Learning and the Basal Ganglia

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Many human behaviors--perhaps more than we would like to think--are, in
essence, reflexes programmed into our brains when we are rewarded or
punished for taking a particular action. New research is showing how
the basal ganglia, deep inside the brain below the cortex, are
important in learning from feedback, in the formation of good and bad
habits, and even in brain disorders as diverse as Parkinson's disease,
ADHD, and addiction. Reflexes deserve respect, writes the author, and
understanding how people differ in learning from positive or negative
feedback may have implications for education as well as for treating
diseases in which the basal ganglia's systems go awry.


Before you read any further, grab a glass of your favorite beverage
and set it down (no drinking yet). Done? Now, reach both hands around
your back and touch your pinkies together. Then quickly take a sip of
your drink. Go on.

Did you do it? If so, the next time you find yourself in a similar
environment you will have a greater chance of spontaneously repeating
this round-the-back pinkie act. Although that possibility may sound
strange, your brain is actually programmed to reinforce actions that
are immediately followed by rewards. This is especially true when the
reward is unexpected (you probably did not expect to have a treat when
you began to read this article).

Although most of us feel like we are in control of our actions, many
of those actions can also be explained by principles of learning that
are embedded in our neural machinery. Of course, this machinery is
inordinately intricate and complex, The more a behavior is ingrained,
the more its neural representations in the basal ganglia are
strengthened and honed. involving several interacting systems, each
with millions of neurons, billions of connections, and multiple
neurotransmitters, all evolving dynamically as a function of genes,
time, past experience, and current environment. But neuroscience is
shedding light on how circuits linking two parts of the brain, the
basal ganglia and the frontal cortex, contribute to learning both
productive and counterproductive behaviors, and even to some
neurological disorders. Those circuits can, for example, help account
for genetically driven individual differences in whether we learn best
from positive or negative reinforcement, and understanding them
provides insights into decision making in people with Parkinson's
disease, attention-deficit hyperactivity disorder, and addictions.

Basal Ganglia Basics
The basal ganglia are a collection of interconnected areas deep below
the cerebral cortex. They receive information from the frontal cortex
about behavior that is being planned for a particular situation. In
turn, the basal ganglia affect activity in the frontal cortex through
a series of neural projections that ultimately go back up to the same
cortical areas from which they received the initial input. This
circuit enables the basal ganglia to transform and amplify the pattern
of neural firing in the frontal cortex that is associated with
adaptive, or appropriate, behaviors, while suppressing those that are
less adaptive. The neurotransmitter dopamine plays a critical role in
the basal ganglia in determining, as a result of experience, which
plans are adaptive and which are not.

Evidence from several lines of research supports this understanding of
the role of basal ganglia and dopamine as major players in learning
and selecting adaptive behaviors. In rats, the more a behavior is
ingrained, the more its neural representations in the basal ganglia
are strengthened and honed.1 Rats depleted of basal ganglia dopamine
show profound deficits in acquiring new behaviors that lead to a
reward. Experiments pioneered by Wolfram Schultz, M.D., Ph.D., at the
University of Cambridge have shown that dopamine neurons fire in
bursts when a monkey receives an unexpected juice reward.2 Conversely,
when an expected reward is not delivered, these dopamine cells
actually cease firing altogether, that is, their firing rates "dip"
below what is normal. These dopamine bursts and dips are thought to
drive changes in the strength of synaptic connections--the neural
mechanism for learning--in the basal ganglia so that actions are
reinforced (in the case of dopamine bursts) or punished (in the case
of dopamine dips).


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