You're about to leave work at the end of the day when
your cell phone rings: It's your spouse,
asking that you pick up a gallon of milk on the way home.
Before you head out the door, though, your
spouse calls again and asks you to stop by the hardware
store, too. Based on your knowledge of the
area and rush hour traffic, you decide to get the milk
first and the toilet plunger second. But whoops!
The phone rings again. This time, it's your boss, asking
you to work late. That means another change of
plans.
Adjusting our behavior to such changing circumstances
enables us to achieve our goals. But how,
exactly, do our brains switch so elegantly and quickly from
one well-entrenched plan to a newer one in
reaction to a sudden change in circumstances? In the
milk-hardware-boss example, do we simply
remember a list of streets and turns, or do we remember a
more abstract set of "rules" governing the
web of relationships between the items we want to buy, our
driving route and our relationships with
spouse and employer?
The answer is both, according to researchers at Johns
Hopkins, who have learned that two
different areas of the brain are responsible for the way
human beings handle complex sets of "if-
then" rules. The researchers, led by
Susan Courtney, associate professor of
psychological and brain
sciences in the Krieger School of Arts and Sciences,
learned that rules that people must actively
remember (in other words, which are not part of their
everyday habits) are controlled primarily
through the prefrontal cortex, which is in the very front
of the brain, beneath the forehead.
"This discovery may eventually lead to enhanced
understanding of psychiatric diseases such as
schizophrenia, obsessive-compulsive disorder and attention
deficit disorder, all conditions in which a
person's ability to remember and change such rules is
impaired," said Courtney, lead author of a paper
in a recent issue of Neuron.
Courtney and her team used mental math tasks (a good
working example of "if-then" rules) and
functional magnetic resonance imaging to investigate which
areas of the brain are used for different
functions. Before beginning the study, participants
memorized the numbers 47 and 53 and the
operations (rules) "add" and "subtract." Only one of the
two numbers and one of the two operations
were relevant to any given trial. For example, participants
would begin by remembering either 47 or 53
and the instructions to either "add" or "subtract." They
then would be given a second number, which
they would add to or subtract from the first until
instructed to make a change. That change could
involve keeping the add or subtract rule and switching the
number, keeping the number and switching
the rule, or switching both the beginning number and the
rule.
Courtney said that if we hold both rule and number in
our memory in the same way, then there
would be no difference in the pattern of activity when
people were asked to switch the rules
compared to when they changed numbers because both rules
and numbers would be in the same place
in memory.
But that's not what they found.
Instead, they discovered that the prefrontal cortex
became more active when participants had
to switch rules, and a different part of the brain —
the parietal cortex, which is near the back of the
head — became more active when the participants were
asked to switch numbers.
"This indicates that different parts of our brains
store different kinds of memories and
information," Courtney said. That, she said, "provides
clues about how the human brain accomplishes
complex, goal-directed behaviors that require remembering
and changing abstract rules, an ability
that is disrupted in many mental illnesses."
Caroline Montojo, a graduate student in the Department
of Psychological and Brain Sciences,
was a co-author on this study.
This work was supported by the National Institutes of
Health and the National Science
Foundation.