(A) The generating process of the narrow-correct trials, for each narrow (brown) and broad (blue) stimuli sample. A full stream sequentially presents 8 such stimuli, each for 200ms with a 50ms inter-sample interval in between. In each trial where the narrow choice is correct, the generating mean of the narrow stream, , is uniformly sampled from [48,60]. The generating mean of the broad stream, , is then set to be . For all trials, the generating standard deviation of the narrow and broad streams are , respectively. The lines above the distributions denote the ranges of and . The particular values of and in this figure are shown for one trial, and chosen arbitrarily for illustration purpose. Given the generating means and standard deviations in a trial, a sequence of 8 stimuli samples are generated from a Gaussian process with certain constraints, for each of the narrow and broad options (See Materials and methods). (B) Sampled distribution of the mean evidence of the narrow and broad streams, across all trials for both monkeys where the narrow option is correct. (C, D) Same as (A, B) but for broad-correct trials. Here, is uniformly sampled from [48,60], and is set to be . (E, F) Same as (A, B) but for ambiguous trials. Here, and are equal and uniformly sampled from [44,56]. (G) The accuracy of Monkey A in the narrow-correct and broad-correct trials. Monkey A was significantly more accurate on ‘Broad-correct’ trials (Chi-squared test, chi = 38.39, p = 5.80x10−10). Errorbars show the standard error. (H) The probability for Monkey A to choose the broad option in ambiguous trials. Monkey A was significantly more likely to choose the broad option (Binomial test, p < 1x10−10). (I) Same as (G) but for Monkey H (Chi-squared test, chi = 59.46, p < 1x10−10). (J) Same as (H) but for Monkey H (Binomial test, p = 3.00x10−6).