Welcome to Your Brain (10 page)

they can recognize them later in a group of people. In May’s case, his fusiform face area has not had a

chance to develop as it does in people who have grown up with vision.

Right after he had his sight restored, Mike May had to ski with his eyes closed. His brain’s

motion-detecting cells are as sensitive as a normal person’s, but that’s a mixed blessing for him.

Skiing down the mountain was no longer exhilarating but became frightening as he watched the world

zoom past him. For the first time in his life, he became uncomfortable with his wife’s driving because

he found the sensation of other cars zipping past on the road overwhelming.

Did you know? The neuron that loved Michael Jordan

What does it mean to be a fan of a celebrity? One study suggests that it literally means

giving space in your brain to that person. There’s an old idea that activity in one or a few

neurons might signal the identification of a certain object or person, but most

neuroscientists don’t believe that the brain does it that way. This is because there just aren’t

enough neurons to account for everything that we can recognize—and because people don’t

have strokes that eliminate their ability to recognize some people but not others (though

some patients lose their ability to recognize people in general, as discussed earlier in this

chapter).

In this study, scientists recorded single neurons from the brains of eight people with

intractable epilepsy. Surgeons implanted electrodes in the temporal lobe of each patient’s

brain to help identify the origin of the seizures, and the scientists used these electrodes to

record from neurons while the patients were looking at pictures. Some neurons responded

specifically to images associated with a particular celebrity (usually an actor, politician, or

professional athlete). For example, one neuron fired spikes in response to all photos of

Jennifer Aniston—except the one where she appeared with Brad Pitt—and did not respond

to pictures of anyone else. Another neuron was activated by photos and drawings of Halle

Berry, and even by her printed name. Although this neuron responded to a picture of Halle

Berry dressed in her Catwoman costume, it did not respond to the photo of another woman

in a Catwoman costume. Other neurons responded to Julia Roberts, Kobe Bryant, Michael

Jordan, Bill Clinton, or even famous buildings like the Sydney Opera House. No one is sure

what these neurons actually do, though one brain region where they’re found is involved in

the formation of new memories.

No one is sure why the brain’s motion system is so robust that it can function after forty years of

blindness, but it might be because motion detection is so important for survival. Whether you’re a

hungry wolf or a terrified rabbit, there’s nothing better than motion for finding the other living things

in your visual world.

The brain areas that analyze motion are separate from the ones that analyze shape. In fact, they’re

in a different part of the brain. The basic motion area detects object movement in a straight line, while

higher areas detect more complicated patterns, including expansion (like rain seen through the

windshield of a moving car or the opening sequence of
Star Trek
) and spiral motion (like the water

swirling down your bathtub drain). These signals are probably important for navigation, as your

retina experiences these sorts of motions as you move through the world.

Damage to these brain regions causes motion blindness. People with this disorder see the world

as if they were under a strobe light at a disco: first a person is here; then suddenly he’s somewhere

else. As you can imagine, it’s very dangerous to live in a world where it seems like all the other

people and objects are capable of random teleportation, so these people have a lot of trouble getting

around.

Myth: Blind people have better hearing

People have long attributed special powers—even magical powers—to blind people.

One common idea is that the blind have extra-sharp hearing. However, when tested, blind

people are no better at detecting faint sounds than sighted people.

But one old belief about blind people’s special abilities is correct. In ancient times,

before the invention of writing, the blind were known for their accurate memories of

biblical interpretations, which were passed down from one generation to the next as oral

traditions. Indeed, blind people do have better memory, especially for language. Since they

can’t rely on vision to tell them things like “Did I set that glass down on the counter?” they

have to use their memory constantly (or else knock a lot of drinks to the floor). Presumably,

constant practice helps them sharpen their spatial memory. They also do better than sighted

people at other language tasks, including understanding the meaning of sentences. In

addition, blind people are better at localizing sounds, which may be another way of keeping

track of where things are.

Blind people seem to improve these abilities by taking advantage of brain space that

isn’t being used for vision. In blind people, verbal memory tasks activate the primary

visual cortex, which is involved only in vision in sighted people. Researchers can

temporarily turn off a region of the cortex by applying magnetic stimulation to the outside of

the skull to interfere with the brain’s electrical activity. This interference impairs blind

people’s ability to generate verbs, which is one of the language tasks that they do especially

well, but it has no effect on this task in sighted people (though it does, of course, interfere

with their ability to see).

So far we’ve talked as if our eyes were taking in a continuous scene, something like a movie

playing on the retina, which is certainly what it feels like. This is because the brain has ways of

smoothing over the world to make your experience feel continuous even when it isn’t. However, by

now you’ve probably guessed what comes next: your brain is lying to you again. All the time you’re

awake, your eyes are jumping around the visual world in abrupt movements called saccades, which

occur three to five times per second. You can see these movements by watching a friend’s eyes. Each

eye movement gives the retina a “snapshot” of some part of the visual scene, but the brain must put

these still pictures back together to create the illusion of a continuous world. Even neuroscientists

don’t have much of an idea about how this complicated process works.

