Sex Sleep Eat Drink Dream (4 page)

The secret of caffeine's power as a stimulant is this: The drug binds tightly to the body's receptors for adenosine, a natural chemical important in sleep and wakefulness. As the cells in your body use up energy, they make adenosine as a byproduct; the harder they work, the more of the chemical they make. The adenosine attaches to receptors on cells everywhere in the body and quiets their activity. In this way it calms heartbeat, lowers blood pressure, decreases the release of stimulating neurotransmitters, and induces sleepiness. Caffeine enhances alertness by binding to adenosine receptors and inactivating them—thus preventing the chemical from exerting its quieting effects. So snug is the drug's fit with adenosine receptors that it's powerful even at low doses.

Caffeine works, then, not by exciting our nerve cells but by foiling the process by which they are calmed. Whether it actually perks up our brain function remains a topic of debate.

In 2005, a team of Austrian scientists used fMRI to watch caffeine's action in the brain. Prior to the test, a group of volunteers abstained from coffee for twelve hours. Then half the group drank two cups of coffee, and the other half drank a placebo. After twenty minutes, the subjects underwent fMRI scans while performing tasks involving memory and concentration. In all the participants, the brain regions involved in short-term or working memory fired up. But those who ingested caffeine also showed greater activity in the parts of the brain involved in attention and concentration (at least until about forty-five minutes into the experiment, when the activity tailed off). The researchers suspect that caffeine's effect on adenosine may be responsible for these regional boosts in neural activity.

There are naysayers, however. Roland Griffiths, a neuroscientist at Johns Hopkins University, suggests that the perceived mental benefits of caffeine many people experience from their morning coffee are an illusion: The coffee simply reverses the symptoms of withdrawal after overnight abstinence. Without coffee, says Griffiths, alertness would likely improve on its own an hour or two after waking.

Perhaps. But I can't wait. Illusion or not, I'm wedded to the quick chemical hit that jolts me out of morning muzziness and helps me make sense of the day.

2. MAKING SENSE

"C
OFFEE?" I WHISPER
to my sleeping husband. Though loath to startle him, I know my whisper is preferable to the shock of a bright light or the 70-decibel ring of his alarm. Morning comes to consciousness of sensory experiences, soft or jarring. Within a few seconds of waking, you can see stars, smell the dewy morning air, feel the light pressure of a sheet or the soft cotton of a shirt just slipped on, and recognize a partner's face or sleepy reply. Scent molecules waft up the nasal passages and latch on to receptors lodged in a tiny patch of tissue below and between your eyes. Nerve endings just beneath the surface of your skin detect the weight and texture of clothing, gentle as it is, and convert this mechanical energy into nerve impulses that your brain reads as touch, heavy or light, silky or scratchy. The sound of voice or buzzing alarm arrives on moving waves of air, which are eventually translated with exquisite efficiency into electrical signals interpreted as speech or birdsong or music. And even in the dim light of a dark bedroom, the forest of vision cells embedded in the retina captures the image of a face and flashes it to your brain.

At first it seems nothing could be simpler than this: the reliable registering of the world in one wide sweep through the discrete conduits of your five senses. Though it's a task that even the most powerful computer performs poorly, to you it seems as natural, as straightforward, as breathing. But as science has lately learned, there is nothing simple about it. A host of eye-opening new discoveries is complicating our view of perception, radically shifting it like the sudden twist of a kaleidoscope.

Take smell. Not long ago, your ability to smell—say, the overripe garbage or the fumes of your car warming up in the driveway—was considered a subpar skill of only minor importance, poorly understood and thought to engage only limited bits of your "lower" brain. Now smell is regarded as a highly sophisticated and sensitive system that can identify thousands of different odorants with some 350 distinct types of receptors, and analyze their dimensions in various regions of the brain to warn of danger or evaluate food. Our thresholds for detection of many odors are commonly in the parts-per-billion range, says Jay Gottfried, a neuroscientist at Northwestern University, "and we can readily discriminate between two different odors distinguished by only a single molecular component."

