NEW YORK, March 13 (Reuters) -- Scientists may have come close to
understanding a biochemical process in the brain that influences the
ability to make the right decisions -- to predict what behavioral
choices may be most useful for survival.
A series of experiments on monkeys reported in the journal
Science indicate that brain cells which secrete the neurotransmitter
dopamine play a key role in this process, signaling the best course
of action for a given situation.
The new explanation for how people come to read sensory clues
around them and choose from a number of behaviors for maximum
benefit are based on more than 15 years of primate experiments in
Switzerland, says Dr. P. Read Montague, professor of neuroscience at
Baylor College of Medicine in Houston, Texas.
In these experiments by Montague's co-author Dr. Wolfram Schultz
of the University of Fribourg, electrodes were attached to the
brains of monkeys to record electrical activity in
dopamine-secreting brain cells (neurons). The monkeys were trained
to press a level in response to a certain pattern of light to
receive a reward (a squirt of juice).
"And the electrical activity in these neurons is known to reflect
the delivery of this chemical, dopamine, to the frontal cortex.
Dopamine is one of several neurotransmitters thought to regulate
emotional response, and is suspected of playing a central role in
schizophrenia, Parkinson's disease, and drug abuse," Montague says.
"We think these dopamine neurons are making guesses at likely future
rewards. The neuron is constantly making a guess at the time and
magnitude of the reward."
"If what it expects doesn't arrive, it doesn't change its firing.
If it expects a certain amount of reward at a particular time and
the reward is actually higher, it's surprised by that and increases
its delivery of dopamine," he explains. "And if it expects a certain
level (of reward) and it actually gets less, it decreases its level
of dopamine delivery."
Thus, says Montague, "what we see is that the dopamine neurons
change the way they make electrical impulses in exactly the same way
the animal changes his behavior. The way the neurons change their
predictions correlates with the behavioral changes of the monkey
almost exactly."
Montague and MIT co-author Dr. Peter Dayan pulled together
Schultz's findings and created a mathematical theory based on the
idea of those neuronal guesses and then compared it to what was
actually recorded in primates. "It dead-on predicts the way those
cells are going to fire," Montague says.
The researcher notes further evidence supporting the theory comes
from recent brain-scan studies of people who are missing parts of
their frontal cortex.
"They're missing the part of their frontal cortex that provides
input to these dopamine neurons in their midbrain," says Montague.
"And interestingly enough, these people lack the ability to make
correct decisions about the future when you give them psychological
tasks asking them that -- which is exactly what you'd predict from
the way we construe these neurons."
Montague says the findings in primates may also increase our
understanding of the brain mechanisms affected by drug abuse. "This
is an important piece of the puzzle because we know dopamine is
involved in the same systems that are usurped by drugs of abuse,
like cocaine."
SOURCE: Science (1997;275:1593-1598)
