Abstracts

Rationale: People do not always make rational decision People tend to overweight low probabilities and underweight high probabilities. For example, someone buys lottery tickets with low probabilities, and someone fears a plain crash than a car crush. This tendency is called nonlinear probability weighting . And people tend to decrease the subjective value of reward in the future. For example, someone prefers to $90 available now than $100 available one month later. This is called temporal discounting . Peters and Buchel (2010) hypothesized that hippocampus codes temporal discounting and engages in episodic future thinking. We plan to confirm this hypothesis in patients with mesial temporal epilepsy, who are supposed to have hippocampal dysfunction. Methods: The subjects were nine patients with mesial temporal lobe epilepsy who underwent unilateral hippocampectomy or hippocampal MST. The probability weighting task and time-discounting task was performed before and after surgery. In the probability weighting , probability weighting function was evaluated. Two choices, one was a certain reward and the other was an uncertain and larger reward were presented, and the subjects were asked to select favorable one. In the time-discounting task , temporal discounting was investigated. Two choices, one was a certain reward and the other was a delayed and larger reward were presented, and the subjects were asked to select favorable one. The informed consents were taken from all the subjects. Results: In the probability weighting task, patients showed a pronounced tendency to overweight low probabilities and to underweight high probabilities after surgery compared with before surgery. In the time-discounting task, patients showed a higher temporal discounting tendency after surgery. Conclusions: The patients tended to show more irrational decision-making after surgery in probability weighting task. It may be suggested that hippocampectomy changes the probability weighting function and increases gambling tendency. The patients also tended to show more pronounced time-discounting after surgery. It may reflect increased impulsiveness. Considering with the previous study, hippocampectomy might affect adversely to the function of episodic future thinking.