Welcome to Your Brain (28 page)

One group of intelligent animals stands out for sheer weirdness: octopuses. The brain of

a common octopus weighs less than a dime and is only half as wide, but the octopus is

capable of learning, imitation, puzzle-solving, and deception. For example, octopuses can

be trained to distinguish between a red and a white ball. When a trained octopus is placed

with one that’s new to the task, the second octopus imitates the demonstrator’s preference

after watching just four times, on average. Octopus keepers often create puzzles for their

charges to give them something to do. At the Oregon Coast Aquarium, octopuses had to

manipulate a three-part sliding puzzle made of PVC pipe to get at a tube packed with squid

—and they did, in less than two minutes.

Invertebrate brains, which are wildly different from those of vertebrates, usually

consist of a few clumps of neurons connected to one another by small yarns of nerve. The

central brain of an octopus grows by a factor of more than one hundred during the animal’s

lifetime, a growth rate unmatched in any vertebrate. The human brain is six hundred times

the size of an octopus brain, but the octopus also has many neurons in its arms, which may

help it to process information.

These observations suggest that the same principles of learning have arisen

independently during evolution in invertebrates and vertebrates. Evidently the view that a

forebrain is the substrate of intelligence is too parochial. Understanding what octopus,

crow, and human brains have in common may help us figure out what it takes to be

intelligent.

These scientific errors mattered because of their effects on public policy. Many early intelligence

researchers were drawn to the possibility that humans could be selectively bred like dogs or cattle to

create an improved race of man, an idea called eugenics. Of course, how you go about this project

depends a lot on your definition of “improved,” and it only works at all if the trait that you want to

improve depends on genes in some straightforward way. The attempt to breed people for traits like

“being respected in society” suffered on both counts. Scientifically, it would have been laughable if it

hadn’t led to outcomes like forced sterilization of people institutionalized for reasons as diverse as

poverty, mental illness, and sexual misbehavior. Many states still have these laws on the books,

though they’re rarely enforced anymore.

As the study of intelligence has become more rigorous, much work has focused on the factors that

affect individual performance. Individual differences in intelligence are much larger than any known

differences between groups of people, but one person’s intellectual performance can vary over time

and across circumstances or tests.

Many subtle situational factors, which are often group-specific, can influence how well someone

does on any sort of test. Most people don’t appreciate how common or powerful these influences are

( s e e
Practical tip: How expectations influence test performance
). For this reason, although

differences in intelligence strongly influence performance across many tasks, these differences are not

fixed across the human lifespan. Even more importantly, environmental influences make a strong

contribution to the development of intelligence, so group differences that exist in one generation may

not carry over to the next. Even if we ignore the ethical problems with the idea, these facts greatly

undermine the validity of any attempts to breed people based on the results of intelligence tests.

There are multiple aspects of intelligence, but in this chapter we’ll focus on what psychologists

call “fluid intelligence,” the ability to reason your way through a problem that you’ve never seen

before. This ability is the best general predictor of performance on many different tasks, and it is

distinct from the skills and facts (such as vocabulary words) you have already learned. The best

measure of fluid intelligence is Raven’s Advanced Progressive Matrices, a test that avoids

vocabulary discrepancies by using no words at all. Instead, people are shown a set of geometric

shapes with common characteristics and asked to choose another shape that fits into the set.

Which parts of your brain are responsible for this ability? The strongest candidate is the

prefrontal cortex. Damage to this region leads to difficulty with many forms of abstract reasoning. In

normal individuals, prefrontal cortex volume also correlates with fluid intelligence. Finally, the

lateral prefrontal cortex is activated by multiple different intelligence tests taken during brain

scanning. However, the prefrontal cortex is probably not the only brain region that is important for

fluid intelligence. Parietal areas of the cortex are also active during many brain-scanning studies of

abstract reasoning and intelligence.

Myth: Brain folding is a sign of intelligence

The idea that folds on the brain’s surface might be related to brain function dates back

at least to the seventeenth century. This idea was further popularized by scientists based

solely on the evidence that human brains are more folded than other available brains, such

as those of cows and pigs.

The myth was contradicted when several eminent thinkers left their brains to science for

measurement after death. Their brains looked very similar to one another, with no physical

feature that correlated with intelligence. The distinguished brains were all equally folded

and did not look different from less-distinguished brains.

Likewise, in other mammals, brain folding is related not to cognitive sophistication but

to absolute brain size. The most folded brains belong to whales and dolphins, the least

folded to shrews and rodents. A leading hypothesis of how these folds form is that the

connections between nerves pull together the cortical surface, like sloppy stitches bunching

up a big sheet. One useful consequence of a folded surface may be to reduce the amount of

space taken up by brain wiring: large amounts of axon are not only bulky but also create

long distances for signals to travel, making processing times longer. In bigger brains, the

cerebral cortex also has more white matter, made up of the axonal wiring that links distant

regions to one another. Increased folding and white matter are seen in all large-brained

mammals, regardless of their mental sophistication, including humans, elephants, … and

cows. (The only exception to the rule is the manatee, which has a brain the size of a

chimpanzee’s but is far smoother. This may be because manatees, otherwise known as sea

cows, are unbelievably slow moving and therefore don’t need signals to get across the

brain quickly, but nobody knows for sure.)

