Figure 6.

Quantile probability functions. The figures show possible outcomes for experiment 1 in which there are six levels of coherence (from 5% to 50%). Predicted quantile RTs for the .1, .3, .5 (median), .7, and .9 quantiles (stacked vertically) are plotted against response proportion for each of the six conditions. Correct responses for left- and right-moving stimuli, combined, are plotted to the right, and error responses for left- and right-moving stimuli combined are plotted to the left. The bold horizontal line in each figure connects correct and error median RTs for the third most accurate condition in order to highlight whether error responses are slower or faster than correct responses. The drift rates from which the data were simulated are those obtained in experiment 1. For all six panels, the starting point (z) was halfway between the boundaries. Across the six panels, boundary separation a takes on values of 0.16, 0.11, or 0.08; across-trial variability in starting point rate s_z takes on values of 0 or 0.07; across-trial variability in T_er, s_t, takes on values of 0 or 0.20; and across-trial variability in drift rate, η, takes on values of 0 or 0.12. T_er is the mean time taken up by the nondecision components of processing is set at 300 ms in the plots.