The odorants—complex organic molecules carried into the inner nose with inhaled air—meet the receptors in the mucus lining of the nose. Millions of olfactory nerve endings, each bearing dozens of identical receptors, poke into the mucus to interact with the world. The signal received by the receptor travels along the nerve by way of a long fiber, or axon, that threads through a tiny hole in the bone above it to the olfactory bulb region of the brain. In an amazing act of self-organization, the axons sort themselves so that the thousands of axons linked with neurons sporting the same receptors all converge in clumps at the same spot in the olfactory bulb. Each aroma sparks a constellation of these clumps, which the brain then interprets in various regions.

The character of a smell (fresh or foul? good or bad?) is sorted out in the orbitofrontal cortex, that all-important portion of the frontal lobe thought to be involved in decision-making, mood control, and drive. Its strength (how pungent?) is sometimes interpreted in the amygdala, the almond-shaped structure important in fear and other emotions—"but only when the smell is emotionally arousing," says Gottfried (for example, the reek of lion for a gazelle as opposed to the scent of a tree).

Identifying an odor, whether strong or faint, good or bad, enlists the regions of the brain involved in memory. A French study in 2005 showed that odor processing activates the memory regions in both hemispheres—probably, say the researchers, to help the mind gather relevant associations that assist in identifying the scent. As one researcher said, "We must first remember a smell before we can identify it."

Some odors may take you back along a deep groove of precise, personal memory. The aroma of bacon does for me, to waking summer mornings in childhood to the scent of thickly sliced farm bacon and smelts, perfect little fish my grandfather had caught fresh that morning from the dark waters of Lake Michigan and was lightly frying for his grandchildren's breakfast. For years anecdotal evidence has suggested that odors are especially powerful reminders of experience, an effect known as the Proust phenomenon, after the author's famous madeleine that summoned his childhood recollections. Scientists have found that olfactory stimuli do indeed evoke autobiographical memories more effectively than cues from other senses. And they fall away less rapidly than other sensory memories. This is even more astonishing when you consider that olfactory cells in your nasal epithelium survive for only a couple of months before they're replaced by new cells, which have to form new connections with cells deep in the brain.

What could account for the deep remembering of scents? Smell memories endure, according to the neurobiologist Linda Buck, because the olfactory cells that carry a receptor for a certain odor, whether they are new or old, always send their axons to the same spot in the brain.

The remarkable wiring of the olfactory system, it turns out, is also essential to taste.

There's nothing like that first sip of coffee. To get maximum pleasure from your mug, take a moment to savor the scent before sipping. The coffee vapors will pass from your mouth, around the soft palate, up into the nasal cavity, and thence to your olfactory bulb to whisper
java
to your brain.

Perhaps you thought your tongue responsible for the rich taste of coffee. But coffee's flavor—or any other flavor, for that matter—is mostly smell, about 75 percent in fact. Slurp a little Sumatra and your tongue will tell you only that it's bitter; that pleasant coffee taste, says Dana Small, is actually a pleasant odor pegged as taste because it's perceived to be coming from the mouth.

Small and her colleagues at Yale University discovered that the brain has a special sensory system devoted to odors delivered through the mouth. The team inserted small tubes into the noses of volunteers, one into the nostrils and one into the back of the throat. Then they introduced four odors into one tube or the other and scanned the subjects' brains using fMRI. The team found that for food-related scents, the two different delivery routes engaged different brain regions—suggesting that the brain possesses at least two distinct olfactory subsystems, says Small, "one specialized for sensing objects at a distance and one for sensing objects in the mouth." The latter is activated only when we breathe out through the nose between chewing or swallowing.

"A key fact about taste stimuli is that they elicit the most basic human emotions of pleasure (sweet) and disgust (bitter)," writes Gordon Shepherd, a neurobiologist at Yale. These are hard-wired in the brain stem from birth. By contrast, the responses to the odor component of taste "seem to be mostly learned," he notes, "which presumably accounts for the enormous diversity of flavours in the world's cuisines."

