Thousands of
words, big and small, are crammed inside our memory banks just waiting to be
swiftly withdrawn and strung into sentences. In a recent study of epilepsy
patients and healthy volunteers, National Institutes of Health researchers
found that our brains may withdraw some common words, like “pig,” “tank,” and
“door,” much more often than others, including “cat,” “street,” and “stair.” By
combining memory tests, brain wave recordings, and surveys of billions of words
published in books, news articles and internet encyclopedia pages, the
researchers not only showed how our brains may recall words but also memories
of our past experiences.
“We found that some words are
much more memorable than others. Our results support the idea that our memories
are wired into neural networks and that our brains search for these memories,
just the way search engines track down information on the internet,” said
Weizhen (Zane) Xie, Ph.D., a cognitive psychologist and post-doctoral fellow at
the NIH’s National Institute of Neurological Disorders and Stroke (NINDS), who
led the study published in Nature Human Behaviour. “We hope
that these results can be used as a roadmap to evaluate the health of a
person’s memory and brain.”
Dr. Xie and his colleagues first spotted these words
when they re-analyzed the results of memory tests taken by 30 epilepsy patients
who were part of a clinical trial led by Kareem Zaghloul, M.D., Ph.D., a
neurosurgeon and senior investigator at NINDS. Dr. Zaghloul’s team tries to
help patients whose seizures cannot be controlled by drugs, otherwise known as
intractable epilepsy. During the observation period, patients spend several
days at the NIH’s Clinical Center with surgically implanted electrodes designed
to detect changes in brain activity.
“Our goal is to find and eliminate the source of these
harmful and debilitating seizures,” said Dr. Zaghloul. “The monitoring period
also provides a rare opportunity to record the neural activity that controls
other parts of our lives. With the help of these patient volunteers we have
been able to uncover some of the blueprints behind our memories.”
The memory tests were originally designed to assess
episodic memories, or the associations — the who, what, where and how details —
we make with our past experiences. Alzheimer’s disease and other forms of
dementia often destroys the brain’s capacity to make these memories.
Patients were shown pairs of words, such as “hand” and
“apple,” from a list of 300 common nouns. A few seconds later they were shown
one of the words, for instance “hand,” and asked to remember its pair, “apple.”
Dr. Zaghloul’s team had used these tests to study how neural circuits in the
brain store and replay memories.
When Dr. Xie and his colleagues re-examined the test
results, they found that patients successfully recalled some words more often
than others, regardless of the way the words were paired. In fact, of the 300
words used, the top five were on average about seven times more likely to be
successfully recalled than the bottom five.
At first, Dr. Zaghloul and the team were surprised by
the results and even a bit skeptical. For many years scientists have thought
that successful recall of a paired word meant that a person’s brain made a
strong connection between the two words during learning and that a similar
process may explain why some experiences are more memorable than others. Also,
it was hard to explain why words like “tank,” “doll,” and “pond” were
remembered more often than frequently used words like “street,” “couch,” and
“cloud.”
But any doubts were quickly diminished when the team
saw very similar results after 2,623 healthy volunteers took an online version
of the word pair test that the team posted on the crowdsourcing website Amazon
Mechanical Turk.
“We saw that some things — in this case, words — may
be inherently easier for our brains to recall than others,” said Dr. Zaghloul.
“These results also provide the strongest evidence to date that what we
discovered about how the brain controls memory in this set of patients may also
be true for people outside of the study.”
Dr. Xie got the idea for the study at a Christmas
party which he attended shortly after his arrival at NIH about two years ago.
After spending many years studying how our mental states — our moods, our
sleeping habits, and our familiarity with something — can change our memories,
Dr. Xie joined Dr. Zaghloul’s team to learn more about the inner-workings of
the brain.
“Our memories play a fundamental role in who we are
and how our brains work. However, one of the biggest challenges of studying
memory is that people often remember the same things in different ways, making
it difficult for researchers to compare people’s performances on memory tests,”
said Dr. Xie. “For over a century, researchers have called for a unified
accounting of this variability. If we can predict what people should remember
in advance and understand how our brains do this, then we might be able to
develop better ways to evaluate someone’s overall brain health.”
