The stimulus arrangement was similar to that used by Formankiewicz and Mollon
(2009) in their study of dichoptic difference thresholds for luminance contrast. The stimulus is illustrated in
Figure 3b. It consisted of eight colored patches (radius = 0.8°), arranged into two subgroups, four above and four below the fixation point. Each color patch, in both eyes, contained an identical hue (a bluish-purple hue was used, corresponding to a hue angle of 120°), but the saturation (chromatic contrast) of the hue differed, as indicated in
Figure 3a. On each trial each dichoptic pair could potentially have a saturation equal to the highest (
H), lowest (
L), or mean (
M, where
M = 0.5[
H +
L]) saturation. These were randomly assigned to each pair, so that at least two pairs with the
H,
L, and
M saturations were present on each trial. One pair was randomly chosen to be the target (left-most pair on the bottom row in
Figure 3c). The target pair had one patch set to the highest saturation and the other patch set to some multiple of the distance between
H and
L. This multiple could take one of six possible predetermined values, presented according to the method of constant stimuli, allowing psychometric functions to be fitted to the detection data. Thresholds are presented as the length of the vector starting at
H and pointing in the direction of the background hue (i.e., point [0, 0]); this is represented by the double-ended red arrow in
Figure 3a. On each trial the saturation
H and
L (and hence
M) were randomly jittered. Each trial, as with the hue experiment, lasted 750 ms. Binocular luminance contrast, in the form of increments or decrements, was added to the color patches. A binocular version of the stimuli was also employed (shown in
Figure 3c), consisting of eight pairs of colored patches—four pairs presented above and four pairs below fixation. Also, as before, at least two pairs consisted of the
H,
L, and
M saturations. In the target pair (left-most pair on the bottom row in
Figure 3c), there was a within-eye difference in saturation defined in the same way as in the dichoptic condition (i.e., as in
Figure 3a). Initial concerns regarding the cognitive demand of the task proved unjustified, as a pilot study indicated that only about five practice trials were required for each observer to become competent with the task.