Brain Buys (5 page)

We can now begin to appreciate how semantic memory networks emerge. As a child, how did you learn that a particular furry, snobbish, long-tailed, four-legged creature is called a “cat”? Somewhere during your first years of life some neurons in your brain were activated by the sight of a cat, while others were activated by hearing the word
cat
. (Babies initially have no knowledge that the sounds of the word
cat
and the sight of a cat are related.) Somehow, somewhere along the way, your brain figured out that the auditory and visual format of “cat” were in some sense equivalent. How did this arise? Most likely it was thanks to your mother. Because Mom insisted on saying, “Look at the kitty cat” the first ninety-nine times you saw a cat, she ensured that the auditory and visual “cat” neurons were active at roughly the same time. Enter Hebb’s rule and associative synaptic plasticity: because these neurons fired together, they wired together—they became connected to each other with strong synapses. Eventually the neurons activated by the word
cat
became capable of turning on some of the neurons stimulated by the sight of a cat, allowing you to figure out what Mom was referring to when she said “cat,” even when the moody creature was nowhere to be seen.
¹⁷

I first appreciated the importance of associations in child development as the result of an unplanned and undoubtedly unethical psychological experiment on my sister, who is nine years younger than me. From her earliest days, I addressed my sister primarily by the unkind nickname
Boba
, which in Portuguese means “dummy.” On one occasion, when she was about three years old, a friend and I were playing in the front yard and he yelled “oba” (“yeah!”). My sister mistook this exclamation for Boba, and immediately dashed outside and said, “Yes.” I still recall being struck by two thoughts. First, I really should start calling her by her real name, and, second, in retrospect, she would have had no way of knowing that
Boba
was a pejorative term and not her name (or one of her names). If someone generates a specific sound every time they interact with you, your brain cannot help but build an association with that word and yourself—it is what the brain is programmed to do.

One of the ingenious properties of this associative architecture is that it is self-organizing: information is categorized, grouped, and stored in a way that reflects the world in which we live.
¹⁸
If you live in India your “cow” neurons will likely be connected to your “sacred” neurons, whereas if you live in Argentina your “cow” neurons will likely be strongly connected to your “meat” neurons. Because of its self-organizing nature, human memory is in many ways vastly superior to the mindless strategy of precisely capturing experiences with a video camera. The associative architecture of the brain ensures that memory and meaning are intertwined: the links are both the memory
and
the meaning.

PRIMING: GETTING IN THE MOOD

Now that we have some understanding of how memories are stored and organized in the brain, we can return to the phenomenon of priming. The fact that we can nudge people into thinking of a zebra by evoking thoughts of Africa and black and white is not only because knowledge is stored as a network of associated concepts, but because memory retrieval is a contagious process. Entirely unconsciously, activation of the “Africa” node spreads to others to which it is linked, increasing the likelihood of thinking of a zebra. Psychologists often study priming by determining the influence of a word (the prime) on the time it takes to make a decision about a subsequent word (the target). In this type of experiment you sit in front of a computer screen while words and nonwords (plausibly sounding pseudo-words such as “bazre”) are flashed one by one. Your job is to decide as quickly as possible if the stimulus represents a real word or not. If the word butter were flashed, it might take 0.5 seconds to respond. But if bread were flashed before the presentation of butter your reaction time might fall to 0.45 seconds. Loosely speaking this increase in speed is because activity in the group of neurons encoding bread spreads to related concepts, accelerating recognition of the word butter. The ability of “bread” to prime “butter” may not be universal: these words have a strong association for Americans because they often put butter on their bread, and because “bread and butter” is an expression referring to financial support; but it is possible that there would be little or no such increase in speed with people from China, where the custom of buttering one’s bread is less common.

Given its importance, it’s unfortunate that we don’t really know what priming corresponds to in terms of neurons and synapses.
¹⁹
One theory is that during the semantic priming task, when the neurons representing “bread” are activated they continue to fire even after “bread” is no longer visible. Like a dying echo this activity progressively fades away over a second or so, and during this fade out neurons continue to whisper to their partners. Thus the neurons representing “butter” receive a boost, even before “butter” is presented, and fire more quickly.
²⁰

Irrespective of the precise neural mechanisms, priming is clearly embedded within the brain’s hardware. Like it or not, whenever you hear one word your brain unconsciously attempts to anticipate what might be coming next. Thus “bread” will not only prime “butter,” but depending on the specifics of your neural circuits it will also prime “water,” “loaf,” and “dough.” Priming probably contributes to our ability to rapidly take into account the context in which words occur and resolve the natural ambiguities of language. In the sentence, “Your dog ate my hot dog,” we know that the second use of “dog” refers to a frankfurter as opposed to a dog that is hot. The use of the word
ate
earlier in the sentence provides context—it primes the correct interpretation of the second use of “dog,” helping to establish the appropriate meaning of the sentence.

The spread of activity from an activated node to its partners is of fundamental importance because it influences almost all aspects of human thought, cognition, and behavior. Consider a conversation you might have with someone you have never met before. As the dialogue proceeds, the topic changes, establishing a conversational trajectory. What determines this trajectory? Human interactions are influenced by many complex factors, but there are patterns. A conversation might start with geography (Where are you from?). If the answer is Rio de Janeiro, the topic may veer toward soccer or Carnival. If the answer is Paris, the topic could head toward food or museums. The transitions of conversations are often primed by the previous topic. But importantly, these transitions depend on the specific structure of the conversationalists’ semantic nets. Indeed, when you know someone well, it is not difficult to elicit a given story or topic from them (or prevent them from telling the story you have heard a million times) by mentioning or avoiding certain priming words.

