Remembering Smell (19 page)

So lovely, and so familiar. Proust at first couldn't remember the last time he'd smelled this scent. Then it came to him—yes, it was in Combray, the French village of his lost childhood. As a boy he'd made frequent visits to his invalid aunt who lived upstairs in his parents' house and who always treated him to tea and a cookie. Decades later, when Proust's mother offered him the same tea and cookie (in part to provide her son with a brief respite from a melancholic state he described as the deadening effect of dull habit), his forgotten childhood returned. The tea and cookie released old memories as if they'd been captured in a frozen waterfall and then freed by a spring thaw. A wave of euphoria came over him, and then the memories. Torrents of words followed. Proust filled thick volumes with the memories that his adult mind, for reasons both mysterious and practical, magically set free.

The strongest odor-induced memories tend to be of moments of delight. These memories are not quite the same neurologically as memories that seem indelible because one was intensely "present" and alert at the time of the event; memories that haunt war veterans fall into that category. Both the nostalgic and the intense types of memories may be triggered by an odor, but the sudden recollection of traumatic battlefield memories can be problematic. Marcel despised the smell of varnish because it took him back emotionally to those nights when he was a small boy and his mother sent him upstairs to bed without a good-night kiss. That detestable wooden staircase, icon of his banishment, was a potent trigger for separation anxiety all his life—not the staircase itself, but its smell. And yet, though Proust hints at a troubled childhood, his most strikingly detailed odor-induced snapshots are of good times. It's as if the brain intentionally filters out emotional triggers that might add up to an accumulated burden of sadness too heavy to bear.

On the strength of a sniff—okay, a few sniffs—Proust emerged from a long and incapacitating struggle with depression and what would now be called obsessive-compulsive disorder (even those who know nothing else about Proust are usually aware that he lived most of his adult life in a cork-lined room). His mind finally focused on something outside his immediate circumstances, this familiar smell, this all-powerful joy. After several false starts (and as the tea-soaked cookie lost its power), the memories took shape even without the odor, restoring his hope and happiness by rekindling his imagination. The creativity that had been so unselfconscious in childhood brought his "true self" back to him. It endured until the end of his writing life.

There are skeptics, of course. Some dismiss Proust's tale of remembrance as pure fabrication, arguing that he'd been casting about for a fashionable literary topic and seized on Freud's theory of the unconscious mind. Non-navel-gazing storytellers, such as Steinbeck, Flaubert, and Hemingway, who render smells in gorgeous prose, are olfaction's real champions. Science writer Jonah Lehrer disagrees. He thinks Proust may indeed have been influenced by the work of Freud, but he was also influenced by hard science. Up until 1900, scientists thought neurons were linked together, forming a sort of meshlike net. Then the flamboyant Spanish scientist Santiago Ramón y Cajal "stared at thin slices of brain under a microscope and let his imagination run wild," wrote Lehrer in
Proust Was a Neuroscientist.
In a classic example of top-down science, Cajal guessed that the spaces between neurons, called synapses, were the communication centers for electrical signals throughout the brain. He was right. Did Proust, hearing of this, let his imagination run wild, as Cajal's had? Subtle shifts in synapse strength ease the communication process that allows neurons to trigger memories, Lehrer explained. "The end result is that when Proust tastes a madeleine, the neurons downstream of the cookie's taste, the ones that code for Combray and Aunt Leonie, light up. The cells have become inextricably entwined; a memory has been made."

Memory research has until recently used a scientific model based on enzymes and genes; mechanisms could be studied only through a process of training lab animals, "bullying" their neurons into altering their synaptic connections, as Lehrer described it. "Senseless repetition seemed to be the secret of memory." But while such experiments do show how the brain adapts, they don't "encapsulate the randomness and weirdness of the memory we live in," the way memories "change and float, sink and swell [and] disobey every logic" and defy our ability to "know what we will retain and what we will forget."

