To exclude the possibility that our group difference in UE-E RT (i.e. reduced behavioural surprise in ASD) is explainable by increased response caution in the ASD participants we compared the 12 fastest responders from the ASD group (mean RT 418 ms) against the 12 slowest responders in the NT group (mean RT 540 ms) on the primary UE-E RT difference measure. Here the 12 fastest overall responding ASD participants are those who are most impulsive/least cautious in general responding (i.e. have the lowest response thresholds) whereas the 12 slowest overall NTs are the least impulsive/most cautious (i.e. have the highest response thresholds). Indeed, mean reaction time is significantly faster in this subgroup of ASD participants than in the subgroup of NTs (t(22)=3.38, P=0.03). Nonetheless, independent-samples t-tests revealed that the ASD participants (in this subset of fast general responders) still show significantly diminished behavioural surprise (t(22)=2.39, P=0.026) relative to NTs (in this subset of slow responders). ASD, autism spectrum disorder. NT, neurotypical. RT, reaction time. Data points represent individual participants, red lines indicate the mean, shaded regions and error bars show 95% confidence intervals and 1 standard deviation of the mean. Star indicates significance at P<0.05.