Roles of orbitofrontal and insular cortices in processing reward variables

Abstract

Delays and risks are important in evaluation of subjective value. We performed two experiments to examine how these reward variables are processed in our brain. Although choices of both humans and animals are more strongly influenced by immediate than delayed rewards, methodological limitations have made it difficult to estimate the precise form of temporal discounting in animals. In the present study, we sought to characterize temporal discounting in rats and to test the role of the orbitofrontal cortex (OFC) in this process. Rats were trained in a novel intertemporal choice task in which the sequence of delay durations was randomized across trials. The animals tended to choose a small immediate reward more frequently as the delay for a large reward increased, and, consistent with previous findings in other species, their choice behavior was better accounted for by hyperbolic than exponential discount functions. In addition, model comparisons showed that the animal’s choice behavior was better accounted for by more complex discount functions with an additional parameter than a hyperbolic discount function. Following bilateral OFC lesions, rats extensively trained in this task showed no significant change in their intertemporal choice behavior. Our results suggest that the rodent OFC may not always play a role in temporal discounting when delays are randomized and/or after extensive training. Risk is essential information in decision making, because it is a universal phenomenon in the environment. Brain imaging studies in humans have shown that anterior insular cortex (AIC) is affected by risk in decision situations. However previous studies have not examined neural activity related to risk in AIC. Moreover, damages in OFC have yielded conflicting results, even they showed altered risky choice behavior. In the present study, we tested neural responses in AIC and in OFC of rats under risk condition. Rats were trained in a Pavlovian conditioning task in which five different auditory cues (1 s) were associated with five different reward probabilities (0%, 25%, 50%, 75% and 100%). Since reward probabilities were symmetric, riskiness is divided into three levels, with the highest risk associated with 50% reward probability, a medium risk with 25% and 75% probabilities, and the lowest risk with 0% and 100% probabilities. Accordingly, neural responses to risk can be dissociated from those to reward probability and expected reward value. Both brain regions represented signals related to risk as well as signals related to expected value, and only the AIC showed multi-phasic activities responded to reward. These results suggest that both AIC and OFC can participate in evaluating subjective value, through conveying signals associated with expected value and risk and through conveying unique signals responded to reward.

인간과 동물의 선택은 지연된 보상보다 즉각적인 보상에 의해 더 큰 영향을 받지만, 방법적인 한계로 인해 동물에서의 temporal discounting 의 정확한 양상을 측정하는 것이 어려운 일이었다. 본 연구에서 우리는 쥐에서 temporal discounting 의 특징을 묘사하고 이 과정에서의 안와전두피질의 역할을 규명하고자 했다. 쥐들은 지연된 시간의 순서가 매 시행에서 무작위적으로 제시되는 독창적인 intertemporal choice task 에서 훈련을 받았다. 동물들은 큰 보상을 얻기 위해 기다려야 하는 시간이 길어질수록 작은 양이지만 즉각적인 보상을 더 자주 선택하는 경향을 보였다. 이러한 경향은 다른 동물 종에서의 이전 연구들과 마찬가지로 exponential discount function 보다 hyperbolic discount function 에 의해 더 잘 설명이 되었다. 또한, 모델 비교를 통해 추가적인 계수가 포함된 더 복잡한 모델이 hyperbolic function 보다 동물의 행동을 더 잘 설명한다고 나타났다. 본 행동 과제에서 매우 충분히 학습된 쥐 뇌의 양쪽 안와전두피질을 손상시켰을 경우, 그들의 선택 경향에 유의미한 변화를 보이지 않았다. 우리의 결과는 충분한 학습과 지연되는 시간이 무작위적으로 제시될 때 설치류 안와전두피질이 temporal discounting 에 역할을 하지 않을 수도 있다는 것을 나타낸다. 위험성은 환경에서 일반적인 현상이므로, 의사 결정을 할 때 중요한 정보이다.