To see what is in front of one’s nose needs a constant struggle.

—George Orwell

Mike May’s experiences illustrate that although vision appears to be one sense, it is really

composed of many functions. To most of us, these functions are woven together to form a seamless

whole, thanks to a lifetime of development and experience. May’s brain has not learned how to lie, or

even to tell the truth, fluently. As a result, he can navigate visually 90 percent of the time. That’s not

as useful as it sounds, though, since he never knows which 10 percent of his perceptions are wrong.

Now that he has vision, he’s discovered that he can’t always trust it. Four years after his sight was

restored, Mike May finally figured out how to deal with these problems: he got his first seeing-eye

dog since his operation.

Chapter 7

How to Survive a Cocktail Party: Hearing

We often think of vision as our most important sense, but perhaps equally essential is hearing. For

obvious reasons, deafness makes it difficult to communicate with other people. Deaf people have

risen to this challenge by creating their own unique form of language, which uses the hands and eyes

instead of the mouth and ears. The barriers to communication between deaf and hearing people are so

profound that distinctive deaf cultures have arisen. (For example, in the movie
Children of a Lesser

God
, when a deaf woman falls in love with a hearing teacher at the school where she works, the

conflict with her loyalty to deaf society threatens their relationship.) How your brain identifies

complex sounds like speech is still something of a mystery, although scientists understand quite a bit

about how we detect and locate auditory signals.

Whether we’re listening to music, birdsong, or the chatter of a cocktail party, hearing begins with

a set of pressure waves in the air that we call sound. If we could see the waves caused by a pure tone

(a flute note would be the closest everyday example) as they moved through the air, they would look

like the ripples you produce when you throw a rock into a pond. The density of the ripples (called

frequency) determines the pitch of the tone—shorter distances between waves make high sounds,

longer ones make low sounds—and their height determines sound intensity. More complicated

sounds, like speech, contain multiple frequencies with different intensities mixed together.

The outer ear transmits these sound waves to an organ in the inner ear called the cochlea (Latin

for
snail
because it’s shaped like one, as you can see in the drawing). The cochlea contains the ear’s

sound-sensing cells, which are arranged in rows along a long, coiled membrane. Sound pressure

moves the fluid in the ear, causing the membrane to vibrate in different ways depending on the

sound’s frequencies. This vibration activates the sensors, called hair cells because they have a bundle

of fine fibers that stick up from the top of the cell like a punk hairdo. Movement of these fibers

transforms the vibration signal into an electrical signal that can be understood by other neurons. Hair

cells can sense movement the size of an atom and respond very rapidly (more than twenty thousand

times per second).

Hair cells at the base of the cochlear membrane sense the highest frequencies. As you move

around the coil toward the other end, hair cells become sensitive to lower and lower frequencies.

(Imagine the sequence of keys on a piano.) This organization forms a map of sound frequency, which

is maintained in many of the brain areas that respond to sound.

Sound information from the two ears is brought together in the neurons of the brainstem. Doctors

use this knowledge to help diagnose the causes of hearing loss, based on whether it occurs in one ear

or in both. Because neurons within the brain get sound information from both ears, any damage to

parts of the brain that process sound causes hearing problems in both ears. For this reason, if you

have difficulty hearing in only one ear, the problem is likely to be damage to the ear itself or to the

auditory nerve. Hearing loss can also be caused by mechanical problems that interfere with the

transmission of sounds from the outside of the ear to the cochlea. This type of hearing loss can be

treated with a hearing aid, which amplifies sounds entering the ear. Hearing loss caused by damage to

hair cells can only be helped by a cochlear implant (see
Practical tip: Improving hearing with

artificial ears
).

The brain has two major goals for sound information: to locate a sound in space, so you can look

toward the sound’s source, and to identify the sound. Neither of these tasks is easy, and each is

accomplished in different parts of the brain. Therefore, some brain-damaged patients have difficulty

locating sounds but not identifying them, and vice versa.

Practical tip: How to prevent hearing loss

Remember your mother warning you not to listen to loud music because you’d ruin your

ears? She was right. In the U.S., one-third of people over sixty and half of those over

seventy-five have hearing loss. The most common cause is long-term exposure to loud

noises. Baby boomers are losing their hearing earlier than their parents and grandparents

did, presumably because our worlds are noisier than they used to be. Some experts are

particularly worried about portable MP3 players like the iPod, which can produce very

loud music for hours without recharging.

It’s not just rock and roll, of course. Hearing loss is caused by any loud noise that

persists over time—a lawnmower, motorcycle, airplane, ambulance siren, or firecracker

show. Even brief exposure to a very loud sound can damage your hearing. In these

situations, where the noise isn’t the point of the experience, you can protect yourself by