Welcome to Your Brain (12 page)

occurrence of an individual person’s sneeze follows the same course over time, without

variation. The explosive beginning of a sneeze expels air at remarkable speeds, up to one

hundred miles per hour. A powerful synchronized reproducible event like this can only be

generated by positive feedback within some circuit somewhere in the brain, one that leads

to a runaway burst of activity, reminiscent of the onset of epileptic seizures. However,

sneezes are different in that they have a preset mechanism for ending, and they don’t spread

in an uncontrolled way to other bodily movements or activities.

The sneezing center is located in the brainstem, in a region called the lateral medulla;

damage to this site causes us and other mammals to lose the ability to sneeze. Usually

sneezing is triggered by news of an irritant that is sent through brain pathways and into the

lateral medulla. This information comes from the nose to the brain through several nerves,

including the trigeminal nerve, which carries a wide variety of signals from the face into

the brainstem. The trigeminal nerves (we have one on each side) are cranial nerves with

multiple functions: they process noxious and tactile stimuli from the face and much of the

scalp, as well as from the conjunctiva and cornea of the eye. The trigeminal nerve even

carries motor signals in the opposite direction, out of the brain, including the commands to

bite, chew, and swallow; it’s a really crowded nerve.

This crowded arrangement might explain why bright light could erroneously induce a

sneeze. A bright light, which would normally be expected to trigger pupil contraction, might

also spill over to neighboring sites, such as nerve fibers or neurons that carry nose-tickling

sensations. Bright light isn’t the only unexpected sensation that is known to trigger sneezes;

male orgasm can also trigger sneezes (in the male who is having the orgasm).

Fundamentally, a crossed-wire phenomenon like the photic sneeze reflex is made

possible because the circuitry of the brainstem is a jumbled, crowded mess. The brainstem

contains critical circuitry for a wide variety of reflexes and actions, including almost

everything our bodies do. The basic layout of the brainstem was worked out early in the

history of the vertebrates. Thirteen pairs of cranial nerves are found in nearly all

vertebrates (though fish have three additional pairs that carry signals such as those from the

lateral line receptors along their sides). The cranial nerves lead to a complex network of

specific clusters of neurons, or nuclei, which are basically arranged the same way and

serve similar functions among all vertebrates. Indeed, looking at nervous systems in

nonhuman animals is an extremely good way to guess at how the structures in our brains

work.

The reason that brainstem structures are so similar across species is that the whole

system is intricately constructed. From an evolutionary standpoint, it would be disastrous to

move anything around on a wholesale basis. As successors to the earliest, simplest

vertebrates, today’s vertebrates (including fish, birds, lizards, and mammals) are all

doomed to use a wiring layout that can be modified in small ways but not fundamentally

changed. It’s reminiscent of the New York City subway system, which was simple at one

point, but is now hopelessly convoluted after planners added layer upon layer of

complexity. Some parts of the brainstem aren’t used any more, and the original core is now

so jury-rigged and patched together, it can’t possibly be replaced for fear of stopping the

whole system cold. Frankly, the brainstem is about as good an argument against intelligent

design as one could ever hope to find in nature.

Taste works the same way, except that flavor receptors are in your tongue. Taste is simpler since

there are only five basic flavors: salty, sweet, sour, bitter, and umami. (What’s umami, you say? It’s

the savory taste that’s found in cooked meat or mushrooms or in the food additive monosodium

glutamate, MSG. There’s no word for it in English, which is why we use the Japanese term.) Each of

these basic tastes has at least one receptor, sometimes more. Bitterness, for instance, is sensed by

dozens of receptors. As animals evolved, they needed to detect toxic chemicals in their environments.

Because toxic compounds came in many forms, it was necessary to have receptors that could detect

all of them. This is why we have a natural repulsion to bitter flavors. This distaste can be overridden

by experience; look at all the lovers of tonic water and coffee.

Why do we call spicy foods hot? The chemical that gives chili and hot sauce their zest is

capsaicin. Your body also uses capsaicin receptors to detect warm temperatures. This is why you

sweat when you eat spicy food—the receptors have what you might call a “hotline” into your brain to

trigger responses to cool you off. You have capsaicin receptors not only in your tongue, but all over

your body. One way to discover this is by cooking with hot peppers and then putting in your contact

lenses. Ouch!

Did you know? Why mice don’t like Diet Coke

The ingredient that makes Diet Coke sweet is aspartame (NutraSweet). It works by

binding to sweet receptors in your tongue. In humans, the sweet receptor binds not only to

sugar, but also aspartame, saccharin, and sucralose (Splenda). In mice, sweet receptors

bind to sugar and saccharin, but not aspartame. They don’t prefer water with NutraSweet to

plain water, suggesting that to a mouse, Diet Coke wouldn’t taste sweet. (It’s a similar story

for ants, which are not attracted by diet soda.)

Scientists have used genetic technology to replace the mouse’s sweet receptor with the

human sweet receptor. These transgenic mice like aspartame—and presumably Diet Coke.

This proves that they use the same brain pathways to taste sweet things as we do, just with

different receptors.

If you have pets, there’s an experiment you can do at home. See how they like different

kinds of sweet beverages—juice, sugared soda, and diet soda. Put out one dish of each and

see what your pet goes for. You might be surprised at the results!

Minty foods taste cool for a similar reason. A receptor has recently been identified that binds to

menthol. Plants may make menthol for the same reason that they make capsaicin—to make themselves

taste bad to animals.

