Welcome to Your Brain (17 page)

adult life. Every day, your kids come home from school—or from basketball practice—with brains

whose neurons are connected slightly differently from when they woke up in the morning.

Did you know? Why are some things easier to learn than others?

Sooner or later, most people discover that a single experience can lead to intense and

sometimes permanent learned responses. For Sandra, it’s orange juice, which didn’t taste

good again for years after the unfortunate college party where it was mixed with excessive

amounts of vodka. For you, it might be the shellfish that you can’t eat anymore, ever since

you ran across that bad oyster at lunch last year. Taste aversion is a vivid example of

prepared learning. It’s easy to develop an intense dislike of what you ate before you got

sick, even if it happened only once, but you never hear anyone say, “I can’t bear to look at

the shirt that my date was wearing the night I was ill.” Logically, this makes sense because

fashion choices are unlikely to make you physically sick (though fashionistas might make a

few exceptions to this rule).

Many types of illness are caused by food. How does the brain know that food has a

special connection to illness? We said in
Chapter 10
that babies’ brains are not sponges

waiting to soak up anything that happens to them. It probably comes as no surprise to hear

that adults also have distinct predispositions for learning. Many of these tendencies—to

learn some things easily and others not at all—seem to be built in at birth, in humans and

other animals. Because evolution selects for outcomes, this approach can be an efficient

way to make sure that an animal is a good fit for its environment, especially when the

details of the surroundings can’t be predicted in advance.

Remember from
chapter 3
that when an electrical signal arrives at the end of an axon, it triggers

the release of a chemical neurotransmitter, which binds to receptors on the neuron at the other side of

the synapse. In most cases, multiple synapses need to be active at once to trigger an action potential in

the next neuron in line. When this happens, all the active synapses are strengthened so that they will

have more influence on the recipient neuron the next time, either by releasing more neurotransmitter

or by having more receptors available to receive the signal. This strengthening process is called long-

term potentiation or LTP. At most synapses, the rule for inducing LTP is similar to one of the common

behavioral rules for learning: stimuli will be associated if they occur close to the same time. In

neurons, by analogy, synapses will be strengthened if they are active at the same time, which is often

the result of receiving two stimuli simultaneously.

Of course, synapses can’t be strengthened indefinitely, or eventually they’d all be maxed out, and

the brain would lose its ability to learn new information. There are a few tricks that the brain uses to

avoid this problem, but the most straightforward is a use-dependent weakening of synaptic

connections called long-term depression or LTD. Synapses are weakened if they become active at a

time when the recipient neuron isn’t receiving enough stimulation to fire an action potential. Another

trick is that on long timescales, new synapses can form and old ones can go away, which allows

connections to be redistributed.

These changes, which are collectively called synaptic plasticity, occur more easily at certain

times, such as infancy. In adults, synaptic plasticity comes more easily to particular parts of the brain

such as the hippocampus, which we’ll discuss further in
Chapter 23
. Your brain has about a dozen

different known ways of learning information, each of which uses a somewhat different combination

of brain regions. For example, learning new facts and places causes changes in your hippocampus and

cortex, while learning a new dance step changes your cerebellum.

Researchers know a lot about the signaling pathways and molecules that are involved in synaptic

plasticity. Scientists have been able to use this knowledge to produce mice that find it harder or

easier to learn, simply because they are missing a single gene from their DNA. This work suggests

that modifying synapses is one of the brain’s most important jobs. There are literally hundreds of

genes that affect learning and dozens that affect overall intelligence. Many pathways perform similar

jobs and can substitute for one another if the need arises, giving a measure of protection against the

complete failure of learning, which would be devastating to an animal.

A particularly well-understood and important type of learning is fear conditioning, the process of

learning to become afraid of stimuli in the environment that predict bad things are about to happen. A

common type of fear-conditioning experiment goes like this: a rat is placed in an unfamiliar cage, a

tone comes on, and then the animal receives a mild electric shock. After a few experiences of this

sort, the rat learns to anticipate the shock by freezing (a typical rodent fear response) whenever it

hears the tone.

Practical tip: Put it out of your mind

Practice makes perfect, or so we hear. Many elite performers, from athletes to actors,

learn to start their training by mentally rehearsing the outcomes that they would like to

achieve. Repeated visualization of a desired experience can be a very effective way to

create a strong mental image in your brain.

Unfortunately, a lot of people end up using essentially this same rehearsal strategy when

remembering bad experiences. It’s unintentional, of course, but the effect of mentally

rehearsing an experience over and over is the same, whether you’re deliberately trying to

increase the intensity of the memory or just doing it by accident because you’re naturally

inclined to think about the bad things that happen to you. Some doctors think that post-

traumatic stress disorder, which we discuss in
chapter 17,
is partly caused by this sort of

mental rehearsal.

