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IOWA CITY, Iowa -- Decision making highly depends on one's emotions. When
a person cannot generate emotional responses to different circumstances,
which can happen when certain parts of the brain become damaged, the person's
ability to make good decisions may be disturbed.
But, depending on the
location of the brain damage, how decision making is affected may be different,
according to findings from a recent University of Iowa Health Care study.
Researchers found that lesions of the amygdala disrupt emotional conditioning,
whereas lesions to the ventromedial prefrontal (VMF) cortex cause an individual
to have difficulty with conflict situations. These results confirm the
Somatic Marker Hypothesis, a theory advanced by Antonio Damasio, M.D.,
UI professor and head of neurology. Damasio had proposed that there is
a neural circuit critical for processing emotional signals. He predicted
that lesions in any elements of that circuit would result in defects in
decision making. "In essence, damage in either of the two structures (the
amygdala and VMF cortex) lead to impairments in decision making," said
Antoine Bechara, M.D., Ph.D., UI assistant professor of neurology and
lead author of the study that appears in the July issue of the Journal
of Neuroscience. "However, the underlying mechanisms responsible for the
impairments are different.
In real life, the two types of patients exhibit
two types of decision-making deficits." Individuals with amygdala damage
have trouble attaching emotional significance to a previously neutral
event. For example, walking along a road is a neutral, routine event.
However, if a person were mugged while walking along this road, that person
would inevitably experience fear when he or she walked along the road
again. Now, imagine that the individual could not associate this particular
road with fear. The person may decide to continue to walk along the road
and possibly subject himself or herself to another harm in the future.
The decision-making difficulties for individuals with VMF cortex damage
are a bit more complex, Bechara said.
These patients have no trouble learning
simple associations with emotions like the patients with amygdala damage.
The problems for the patients with VMF cortex damage arise when they have
to deal with conflict situations involving immediate reward and distant
consequence. Suppose a spy offered a bribe to sell government secrets.
On one hand, there is the thought of getting caught, being fired and possibly
going to jail. When confronted with this type of conflict, Bechara and
his colleagues believe that people generate two emotional signals -- a
positive signal triggered by the rewarding impact of money resulting from
the bribe, and a negative signal resulting from the fear of being caught.
If the negative/fear emotion is stronger than the positive/reward emotion,
the person will likely turn down the bribe. The UI study suggests that
the immediate effect of any action influences patients with VMF cortex
damage, regardless of whether the immediate effect is correct. In other
words, patients with VMF cortex damage would likely take the bribe in
this situation. "They are oblivious to any consequence that the action
may have in the future," Bechara said.
The decision-making deficit from
VMF cortex damage is usually confined to financial matters and social
relationships. Patients with this type of damage usually do not make decisions
that lead to physical harm to themselves or to others, Bechara explained.
The patients with amygdala damage have similar troubles to the VMF cortex
patients, but in addition, they have more profound problems that can cause
harm to themselves and others, he said. Bechara and his colleagues had
two goals for their study: to determine whether amygdala damage would
interfere with decision making and to find out whether there was a difference
between the roles that the amygdala and VMF cortex played in decision
making.
Bechara and his colleagues based their findings on a study of
23 subjects -- 13 individuals with no brain damage and 10 individuals
with brain damage (five with amygdala damage and five with VMF cortex
damage). The study involved a computerized gambling task test. The subjects
saw four decks of cards -- labeled A, B, C, and D -- on a computer screen.
The goal of the game was to win as much money as possible. If the subjects
found themselves unable to win, they needed to try to avoid losing money
as much as possible. The subjects were free to switch from one deck to
another any time they wished. The experiment shut off automatically after
100 card selections; however, the researchers did not tell the subjects
in advance how many cards they would be able to pick. Using a mouse, the
subjects could click on a card in any of the four decks. The computer
tracked the sequence of selected cards from the decks. Every time the
subjects chose a card, a green bar on the top of the computer screen changed
according to the amount of money won or lost after each selection. Decks
A and B had immediate higher gains, but the later losses were even greater,
for an overall net loss. Decks C and D had immediate lower gains but the
later losses were even lower for an overall net gain.
The researchers
attached electrodes to the subjects' palms to measure skin conductance
responses (SCRs) -- an index of emotional state activation. Each time
the subjects clicked the mouse, the action was recorded as a mark on a
polygram. The SCRs generated during the task were divided into three categories:
reward SCRS, which were generated after winning a sum of money; punishment
SCRs, which were generated after losing a sum of money; and anticipatory
SCRs, which were generated previous to turning a card (i.e., when individuals
were pondering from which deck to choose.) As the task progressed, the
control group (people without brain damage) gradually began choosing more
from the good decks (C and D) and less from the bad decks (A and B). By
contrast, both groups of patients with brain damage failed to make the
shift in behavior. They selected more cards from the bad decks than from
the good ones.
However, the results of the polygram showed that there
was a difference between patients with VMF cortex damage and those individuals
with amygdala damage in the ability to generate SCRs after wins (reward)
or losses (punishment) were received. This physiological difference reflects
a difference in the contribution of each of the two brain structures to
the global process of decision making.
In addition to Bechara, the other
researchers involved in the study included Antonio Damasio; Hanna C. Damasio,
M.D., UI Foundation Distinguished Professor of Neurology; and Gregory
P. Lee, from the section of neurosurgery, Medical College of Georgia.
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