In 2003, Kausik Si, a researcher for whom Lehrer worked in Nobel laureate Eric Kandel's lab at Columbia University, published in
Cell
what Si hoped would be a breakthrough theory of memory. Si had earlier discovered that a certain protein called a prion can erase or create memory by changing shape. Prions are stimulated by serotonin and dopamine, two neurotransmitters produced by thinking. How could Si resist hypothesizing a link between such renegade particles and the kind of memories that "disobey logic"? Having been activated by the first taste of, say, a madeleine dipped in linden-scented tea, the protein marks a specific dendritic branch as a memory, Lehrer wrote.

The protein will patiently wait, quietly loitering in your synapses. One could never eat another madeleine, and Combray would still be there, lost in time. It is only when the cookie is dipped in the tea, when the memory is summoned to the shimmering surface, that [the protein] comes alive again. The taste of the cookie triggers a rush of new neurotransmitters to the neurons representing Combray, and, if a certain tipping point is reached, the activated [protein] infects its neighboring dendrites. From this cellular shudder, the memory is born.

Gordon Shepherd dug up an old article for me to read in which he outlined why Proust's successful efforts to evoke the smell of the tea-soaked cookie were in sync with what scientists have come to understand about the neurology of smell. Scholars had tended to regard this effortful conjuring as Proust's prerogative to describe the episode as he saw fit. He was an artist, not a scientist. The suspenseful telling only made the episode, and its essential insight, more powerfully thought-provoking. Now scientists know that Proust's recollection of his recollection was spot-on.

The narrator, Marcel, freely admits that intellect did the heavy lifting after smell waltzed by. When he seems to later disqualify intellect's power, Proust is really just asking his readers to focus on the power of memory itself, the way it galvanizes our artistic souls by bringing us back to childhood. By dramatizing the
difficulty
of relying on intellect to stimulate memory, he shows us the breach, created by the thinking brain, between present and past, between our true selves and the strangers we so often become.

A
BSENCE OF SMELL
was better than phantosmia after all. A brain filled with vile odors can't keep track of the car keys, let alone ask reasonable questions. Questions raised by phantosmia, for instance. The questions were becoming more persistent, like early-morning dreams, the most vivid and usually horrific, that you can't shake off. Why did you keep falling down when the monsters were chasing you? Why did they all look like your boss?

The question nagging at me now was this: Why does the brain
care
so much about smell? It doesn't go nuts when you lose a toenail. Well, for one thing, the toenail will grow back on its own. Toenail tissue, like all human tissue except the kind in the brain, quickly regenerates new cells. Brain cells are iffier. New research suggests that some brain cells may repair themselves some of the time, and although this may one day allow science to manipulate genes and harness neurons' ability to divide and multiply, for the moment people with paralyzing spinal cord injuries are still confined to wheelchairs. The damaged neurons, no matter how threatening their loss is to the individual's survival, do not automatically begin regenerating.
That cells in the olfactory system do have the capacity to heal suggests—even more powerfully than phantosmia does—that the brain cares a lot about smell. We think of the nose as a highway for odor molecules, but it's actually a filter. That's why a dog's snout is huge compared to a human's nose. The most hazardous airborne molecules are big and heavy, so they hover just above the ground. A dog's big snout is designed to filter them out before they can reach the brain and lungs. Gordon Shepherd thinks the human nose shrank in size as our ancestors took to walking on two legs instead of four. We don't need big filters to protect us from lightweight airborne volatiles.

Or do we? As mankind spews more and more toxic and extremely tiny particulates into the air, our nasal filters may not be up to the task of keeping our brains pollution-free. There's no telling how our brains—our genes, actually—are responding to this new type of outside threat. It hasn't escaped the attention of brain scientists that the olfactory system is the only brain function able to repair itself, and this has enormous implications for neuroscience as a whole. For one thing, it suggests that Mother Nature wasn't willing to risk exposing the brain to irreparably damaging substances capable of rising to human nose level. But not all toxic odorants are big and heavy. And people aren't always high above them. What about infants? They spend a lot of time rolling around on the ground. A young and developing brain is more vulnerable than an adult brain. Scientists now think smell cells regenerate because of such factors—and because they're being destroyed more frequently than we realize.