The best strategy is easy to state. To develop a strong mental image of something you

want to accomplish, visualize it repeatedly in as much detail as you can. If something is

making you unhappy and you want to get it out of your head, try not to think about it too

much. This is especially true of things that make you afraid.

This strategy can be hard to achieve in practice. One approach to try is to distract

yourself. The approach can be direct: some psychologists recommend wearing a rubber

band around your wrist and snapping it every time the persistent thought enters your head.

Or it can simply involve doing something that you find engaging, whether that’s playing a

team sport or listening to music or going to the races. It will probably help to tell your

friends and family that you’ve decided not to dwell on the problem anymore and ask them

to remind you of that decision if you bring up the subject again. Then go do something

productive or fun, as long as it’s challenging. If the intrusive thought persists, it might be

time to see a therapist, as we discuss in
chapter 17
.

Scientists at New York University showed that auditory signals travel directly from the thalamus

to the amygdala, a small region that is important for emotional responses, particularly fear. Neurons

in a particular region of the amygdala fire more action potentials in response to the tone after

conditioning than they did before the animal had learned to fear the tone. These changes in the

electrical responses of neurons occur around the time that the animals start to show fear behavior,

suggesting that they may cause fear-induced learned freezing. Similarly, rats or people with damage to

the amygdala don’t form normal fear memories.

Fear conditioning can be counteracted by a process called extinction, which is induced by

repeatedly exposing a conditioned animal to the tone without the electrical shock. If this happens

often enough, the animal will learn to stop freezing when it hears the tone, and the amygdala neurons

will stop firing so strongly in response to the tone as well. However, extinction is a second form of

learning that is overlaid onto the original fear conditioning; it does not restore the brain to its original

state. Extinction seems to involve learning in the prefrontal cortex, a brain region that selects

appropriate behaviors in context. Neurons in the prefrontal cortex become more active after

extinction training, and they then suppress the activity of the amygdala neurons in response to the tone.

Rats with damage to the prefrontal cortex can learn to fear a tone, but although extinction reduces

their responses temporarily, the learning doesn’t last, so the next day they act as though the extinction

training never happened. Like other types of learning, extinction is influenced by an animal’s natural

tendencies. It is much more difficult to extinguish fear of stimuli, like snakes or spiders, that were

important sources of danger in the evolutionary history of our species.

The amygdala also mediates the effects of emotions on other types of learning. Emotional arousal

facilitates attention to the most important details of an experience. For instance, victims of armed

robbery usually remember what the gun looked like. Patients with amygdala damage, on the other

hand, may fail to concentrate on relevant details at stressful moments. In rats and humans, transient

stress enhances learning in two ways: via the release of adrenaline and glucocorticoids. Both

hormones act on receptors in the amygdala and hippocampus to enhance synaptic plasticity. However,

chronic stress can greatly impair learning ability. This is another fact that’s worth remembering when

you’re trying to train your dog.

Each of the brain’s different learning systems has its own special properties. In the case of fear

conditioning, the amygdala system allows you to learn one-time occurrences if they induce enough

fear. At the other extreme, consider the number of repetitions necessary for most people to remember

long lists of facts, an incredibly dull task that uses a different system, the hippocampus.

Most tricks for learning facts take advantage of the natural ways that human beings learn. Just as

pigs gravitate toward burying things and chickens tend to peck, we have ways of learning in the

natural world that come more easily to us. As described in
Chapter 6
, humans are exceptionally

visual animals, and at least one-third of our cortex works with visual information in one form or

another. In addition, sequences of events and the physical proximity of objects to one another are

natural groupings for us, since these are the ways that we experience the world. The hippocampus

handles both the learning of facts as well as the learning of events and sequences. One effective

strategy combines several of these tricks: imagine that you are walking through a house, and each fact

that you want to remember is associated with a particular place in the house. If this seems like a

tedious business, you could achieve single-trial learning with the amygdala system. Unfortunately, this

would require you to experience intense fear with every fact that you learn. It’s not worth it.

Chapter 14

Reaching the Top of the Mountain: Aging

We hadn’t been paying much attention to the research on aging and how to improve our chances of

keeping our brains healthy for as long as possible. Now we’re glad we wrote this book, because it’s

time for us to make some lifestyle changes that should make our retirement years happier. Let’s start

with the bad news. Even putting aside diseases of aging like dementia, your brain’s performance is

likely to get worse as you get older. There are two main problem areas. The one that everybody

knows about is memory. You may have more trouble keeping track of your car keys than you used to;

this ability starts to deteriorate in your thirties, on average, and continues to decline as you age.

Spatial navigation relies on a part of the brain involved in memory, the hippocampus, and this ability

is also impaired with age in many animals, including humans.

The other problem area is what scientists call “executive function,” which is the set of abilities

that allows you to select behavior that’s appropriate to the situation, inhibit inappropriate behavior,