Figure 5. Physiological responses reflect uncertainty and surprise.

(a) Median splits indicate that both irreducible uncertainty and surprise increase pupil diameter (both P<0.001). Shading is s.e.m. across participants. (b) Baseline pupil diameter and skin conductance on each trial displayed a clear inverted-U relationship with belief, as seen for subjective stress. A curve describing the variance of a Bernoulli distribution fit well (Pearson correlations: pupil diameter: r=0.96, P<0.001; skin conductance: r=0.84, P=0.002). Error bars represent s.e.m. (c) A multiple regression model demonstrated a role for uncertainty throughout the trial for both pupil diameter (robust regression β=0.11, single-sample t-test, t₂₁=4.72, P<0.001) and skin conductance (robust regression β=0.044, single-sample t-test, t₃₆=2.25, P=0.031). Error bars represent s.e.m. (d) Across subjects, the sensitivity of subjective stress to irreducible uncertainty correlated with the sensitivity observed in pupil diameter (Pearson correlation, n=22, r=0.52, P=0.013) and skin conductance (Pearson correlation, n=37, r=0.38, P=0.021) models. Each data point is one participant.