Right
amygdala
Jenison’s group found 16 individual cells
that responded in a way consistent with
the values that participants assigned to
individual foods. The study, published
January 5 in the Journal of Neuroscience ,
is the first to show how single neurons in
the human amygdala work to assess the
value of an object (SN: 1/29/11, p. 8). In
addition, the study shows how amygdala neurons compute value in real time,
while decisions are being made.
“These findings suggest that the
human amygdala’s involvement in decision making happens at the time of the
choice and is not restricted to familiar
situations,” he says.
Many of the animal studies, on the
other hand, are based on learned tasks, so
value could be assigned over a number of
trials, Jenison says.
Using functional MRI to map the location of the electrodes, Jenison’s group
created an anatomical map showing
the precise location of the value-coding
neurons. Such information may serve as
a starting point in figuring out how such
neurons connect to other structures in
the brain.
Because value-coding neurons in the
amygdala intermingle with neurons that
do other things, sorting out connections
could be crucial for understanding how
people respond to particular cues, says
neuroscientist Patricia Janak of the University of California, San Francisco, who
studies the amygdala’s role in reward
and addiction. “The idea would be to
see where those [value-coding] neurons
are going, and then hence what behavior
they affect,” she says.
both: wr. jenison et al/journal of neuroscience 2011
Untangling connections
While recent studies show that the
amygdala plays a role in assigning value
to actions or things, other findings show
that parts of the prefrontal cortex also
signal what things are worth.
“We were really interested to know: Is
the amygdala the thing which supplies
that value information or is it the other
way around — the prefrontal cortex that
tells the amygdala?” says Peter Rudebeck,
a postdoc in Murray’s lab. “The anatomy
suggests that it’s the amygdala telling
Electrode
location
Nerve cell response during value rating
Discharge rate (spikes per second)
25
spike rates
Presentation of stimuli
15
5
60 65 85
70 75 80
Tracking chatter by monitoring nerve
cell activity in patients as they rated the value
of various junk foods, researchers were able
to locate specific cells in the amygdala whose
responses corresponded with value ratings.
the graph shows how one neuron’s firing rate
(electrode location shown above) peaked
after junk foods were presented.
Time (seconds)
these other parts of the brain what these
certain things are worth. But we wanted
to test that.”
Rudebeck and his group trained
monkeys to play a computer game in
which they assessed the value of differ-
ent rewards. The animals were shown
two different pictures and allowed to
choose between them. One picture
brought a large juice reward, and the
other brought a much smaller amount
of juice. The animals chose the picture
associated with the larger reward more
than 98 percent of the time.
After turning off the amygdala in some
animals, the scientists used single-cell
recordings to listen in on brain cell
chatter in the orbitofrontal cortex and
anterior cingulate cortex. To the team’s
surprise, the monkeys still chose the picture with the “best” outcome on pretty
much every trial, just as they had done
with a working amygdala.
Though the animals continued choos-
ing in the same manner, the scientists
found that fewer neurons in the ante-
rior cingulate cortex changed their firing
rate in response to an expected reward.
When looking at the animals’ emo-
tional responses — as measured by pupil
diameter and heart rate — researchers
found that monkeys without a working
amygdala didn’t react to a reward in
the typical way, Rudebeck says. “They
seemed to have no idea of what reward
was, despite the fact that they could still
choose perfectly well.”
The findings, reported at the neuro-
science meeting, suggest that the brain
uses various mechanisms to calculate
how much something is worth. While the
amygdala may be important for assign-
ing an emotional value, Rudebeck says,
it may not be the “be-all and end-all” in
valuing objects. “I’m kind of glad that the
brain isn’t as simple as we sometimes
make it out to be,” he says.
Explore more
s c.d. salzman and s. Fusi. “emotion,
cognition, and Mental state representation in amygdala and Prefrontal
cortex.” annual review of neuro
science 2010.
www.sciencenews.org
February 26, 2011 | science news | 25
