Figure 3. Post Outcome Pupil Dilation Tracks Both Environmental Volatility and Outcome Surprise.

Time courses for the effect of trial-wise estimates of (a) volatility and (b) surprise on pupil dilation following presentation of the outcome. The graphs show the mean across participants (n=28) of the beta weights obtained by regressing post outcome pupil dilation against trial-wise estimates of environmental volatility and outcome surprise. Post outcome pupil dilation was greater for trials where environmental volatility was high, F(1,26)=9.8, p=0.004, and the outcome was surprising, F(1,26)=9.2, p=0.005. Asterisks indicate 1s time bins in which the effect of volatility or surprise on pupil dilation post outcome differed significantly from zero (bonferonni corrected for multiple comparisons, ps corrected <.05). The effect of trial-wise volatility was longer lasting and had a later onset than that of outcome surprise. (c) The degree to which an individual’s pupil tracked volatility (calculated as the mean beta weight across the 6 second post-outcome period) predicted change in learning rate between volatile and stable blocks, r(26)=0.37, p=0.05. (d) The degree to which an individual’s pupil tracked surprise predicted extent of surprise-related choice reaction time slowing on the subsequent trial, r(26)=0.44, p=0.02. Shaded regions in panels a and b represent the standard error of the mean. Error bars in panels c and d represent the standard deviations of the regression coefficients (beta weights) from the pupil analysis and the parameter estimates from the behavioral model for each subject.