Recent research found that a diminished sense of smell predicted frontal lobe damage in 231 soldiers who had suffered blast-related injuries on the battlefield. In the Department of Defense study led by Michael Xydakis of the Uniformed Services University of the Health Sciences, subjects with low scores on a smell test were three times as likely to show evidence of frontal lobe damage during brain imaging than those whose sense of smell was normal.
When the sense of smell is working properly, it acts as a matchmaker between odorant molecules in the air and memories stored in the brain. Those memories are not housed in a single place, Xydakis says, but extend across many regions. Because different smell signals have to take a variety of paths to reach their destinations, arranging their travel requires a lot of coordination. “This unique feature makes an individual's ability to describe and verbally name an odor extremely challenging and cognitively demanding,” he says.
A damaged sense of smell, therefore, can indicate that the ability to make those connections has been hampered by disease, a lack of sleep or, as shown in Xydakis's study, injury to the brain. The new results add to a growing understanding of the link between brain damage and an impaired sense of smell. Researchers have been working for years to use olfaction tests to track damage to the brain caused by neurodegenerative ailments such as Parkinson's and Alzheimer's diseases.
Kim Good, an associate professor in the psychiatry department at Dalhousie University in Nova Scotia, is currently recruiting subjects for a cohort study that aims to better understand the link between olfaction and Parkinson's, which could improve early identification and intervention. “Olfactory deficits are as common as tremor in Parkinson's, and they help rule out other competing diagnoses,” Good says.
Smell is also the first sense to be affected by Alzheimer's, with the hallmark protein tangles of the disease appearing early in the olfactory bulb, says psychiatrist Davangere Devanand of Columbia University. Last January he and his colleagues reported the results of a four-year-long cohort study in Manhattan, which found that scores on a multiple-choice scratch-and-sniff test in which participants had to identify 40 scents were good predictors of cognitive decline.
It's not hard to imagine such exams becoming a routine part of primary care for older patients. “The beauty of olfaction,” Good points out, “is that testing is easy and can be done in the family physician's office.” —Ian Chant
Why Smell Is Special
The unique characteristics of our sense of smell make sniff tests ideal for diagnosing brain injury. Here are some of the most interesting scientific findings about this unusual sense:
- The adult brain can generate new neurons in the olfactory bulb, the brain region that processes smells. This area is one of just a few regions that continue to grow new neurons during adulthood.
- Individuals vary in how they perceive odors and whether or not they can detect certain scents, and yet humans seem to universally enjoy the smell of vanilla.
- Anosmia, a condition in which people completely lose their sense of smell, can be debilitating. Sufferers often report feeling disconnected from their surroundings, and many become severely depressed.
- Romantic couples can unconsciously sense their partner's emotional state from their sweat—and the longer they have lived together, the better they are at it.
- Babies locate their mother's nipples in part by learning a smell map of the breasts.