Stimuli and design of signal-in-noise and feature difference tasks.

(A) A representation of stimuli in the signal-in-noise task viewed from above, and an illustrative red-green stereogram of the stimulus. Only a certain percentage of dots are located at the correct depth plane; the remainder are given random disparities. In the left image, blue dots represent ‘signal’ dots that were located at the target disparity, green dots represent distracting ‘noise’ dots, and black dots represent the surrounding pedestal disparity, located at the plane of fixation (horizontal line). In the experiment, all dots were black or white. (B) A representation of stimuli in the feature difference task viewed from above, and an illustrative stereogram of the stimulus. The pedestal was always located at a crossed disparity and the central test plane was titrated around this. Although all dots are given a crossed disparity, there is a central patch of dots that have a smaller disparity and appear behind the surrounding dots; therefore this stimulus is classified as ‘far’. (C) The procedure for a typical trial. The first black bar indicates stimulus onset and duration, the second black bar indicates presentation of the response cue.