(A) Mean reaction time on correct trials (top), accuracy (upper middle) estimated mean drift rate (lower middle), and confidence (bottom) as a function of coherence level. RTs on correct trials and choice accuracy scaled with SNR In both experiments (Exp 3A: RT, F(4, 3012.3)=21.60, p<0.001, error rates, X²(4)=170.74, p<0.001; Exp 3B: RT, F(3, 2173.1)=3.07, p=0.027, error rates, X²(3)=83.311, p<0.001). Correspondingly, drift rates estimated from fits of a hierarchical drift diffusion model also increased monotonically with SNR in both experiments (Exp 3A: Friedman χ2(4)=41.02, p<0.001; Exp 2B: Friedman χ2(3)=31.61, p<0.001). In these model fits, decision bound was not allowed to vary as a function of SNR, and its average was 1.33 (SD = 0.17) and 1.33 (SD = 0.20) in Experiment 3A and 3b, respectively. Similarly, non-decision time was estimated independently from SNR, and its average was 0.28 (SD = 0.08) and. 022 (SD = 0.05). Green crosses show the fits from the DDM. (B) Accuracy as a function of decision confidence. Confidence ratings were closely linked to choice accuracy, even on a trial-by-trial basis while including SNR as a covariate (logistic regression of confidence on accuracy: significant positive slopes for 21 observers, with all ps < 0.001, and non-significant positive slopes for two observers, with both ps > 0.354), showing that confidence predicts accuracy over and above SNR. Dot size reflects the percentage of trials per confidence label, separately for each participant. (C) Accuracy as a function of reaction time. There was no linear relationship between RTs and accuracy, b = −0.001, p>0.545. A polynomial regression did, however, confirm the inverted U-shape seen in the data, t(29.2) = −3.19, p=0.003. (D) Confidence as a function of coherence level, separately for corrects and errors. Confidence judgments only scaled with SNR in Experiment 3A (Exp 3A: F(4,3953.9) = 37.34, p<0.001; Exp 3B: F(3,2867.1) < 1, p=0.141), but there was a highly significant interaction between SNR and accuracy in both versions of the experiment (Exp 3A: F(4,3955.1) = 87.59, p<0.001; Exp 3B: F(3,2867.8) = 48.16, p<0.001): confidence increased with SNR for corrects (linear contrast: Exp 3A: p<0.001; Exp 3B: p<0.001) and decreased with SNR for errors (linear contrast: Exp 3A: p<0.001; Exp 3B: p<0.001).