Moreover, while some people regard anosmia as an insignificant problem, the human limbic system may not share that bias. Its design predates
Homo sapiens,
and as far as the limbic system is concerned, a loss of olfactory function is as hazardous to a human as it is to any animal—and even more hazardous than a spinal cord injury. There is no doubt that our oldest postprimate ancestors—the first humans—depended on smell much more than we do. Even while the genes for smell were adapting to new tasks, these people relied on olfaction to sniff out both dinner and danger. They probably practiced smelling, though not consciously, just as professional noses develop their superior sense of smell through years and years of practice.

It used to be assumed that the blood-brain barrier protected brain cells from toxins that circulated through the rest of the body. The blood-brain barrier is a physiological mechanism that prevents certain substances from entering the brain, minimizing toxic exposure. But brain cells can be negatively affected by substances inhaled through the nose, so all manner of chemicals are now under scrutiny, from secondhand smoke to car exhaust to nano-size particles used to make computers lightning fast (among other applications). Scientists at the University of Rochester Medical Center showed that when rats breathed in such tiny materials, the chemicals made a beeline from the nasal cavity to several regions of the brain. Factory welders routinely inhale ultrafine manganese oxide as well as bundled "rods" of zinc, carbon, and gold particulates used in the miniaturization of electronic, optical, and medical devices. Many consumer products, such as toothpaste, lotions, and some sunscreens, also contain such particles. The University of Rochester study is part of a five-year, five-and-a-half-million-dollar investigation of the effects of nano particles on human cells; the research was launched in 2004 by the Department of Defense.

A study of children in California found that black mold, a malignant fungus that thrives in damp basements (and all over New Orleans since Hurricane Katrina), causes memory problems when it's sniffed. Researchers at Michigan State University had mice sniff minute amounts of black mold. Each mouse suffered a significant loss of smell, as well as inflammation that spread into the higher brain's memory circuits. In some, the olfactory bulb was destroyed. Newer studies report similar links between air pollutants and brain disease. Dogs who died in polluted Mexico City were found to have brains laced with lesions identical to those that cause the symptoms of Alzheimer's, while dogs exposed to rural air only were lesion-free.

Human subjects who volunteered to breathe in diesel fumes for several hours in a lab subsequently had abnormal EEGs. This diesel study points out another interesting proclivity of the smell brain: Sniffers of the toxin all agreed that the fumes smelled bad. Some said they made them feel nauseated. Sniff at your peril, our noses seem to be saying. But one person's bad odor may be another's olfactory delight. For a pair of newlyweds painting their first apartment, the happy experience may even turn a dangerous chemical—the VOCs in household paint—into an aphrodisiac.

Nevertheless, most potentially toxic cleaning agents and home-improvement products, such as bleach, ammonia, solvents, paint strippers, glues, and insulating products, are offensive to the nose. Their naturally off-putting odors have to be masked by synthetic copies of odorants such as pine, lavender, or linen. (Benjamin Moore, the paint brand, has held its own in a tough economy thanks in part to expensive new products that are low in VOCs and marketed as scentless.) What's more, as those marketing types at Procter & Gamble and Clorox know, bad smells tend to linger a long time in the air. Why doesn't the brain adapt to the bad smells—in effect, tune them out—as quickly as it does most smells? Maybe for the same reason the substances smell bad. Whether its response is innate or trained, the olfactory system is saying that danger is present.

Congenital psychiatric and perception disorders are also being linked to olfaction. It's possible that olfactory environmental factors are involved in utero, and even earlier than that.

Patients with Korsakoff's syndrome, a horrific disorder precipitated by thiamine deficiency, suffer from anterograde amnesia, an inability to create new memories; the syndrome may be the result of gaps in the normal storage process. These gaps occur when the shortage of thiamine creates a chemical imbalance that essentially bores holes in parts of the brain tissue, especially the medial thalamus and its connections to the hippocampus and cerebral cortex, the pathway that mediates movement between the biological equivalent of the RAM and the hard drive (that is, the short-term and long-term memory).