Very little was known about the real science of the mouth sense until quite recently. Now, test tubes, gene sequencing machines, and brain scanners are offering hints about what creates the full experience of flavor. The 25 percent wedge that is rooted in taste arises from receptor proteins that reside on taste cells within the taste buds of your tongue. Each of these receptors is dedicated to one of the five tastes: salty, sweet, sour, bitter, and umami. The latter taste quality (from the Japanese
umai,
for "good," and
mi,
for "taste") is responsible for the savory flavor of such foods as chicken broth, Parmesan cheese, mushrooms, and bacon.

Notwithstanding those tongue maps so ubiquitous in textbooks, the ones showing discrete areas sensitive to certain tastes—sweet on the tip, sour on the sides, and so on—cells responsive to the five basic taste qualities are scattered across the whole rolling landscape of the tongue. Though some taste cells are found on the pharynx, larynx, and epiglottis, most are located in the taste buds on the tongue.

Magnified, a taste bud looks like nothing so much as a little onion. Each bud possesses up to one hundred taste cells, which carry the receptors that do the real work of gustation: Chemicals in food slip through small holes in the buds, where they meet the receptors, which send their specific taste-quality message to the taste cortex of the brain. The brain then weds these taste sensations with information about fizziness and texture, the so-called mouth feel of food (which makes a crisp potato chip delicious and a soggy one unappetizing), and, in the case of hot chilies and other spicy foods, sensations of pain, to create the full-blown perception of the sweet, homey taste of banana bread or the savory flavor of squab infused with wine.

Even temperature enters the picture: Warming enhances the perception of sweetness and bitterness (another reason that hot coffee tastes so good). In fact, just changing the temperature of the tongue, cooling or heating it, will trigger an actual taste sensation in one out of every two people. In 2005, a team of researchers reported discovering the secret to the odd phenomenon known as thermal taste. When the tongue's receptors for sweet tastes are stimulated, a special channel opens. It turns out that heat, too, opens this channel, activating the taste receptors even when there's nothing to taste.

That all tasters are not created equal most of us know: Think of the sweet tooth that plagues some of us, and certain people's aversion to cilantro or anchovies. Think of George Bush Sr.'s well-known dislike of broccoli. Think of the flavor of olives, which to some is a divine mix of salty, sour, and bitter, but to others is similar to life at sea as Emerson described it, like being "suffocated with bilge, mephitis, and stewing oil." But only lately has it come to light just how wildly different a taste world each of us inhabits, especially where bitterness is concerned.

We humans possess a variety of some twenty-five bitter taste receptors, which are thought to have evolved to detect toxins in plants and foods. "Virtually every plant, edible or otherwise, contains toxins that can make us ill," says Paul Breslin of the Monell Chemical Senses Center. The picky eating habits of very young children, often directed against bitter-tasting fruits and vegetables, may be an evolutionary device to protect them from poisoning themselves when they're toddlers. Likewise, the nausea and aversion to certain foods experienced in pregnancy may have evolved to reduce a fetus's exposure to natural toxins. More women than men have a heightened reaction to bitter taste, though the sensitivity seems to vary over a woman's lifetime, rising at puberty and peaking during early pregnancy. After menopause, the sensitivity tails off, possibly because there's no longer a need to protect a developing child.

Scientists recently pinpointed small variations in these bitter receptor genes, which result in as many as two hundred slightly different forms of the receptors. Breslin has found, for instance, that people with a variant of one gene rate watercress, broccoli, mustard greens, and other such vegetables (which contain a compound toxic to the thyroid gland) as 60 percent more bitter than people with a different variant. So while you and I may share the template of two dozen or so genes for these bitter taste receptors, each of us carries his own distinctive versions, prompting either wrinkled nose or enthusiastic relish at the prospect of a plate of bitter greens.

Highly individual genes may also shape the way I make the morning's selection of tops and bottoms from my closet. Whether that vermilion shirt looks like a good match for those jade pants depends on the activity of genes that vary not only from person to person but from women to men—offering a possible explanation for heated spousal arguments over clothing and paint color. These genes were shaped in my primate ancestors' distant past.