From love and politics to health and
finances, humans can sometimes make decisions that appear irrational, or
dictated by an existing bias or belief. But a new study from Columbia
University neuroscientists uncovers a surprisingly rational feature of
the human brain: A previously held bias can be set aside so that the
brain can apply logical, mathematical reasoning to the decision at hand.
These findings highlight the importance that the brain places on the
accumulation of evidence during decision-making, as well as how prior
knowledge is assessed and updated as the brain incorporates new evidence
This research was reported today in Neuron.
"As we interact with the world every day, our brains constantly form
opinions and beliefs about our surroundings," said Michael Shadlen, MD,
PhD, the study's senior author and a principal investigator at
Columbia's Mortimer B. Zuckerman Mind Brain Behavior Institute.
"Sometimes knowledge is gained through education, or through feedback we
receive. But in many cases we learn, not from a teacher, but from the
accumulation of our own experiences. This study showed us how our brains
help us to do that."
As an example, consider an oncologist who must determine the best course
of treatment for a patient diagnosed with cancer. Based on the doctor's
prior knowledge and her previous experiences with cancer patients, she
may already have an opinion about what treatment combination (i.e.
surgery, radiation and/or chemotherapy) to recommend -- even before she
examines this new patient's complete medical history.
But each new patient brings new information, or evidence, that must be
weighed against the doctor's prior knowledge and experiences. The
central question, the researchers of today's study asked, was whether,
or to what extent, that prior knowledge would be modified if someone is
presented with new or conflicting evidence.
To find out, the team asked human participants to watch a group of dots
as they moved across a computer screen, like grains of sand blowing in
the wind. Over a series of trials, participants judged whether each new
group of dots tended to move to the left or right -- a tough decision as
the movement patterns were not always immediately clear.
As new groups of dots were shown again and again across several trials,
the participants were also given a second task: to judge whether the
computer program generating the dots appeared to have an underlying
Without telling the participants, the researchers had indeed programmed
a bias into the computer; the movement of the dots was not evenly
distributed between rightward and leftward motion, but instead was
skewed toward one direction over another.
"The bias varied randomly from one short block of trials to the next,"
said Ariel Zylberberg, PhD, a postdoctoral fellow in the Shadlen lab at
Columbia's Zuckerman Institute and the paper's first author. "By
altering the strength and direction of the bias across different blocks
of trials, we could study how people gradually learned the direction of
the bias and then incorporated that knowledge into the decision-making
The study, which was co-led by Zuckerman Institute Principal
Investigator Daniel Wolpert, PhD, took two approaches to evaluating the
learning of the bias. First, implicitly, by monitoring the influence of
bias in the participant's decisions and their confidence in those
decisions. Second, explicitly, by asking people to report the most
likely direction of movement in the block of trials. Both approaches
demonstrated that the participants used sensory evidence to update their
beliefs about directional bias of the dots, and they did so without
being told whether their decisions were correct.
"Originally, we thought that people were going to show a confirmation
bias, and interpret ambiguous evidence as favoring their preexisting
beliefs" said Dr. Zylberberg. "But instead we found the opposite: People
were able to update their beliefs about the bias in a statistically
The researchers argue that this occurred because the participants'
brains were considering two situations simultaneously: one in which the
bias exists, and a second in which it does not.
"Even though their brains were gradually learning the existence of a
legitimate bias, that bias would be set aside so as not to influence the
person's assessment of what was in front of their eyes when updating
their belief about the bias," said Dr. Wolpert, who is also professor of
neuroscience at Columbia University Irving Medical Center (CUIMC). "In
other words, the brain performed counterfactual reasoning by asking
'What would my choice and confidence have been if there were no bias in
the motion direction?' Only after doing this did the brain update its
estimate of the bias.
researchers were amazed at the brain's ability to interchange these
multiple, realistic representations with an almost Bayesian-like,
"When we look hard under the hood, so to speak, we see that our brains
are built pretty rationally," said Dr. Shadlen, who is also professor of
neuroscience at CUIMC and an investigator at the Howard Hughes Medical
Institute. "Even though that is at odds with all the ways that we know
ourselves to be irrational."
Although not addressed in this study, irrationality, Dr. Shadlen
hypothesizes, may arise when the stories we tell ourselves influence the
"We tend to navigate through particularly complex scenarios by telling
stories, and perhaps this storytelling -- when layered on top of the
brain's underlying rationality -- plays a role in some of our more
irrational decisions; whether that be what to eat for dinner, where to
invest (or not invest) your money or which candidate to choose."