At the party, he met Wilma Bainbridge, Ph.D., an
assistant professor in the department of psychology at the University of
Chicago, who, at the time was working as a post-doctoral fellow at the NIH’s
National Institute of Mental Health (NIMH). She was trying to tackle this same
issue by studying whether some things we see are more memorable than others.
For example, in one set of studies of more than 1000
healthy volunteers, Dr. Bainbridge and her colleagues found that some faces are
more memorable than others. In these experiments, each volunteer was shown a
steady stream of faces and asked to indicate when they recognized one from
earlier in the stream.
“Our exciting finding is that there are some images of
people or places that are inherently memorable for all people, even though we
have each seen different things in our lives,” said Dr. Bainbridge. “And if
image memorability is so powerful, this means we can know in advance what
people are likely to remember or forget.”
Nevertheless, these results were limited to
understanding how our brains work when we recognize something we see. At the
party, Drs. Xie and Bainbridge wondered whether this idea could be applied to
the recall of memories that Dr. Zaghloul’s team had been studying and if so,
what would that tell us about how the brain remembers our past experiences?
In this paper, Dr. Xie proposed that the principles
from an established theory, known as the Search for Associative Memory (SAM)
model, may help explain their initial findings with the epilepsy patients and
the healthy controls.
“We thought one way to understand the results of the
word pair tests was to apply network theories for how the brain remembers past
experiences. In this case, memories of the words we used look like internet or
airport terminal maps, with the more memorable words appearing as big, highly
trafficked spots connected to smaller spots representing the less memorable
words,” said Dr. Xie. “The key to fully understanding this was to figure out
what connects the words.”
To address this, the researchers wrote a novel
computer modeling program that tested whether certain rules for defining how
words are connected can predict the memorability results they saw in the study.
The rules were based on language studies which had scanned thousands of
sentences from books, news articles, and Wikipedia pages.
Initially, they found that seemingly straightforward
ideas for connecting words could not explain their results. For instance, the
more memorable words did not simply appear more often in sentences than the
less memorable ones. Similarly, they could not find a link between the relative
“concreteness” of a word’s definition and its memorability. A word like “moth”
was no more memorable than a word that has more abstract meanings, like
“chief.”
Instead, their results suggested that the more
memorable words were more semantically similar, or more often linked to the
meanings of other words used in the English language. This meant, that when the
researchers plugged semantic similarity data into the computer model it
correctly guessed which words that were memorable from patients and healthy
volunteer test. In contrast, this did not happen when they used data on word
frequency or concreteness.
Further results supported the idea that the more
memorable words represented high trafficked hubs in the brain’s memory
networks. The epilepsy patients correctly recalled the memorable words faster
than others. Meanwhile, electrical recordings of the patients’ anterior
temporal lobe, a language center, showed that their brains replayed the neural
signatures behind those words earlier than the less memorable ones. The
researchers saw this trend when they looked at both averages of all results and
individual trials, which strongly suggested that the more memorable words are
easier for the brain to find.
Moreover, both the patients and the healthy volunteers
mistakenly called out the more memorable words more frequently than any other
words. Overall, these results supported previous studies which suggested that
the brain may visit or pass through these highly connected memories, like the
way animals forage for food or a computer searches the internet.
“You know when you type words into a search engine,
and it shows you a list of highly relevant guesses? It feels like the search
engine is reading your mind. Well, our results suggest that the brains of the
subjects in this study did something similar when they tried to recall a paired
word, and we think that this may happen when we remember many of our past
experiences,” said Dr. Xie. “Our results also suggest that the structure of the
English language is stored in everyone’s brains and we hope that, one day, it
is used to overcome the variability doctors face when trying to evaluate the
health of a person’s memory and brain.”
The team is currently exploring ways to incorporate
their results and computer model into the development of memory tests for
Alzheimer’s disease and other forms of dementia.
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