Smells and tastes often have strong emotional associations: your grandmother’s apple pie, burning

leaves, your lover’s shirt, fresh coffee in the morning. Smells can also have negative associations. On

September 11, 2001, and in the days after, Manhattan was permeated by a bitter, acrid smell that

nobody who was there can ever forget. Some smells may be negative for some and positive for

others. (Think of Kilgore’s favorite smell in
Apocalypse Now
: “I love the smell of napalm in the

morning … the whole hill smelled like victory.”) These associations may occur because olfactory

information has a direct connection into your limbic system, brain structures that mediate emotional

responses. These structures are able to learn, raising the possibility that they allow you to associate

smells with pleasurable or dangerous events.

Chapter 9

Touching All the Bases: Your Skin’s Senses

Pickpockets may not spend a lot of time talking about how the brain works, but their profession does

require some practical knowledge of the subject. A common technique involves two partners in

crime. One thief bumps into the victim on one side, to distract him from the other thief’s hand taking

something from the other side. This approach works because it draws the victim’s attention to the

wrong side of his body, which distracts his brain from events on the side where the important action

is.

Expectations do not only influence our responses; they actually influence what we feel. Your

perception of the body’s sensations comes from the interaction of two processes: signals coming from

receptors in your body, and activity in brain pathways that control your response to these signals—

including, in some cases, whether they get passed along to the brain at all. This interaction is apparent

not only in pickpocketing, but also in phenomena as diverse as pain and ticklishness.

Of course, the physical stimuli on your body also affect what you feel. Your skin contains a

multitude of different receptors—specialized nerve endings that sense things like touch, vibration,

pressure, skin tension, pain, and temperature. The brain knows which kind of sensor is activated, and

where it is on the body, because each sensor has a “private line” that uses spikes to carry only one

kind of information to the brain. Some parts of your body are more sensitive than others. The highest

density of touch receptors is found on the fingertips, with the face a close second. Your fingers

contain many more receptors than your elbows, which is why you don’t explore an object with your

elbow when you’re trying to figure out what it is.

Another set of receptors in your muscles and joints gives you information about the positioning of

your body and the tension in your muscles. This system is what allows you to be aware of the position

of your arm when your eyes are shut. When these sensors are damaged, people find all kinds of

movement to be very difficult, and they have to watch themselves as they move to avoid making

mistakes.

Did you know? Why can’t you tickle yourself?

When doctors examine a ticklish patient, they place the patient’s hand over theirs during

the exam to prevent the tickling sensation. Why does this work? Because no matter how

ticklish you may be, you can’t tickle yourself. Go ahead. Try it. The reason is that with

every move you make, part of your brain is busy predicting the sensory consequences of

that movement. This system keeps your senses focused on what’s happening in the world so

important signals aren’t drowned out in the endless buzz of sensations caused by your own

actions.

For instance, as we write, we are unaware of the feel of the chair and the texture of our

socks. Yet we’d immediately notice a tap on the shoulder. If the only information your brain

received was pure touch sensation, you wouldn’t be able to tell whether someone was

punching your shoulder or whether you’d just bumped into a wall. Since you’d want to

react very differently to those two situations, it’s important for your brain to be able to tell

them apart effortlessly.

How does your brain accomplish this goal? To study this, scientists in London

developed, of all things, a tickling machine. When a person presses a button, a robot arm

brushes a piece of foam across the person’s own hand. If the robot arm brushes the hand as

soon as she presses the button to activate it, the person feels the sensation but it doesn’t

tickle. However, the effect can be enhanced by introducing a delay between the button press

and the touch. A delay of one-fifth of a second is enough to fool the brain into thinking the

robot’s touch has been delivered by someone else—and then it tickles.

Even better, if the robot’s touch is delivered in a different direction than the one in

which the person pulls the lever, then a delay as short as one-tenth of a second is enough to

generate a tickling sensation. This experiment shows that, at least for tickling, your brain is

best at predicting the sensory outcome of a movement on the time scale of a fraction of a

second.

So what happens in the brain when you try to tickle yourself? The same scientists used

functional brain imaging, a technique that allowed them to observe how different parts of

the brain respond to various types of touch. They looked at brain regions that normally

respond to a touch to the arm. These regions responded when the experimenters delivered

the touch. However, if someone delivered the touch to his own body, the response was

much smaller—but still there. When the delay was increased, leading the touch to feel

tickly, the brain responses became large once again. It’s as if your brain is able to turn

down the volume on sensations that are caused by your own movements.

This means that some brain region must be able to generate a signal that distinguishes

your own touch from someone else’s. The experimenters found one: the cerebellum. This

part, whose name means “little brain,” is about one-eighth of your total brain size—a little

smaller than your fist—and weighs about a quarter-pound. It’s also scientists’ best

candidate for the part of the brain that predicts the sensory consequences of your own

actions.

The cerebellum is in an ideal location for distinguishing expected from unexpected

sensations. It receives sensory information of nearly every type, including touch, vision,

hearing, and taste. In addition, it receives a copy of all the movement commands sent out by

the motor centers of the brain. For this reason, researchers suggest that the cerebellum uses

the movement commands to make a prediction of the expected consequences of each

movement. If this prediction matches the actual sensory information, then the brain knows

it’s safe to ignore the sensation because it’s not important. If reality does not match the

prediction, then something surprising has happened—and you might need to pay attention.