July 29, 1999 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.
The UI-led study was supported by a grant from the National Institute of Neurological Diseases.
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