Neural Basis Of The Impact Of Effort On Reward Processing

Effective processing of reward information is important for the human kind to adapt the society.Reward processing is a relative process that can be affected by many other factors including effort.There exists two opposite theories: effort discounting and effort valuation.In the theory of effort discounting,efforts are considered as costs for obtaining the reward,and thus effort expenditure decreases reward anticipation and the subjective value of the reward.In the theory of effort valuation,however,effort expenditure increases the subjective value of the reward.In this thesis,we propose to use ERP to further investigate the regulation of effort on reward processing,with innovations in the following three aspects.Firstly,as opposed to previous related ERP studies involves only situations whether there exist reward or not,here in this research we introduce reward levels for both positive and negative rewards,being able to investigate the regulation of effort on the sensitivity to reward levels.Secondly,issues such as risk decision,negative reward,and alternative option are introduced to the experiment in order to investigate on how effort regulates reward processing and the impact of risk on reward processing.Lastly,in this research,the impact of effort and risk on the ERP component P2 which is however not considered as a direct player on reward processing.In Experiment 1,a simple reward context is used to investigate the regulation of effort(with three levels: high,low,none)on the processing of positive reward magnitude,with respect to the separation in time of different processes.It is demonstrated that the effect of effort on P2 is significant,in the sense that high effort evokes more positive P2 compared to low and none effort,while P2 is not sensitive to the magnitude of reward.On another ERP component feedback related negativity(FRN),which is the major player on reward processing,the interaction effect of effort and reward magnitude is of significance.Specifically,under high effort,the effect of rewardmagnitude is significant,and larger reward evokes more positive FRN.Under large reward,the effect of effort is of significance,and as compared to low and none effort,high effort evokes more positive FRN.However,under low or no effort,FRN is not sensitive to reward magnitude,while under low reward,it is not sensitive to effort degree.On a later ERP component P300,the effect of effort is significant in the sense that high effort evokes more positive P300,while P300 is not sensitive to reward magnitude.Experiment 2 involves risk decision and positive/negative reward with two levels of magnitude,as well as a positive/negative reward value on an alternative.It is shown that for P2 component,the effect of reward level is significant,and higher negative reward evokes greater P2.As being well known,the magnitude of FRN is determined mainly by reward value,and it is shown that the interaction between reward and effort is significant in the sense that the effect of reward is significant under both high and low effort,and greater reward evokes more positive FRN;and in the same time,under reward +5 and +25,the effect of effort is significant,while high effort increases FRN for +25,it decreases FRN for +5 instead,so effort improves the sensitivity of FRN on the magnitude of positive reward.For the P300 component,the interaction between reward and the other's result is significant.When the other wins,P300 for +25 is the largest,and P300 under-5 is the smallest when the other loses.If the reward is-5,P300 is greater when the other wins.In summary,in both situations high effort evokes more positive FRN under large reward.This supports the theory of effort valuation.Such effect of effort directly improves the sensitivity of FRN on the magnitude of positive reward,resulting significant difference on small and large positive rewards under high but not low effort,and this is consistent with a recent f MRI result,while it also extends the result of a recent ERP study where a with/without effort(only one positive reward magnitude)situation is considered.The results on the regulation of effort on FRN under risk decision is consistent with results under the simple reward situation,withone exception that for small reward high effort has also an effect on FRN but it makes the FRN more negative.Consistent to well-known results,in this study FRN is also not sensitive to negative reward,and furthermore,effort is not able to improve the sensitivity.The results on P300 are also agree with previous studies,but it is also demonstrated that under complicated risk-decision situation,P300 may be determined by several independent factors including reward magnitude,reward valence,and observation effect.This study reports,for the first time,that under reward processing there is an effect of both effort and reward level on P2 component.When there is no negative reward,effort increases P2,while under risk decision situation involving negative reward,the magnitude of P2 is increased by only the great negative reward but not effort.