words how something smells.
Try it. Pretend that you’re talking to a Martian who speaks English but has never smelled cheese, or a flower, or a cake baking in the oven. You can always try to describe a smell in terms of other smells ("fishy"; "like dirty socks"; "moldy") or in terms borrowed from other senses such as touch or taste ("sharp," "sweet"). But it’s hard to describe a smell on its own terms.
You may be led by the nose, but what you smell depends on your brain
But wait a minute. Isn’t smell in your nose, and not in your brain? Not really. It’s your brain that’s responsible for detecting and interpreting odors. Usually, people think of their senses as located in parts of the body such as the eyes, ears, tongue, or nose. But that’s only partly true. Your eyes, for example, don’t really "see" anything at all. Their rod and cone receptors pick up and relay information that the brain interprets in terms of lightness, darkness, shape, color, and texture. Some people with color blindness, or even complete blindness, have perfectly well-functioning eyes but lack the properly-functioning brain regions to process and interpret data from their eyes.
The fact that something is hard to talk about doesn’t necessarily mean it’s unimportant, though. There’s a lot of evidence that our sense of smell is very important. The main smell center of our brain, called the olfactory bulb, is almost the only part of the human brain that has its cells replaced all the way from birth to death. Just as a shark keeps growing new teeth when any are lost, the part of the brain responsible for interpreting scent has its neurons freely replenished even into old age. So it would be reasonable to think that there’s something special about our ability to detect odors. Indeed, people who suffer from anosmia — an inability to smell — stand a particular risk of injury, sickness, or death due to the fact that their brain’s smell center can’t warn them of dangers such as the smell of gas or fire, or the odor of rotten food.
Smell can influence our brain, and our behavior, in ways we might not even be conscious of. It’s well documented that air-borne chemicals influence our behavior without our being aware of smelling anything at all. Researchers have even recently obtained brain-scan images showing that certain parts of our brain, including brain structures that control emotion and memory, become activated in response to an airborne compound at such a low concentration that we have no conscious awareness of it.
Evidence for a sixth sense: pheromones
One of the ways that animals communicate is by way of pheromones — airborne chemicals produced by one animal and detected by another of the same species. Pheromones influence the behavior of the animal that senses them. For example, androsterone, a hormone produced by male pigs and expelled in their breath, makes female pigs eager to mate. Obviously, perfume and cologne manufacturers would love to find chemical compounds with similar effects on humans. But many researchers have claimed that humans have lost the receptors needed for the brain to receive any pheromone-conveyed messages from our senses.
There’s an organ that seems to be specialized for the detection of pheromones in other animals. This organ, the vomeronasal organ (VNO), is, in humans, connected to the nasal passage by a small opening about an inch behind the nostrils. It’s so small that it wasn’t even discovered until the 18th century, and for a long time after that the human VNO was thought to be an organ whose function had been lost during evolution. So it made sense that pheromones couldn’t play a role any longer in human behavior.
More recent evidence shows that the VNO is intact and functioning in humans, and that it probably does indeed serve to send pheromone-carried signals to the brain. Women rate the male pheromone androsterone (yes, the pig pheromone is also produced by men) as more pleasant-smelling near ovulation, when they’re fertile, than at the beginning or end of their cycle. And pheromones may be influencing the behavior of women whether or not they’re aware of it. In one experiment, researchers sprayed androsterone onto a seat in a dentist’s waiting room, and observed that women were particularly eager to sit there.
Good-smelling may be more important than good-looking
We all know how we can like or dislike the way that someone smells. Sometimes, you might even like someone because of the way they smell, even though you might not be aware that it’s their smell you’re attracted to. Some research has found that women tend to like the smell of men whose genes matches their own in a way that would result in healthier children.
Another recent smell study ties in with evidence for the bizarre fact that women prefer symmetrical men to men with more lopsided faces or bodies. Why? It may be that lopsidedness is a sign of more important imperfections that might harm a mate’s longevity, fertility, and health. Believe it or not, women seem to be able to identify the symmetry of a man through scent alone. In this study, when presented with T-shirts to sniff, women near the peak fertility phase of their cycle preferred the scent of T-shirts that had been worn by symmetrical men. In other words, during times when they would be liable to get pregnant, their sense of smell tended to lead them to good mates, genetically speaking. During other phases of their cycle, or if on a birth control pill, they showed no preference.
So your sense of smell is nothing to sniff at. In more ways than you would think, it affects your behavior, your emotions, and your memory. It may even lead you to your boyfriend, girlfriend, husband, or wife.
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