Gain - specific and probability - independent reward skewness coding in the 1 insula

22 Rewards in the natural environment are rarely predicted with complete certainty. Uncertainty 23 relating to future rewards has typically been defined as the variance of the potential 24 outcomes. However, the asymmetry of predicted reward distributions, known as skewness, 25 constitutes a distinct but neuroscientifically underexplored risk term that may also have an 26 impact on preference. By changing only reward magnitudes we study skewness processing in 27 equiprobable ternary lotteries involving only gains and constant probabilities, thus excluding 28 probability distortion or loss aversion as mechanisms for skewness preference formation. We 29 show that individual preferences are not only sensitive to the mean and variance, but also to 30 the skewness of predicted reward distributions. Using neuroimaging we show that the insula, 31 a structure previously implicated in the processing of reward-related uncertainty, responds to 32 the skewness of predicted reward distributions. Some insula responses increased in a 33 monotonic fashion with skewness (irrespective of individual skewness preferences) whereas 34 others were similarly elevated to both negative and positive as opposed to no reward skew. 35 These data support the notion that the asymmetry of reward distributions is processed in the 36 brain and, taken together with replicated findings of mean coding in the striatum and variance 37 coding in the cingulate, suggest that the brain codes distinct aspects of reward distributions in 38 a distributed fashion. 39