If it’s not brain folding, then does brain size determine cognitive sophistication? Not

exactly. Brain size depends mainly on body size. Comparing species with one another,

brain size increases about three-fourths as quickly as body size. It’s not clear why bigger

bodies need bigger brains, but one possibility is that the musculature of larger animals is

more complex and therefore needs a bigger brain to coordinate movement.

On the other hand, having extra brain mass (relative to body size) does seem to increase

cognitive abilities. For example, humans have the largest brains of the animals in our

weight class. The extra growth is concentrated in the cerebral cortex: our ratio of cerebral

cortex to total brain volume (80 percent) is the highest of any mammal. The runners-up, not

surprisingly, are chimpanzees and gorillas.

Fluid intelligence is closely related to working memory, the ability to hold information in your

mind temporarily. Working memory can be as simple as remembering a house number as you’re

walking from your car to a party, or it can be as complicated as keeping track of the solutions that

you’ve already tried for a logic puzzle while you’re trying to think up new potential answers to the

problem. People with high fluid intelligence are resistant to distraction, in the sense that they tend not

to “lose their place” in what they were doing when they temporarily turn their attention to something

else. A brain imaging study found that this improvement was correlated with lateral prefrontal and

parietal cortex activity at high-distraction moments in people with high fluid intelligence.

Genes account for at least 40 percent of the individual variability in general intelligence overall,

but their influence varies substantially depending on the environment (see
Chapter 15)
. Identical

twins reared separately after adoption into middle-class households show a 72 percent correlation in

intelligence, but this is probably an overestimate of the genetic contribution, since the twins shared an

environment before birth (prenatal environment accounts for 20 percent of the correlation) and are

often placed in similar homes. Intelligence test results are also strongly influenced by factors like

education, nutrition, family environment, and exposure to lead paint and other toxins. Indeed, when

the environment is bad, the influence of genes drops as low as 10 percent. Thus, it seems that genes

set an upper limit on people’s intelligence, but the environment before birth and during childhood

determines whether they reach their full genetic potential.

Interactions between genes and environments can be quite complicated, as we’ve said before.

Genetic influences on intelligence become stronger as people get older, perhaps because people seek

out environments that suit their genetic predispositions. For example, people with high intelligence

tend to be drawn toward professions that require them to exercise their reasoning skills regularly,

which may help to keep these skills sharp.

Taken together, this information suggests that proponents of eugenics took exactly the wrong

approach to improving human intelligence. As a society, we could increase average intelligence much

more effectively by improving the environments of children who don’t have the resources to live up

to their genetic potential. The controversy over group differences in intelligence distracts attention

and resources from a much more productive conversation about how we might do that.

Chapter 23

Vacation Snapshots: Memory

During most of London’s history, which goes back several thousand years, the only way to get around

was on foot or by horse-drawn wagon. Because the city was not planned for cars, its roads are a giant

jumble. Streets bend and jog and run at odd angles, and they are often narrow, allowing only one-way

traffic. Traffic circles and tiny parks are everywhere. Street names change from one block to another.

To visitors who are used to streets and avenues organized in orderly grids, it’s a mess.

A time-honored way to avoid all this confusion is to hire a cab. Drivers of London’s black cabs

are legendary for their ability to get to any destination in the city quickly and efficiently. You arrive in

Piccadilly Circus, say, and find a taxi. You put all your luggage in the main passenger compartment

(“Wow, it’s as big as my whole studio apartment in New York!”) and give the driver your address,

“Grafton Way.” After a number of twists and turns—and for most North American tourists, moments

of seat-gripping fear as you watch traffic rushing at you in the right-hand lane—you are safely at your

destination.

Did you know? Forgetting your keys but remembering how to drive

In the movie
Memento
, Leonard has brain damage that leaves him unable to remember

what has happened to him just a few moments before (see
Chapter 2)
. This injury makes his

life confusing and disjointed. Yet he still remembers how to drive a car perfectly well.

How can this be?

Although we commonly think of memory as a single phenomenon, it really has many

components. For instance, our brains are able to remember facts (like the capital of Peru)

and events (yesterday I had lunch with a friend), and to associate a particular sensation

with danger. We also remember how to get to a place in town, how to solve a mechanical

puzzle, and how to do a dance step. All these abilities use different brain regions. Together

these threads make up the fabric of what we call memory.

Leonard’s trouble learning about new facts and events is a defect in what’s called