MEMORY BUGS

Priming is one of the most valuable features of the brain, but it is also responsible for many of our brain bugs. We have already seen that false memories can be generated because we confuse related words. Given the words
thread
,
pin
,
sharp
,
syringe
,
sewing
,
haystack
,
prick
, and
injection
, people will often insist that “needle” was among them. Remembering the gist of something is a useful feature of memory, because it is often the gist that really matters. Let’s suppose you are setting out on an expedition and are told that the forest contains anacondas, poison ivy, quicksand, scorpions, cannibals, alligators, and rodents of unusual size. When your traveling companion asks you whether you think you should head through the forest or across the river, you may not be able to convey all the reasons why the river is the superior choice, but the general gist will be a cinch to remember.

In many circumstances, however, simply remembering the gist will not suffice. If your significant other asks you to buy a few things on the way home from work it’s not sufficient to remember the gist of the list; family harmony is best achieved by remembering whether bread
or
butter was one of the items. To my embarrassment I once caught myself making a memory error that was clearly due to priming and the common association between crocodiles and alligators. In the pursuit of the plastic footwear named “Crocs,” I found myself asking a salesperson if he sold “alligators.”

Because individual experience sculpts our semantic nets, different individuals would be expected to have different susceptibilities to some types of errors. In a study performed by the psychologist Alan Castel and his colleagues, volunteers were given a list of names of animals to memorize:
bears
,
dolphins
,
falcons
,
jaguars
,
rams
, and so on—all animals that have American football teams named after them. Not surprisingly, people who were football fans were better at memorizing the list (presumably because they had a richer set of links associated with each animal name). But they were also more likely to have false memories, and mistakenly believe that eagles or panthers (also the names of football teams, but that were not on the list) had been presented.
²¹

You probably have your own examples of memory errors caused by the hyperlinked associative networks in your cortex. As annoying as these errors are, they are generally not life-threatening. But in some cases they can be. Paxil, Plavix, Taxol, Prozac, Prilosec, Zyrtex, and Zyprexa are all on the Institute for Safe Medication Practice’s list of frequently confused drug names.
²²
The confusion of medications by doctors, pharmacists and patients is responsible for medical mistakes, and up to 25 percent of medication errors are related to confusing pharmaceutical names. Indeed, as part of the drug approval process, the Federal Drug Administration screens drug names precisely to decrease this type of error. Some of these memory errors arise when medical professionals confuse drugs that are in the same category: Paxil and Prozac are both a specific type of antidepressant, with similar mechanisms of action, so their names readily become linked in our neural nets. Other errors result from drugs having similar names such as Xanax and Zantac, or Zyrtex and Zyprexa. Here the drugs may share associations because the brain represents their pronunciation or spelling by using similar nodes.

Is the rock formation hanging from the ceiling of a cave a stalagmite or a stalactite? Is a bump on a road concave or convex? Is the big guy Penn or Teller? Why do we confuse the words representing distinct but related concepts? Because if two concepts that are not used much share most of their links—similar spelling, pronunciation, contexts, or meaning—they run the risk of becoming so entwined as to be indistinguishable.

We are now in a better position to understand the causes of the Baker/baker paradox, which shows that we are more likely to remember professions than names—even when they are the same word. Throughout your life the profession “baker” has acquired many associations (bread, funny hat, dozen, cake, getting up early). By contrast, the name “Baker” pretty much stands alone (unless, of course, your name happens to be Baker). In other words, the “baker” nodes are well connected, whereas the “Baker” nodes are loners, and that is why “Baker” is more difficult to remember.
²³
When we are introduced to a baker more links are activated than when we are introduced to Mr. Baker; the increased number of links may translate into a more enduring memory because a larger number of synapses are contenders to undergo synaptic plasticity. A common mnemonic device to remember names is to associate them with something more memorable (Richard with being rich or Baker with a baker). This trick may work because it “borrows” links and synapses from nodes that would not otherwise be used, increasing the number of synapses involved in memory storage. Although we will have to await future research to confirm this explanation, we can begin to understand the cause of one of the most maligned characteristics of human memory: the difficulty in remembering names. The associative architecture of the brain offers a powerful way to organize and store knowledge, but like a Web page that nobody links to, a node without many links is difficult to find.

IMPLICIT ASSOCIATIONS

Priming and the associative architecture of our memory can have even spookier and more far-reaching effects than those arising from the confusion of related concepts and words. We generally view memory as a neutral source of information about the world, but it turns out that the way information is stored can sway our behavior and opinions in an entirely unconscious fashion.

A simple example of how the associative architecture of memory influences how we use and access the information stored in our neural nets is illustrated by what’s known as an
implicit association test
. Each word in the list below is either a flower or insect, or a word with a “positive” or “negative” connotation (for example, “helpful” or “nasty”). Your task is to categorize each word as quickly as possible by checking the left column if the word is a flower or can be said to be something positive, and the right column if it is an insect or represents something negative. If you’re in a quantitative mood you can time yourself to find out how long it takes you to complete the first part of this twelve-word test.