Decision making related to risks and rewards has been studied in the rodent by using several behavioral tests including the use of probability discounting tasks. However, it still remains unclear how long-term values and the different levels of risk are involved in decisions with reward uncertainty. In this study, we used a T-maze choice task to investigate the role of expected reward values in decision making under risk in the rat. The task was specifically set up to run with the expected value (EV) being equal to 1 between binary choice options (small-and-certain vs. large-but-risky rewards). The tests were carried out by providing three reward ratios that represented different levels of riskiness. Moreover, by varying the reward probabilities, the EV of the large-but-risky choice option that was set at either 0.5 or 2 was manipulated to determine whether choice could be influenced by the contrast between unequal EVs. Results demonstrated that the rats were able to distinguish different EVs because they clearly chose the option with a relatively larger EV (2 > 1 and 1 > 0.5). By way of contrast, risk-dependent choice appeared when the EVs were equal; in particular, risk-prone choice was made in a low-risk state, whereas risk-averse choice was made in a high-risk state. The systemic injection of d-amphetamine slightly increased the large-but-risky choices only under the high-risk condition in which the EVs were set to be equal; d-amphetamine did not affect the risk choices in the other two EVs set unequal. Overall, the present results implicate that the contrasts for unequal EVs and the different levels of risk are decision parameters critically involved in the rat's choice. And, relatively low doses of d-amphetamine did not have much of an effect on the present model of risk choice.
Keywords: Evaluation of riskiness; Expected value of reward; Psychostimulant drug; Risk-based decision making; T-maze risk choice.
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