A possible explanation for the SPE signal is that it merely reflects a general attentional or salience signal, with subjects deploying greater attention on trials in which a given state was more unexpected, compared to those in which it was less unexpected. However, we might expect that attention would be grabbed equally by the delivery of unexpected rewards or omissions of reward, and certainly more by either of these than the unexpected presentations of the somewhat less behaviorally salient visual stimuli, which denote the different states in our task. Thus, we tested the null hypothesis that the areas identified as correlating with an SPE could be also explained by an unspecific surprise signal by examining the correlations between our fMRI data and the absolute value of our signed RPE signal [abs(RPE)], which exactly captures the unexpectedness of the delivery or omission of reward. This abs(RPE) signal correlated with a number of brain regions including an IPS locus anterior to where we found SPE correlates at p < 0.001 uncorrected (Figure S3). However, a direct comparison between SPE and abs(RPE) revealed
