Illustration of the up-down adaptive staircase procedure. In this fictitious block of trials, the subject had to compare, on each trial, two stimuli, “F” and “D”. The ratio F/D varied from trial to trial and was represented in two interleaved adaptive staircases called α and β. For task A (size comparisons), F/D was initially equal to 2 for staircase α and 1/2 for staircase β. In both staircases, F/D was subsequently multiplied by 21/12 (i.e., approximately 1.059) when subjects perceived F as smaller than D, and divided by the same factor when subjects perceived F as larger than D. The block of trials was terminated when at least 12 reversals in the variation of F/D had occurred for each staircase. Similarly, for task B (duration comparisons), as shown in the figure, the initial values of F/D were always 3 and 1/3, and F/D was always multiplied or divided by a factor of 31/8 (i.e., approximately 1.147) when F was estimated respectively shorter or longer than D.