2.2. Subjects
Twenty-one and ten subjects participated in Experiment 1 and Experiment 2, respectively. All but one subject in both experiments were naïve. That subject, who alone participated in both experiments, was an author (RTE); he is a highlyexperienced subject in color vision experiments, yet his results were very similar to the majority of naïve subjects. In both experiments, subjects performed 10 blocks of each of four different hue boundary tasks, on both the RGC and RGC∗ stimulus sets.
2.3. Experimental Protocol and Data Analysis
The hue boundary task was described in text displayed on the left side of the computer monitor. The instructions encouraged subjects to explore the continuum of colors until they felt confident with the outcome of their selection. The procedure for measuring individual red and green zones consisted of four consecutive tasks, each resulting in a position along the green–red continuum, arbitrarily denoted from 0 (green) to 1 (red). Because the measurements were expected to show some hysteresis, four different tasks were used. The four tasks and four resulting hue boundaries GR1–GR4 (Fig. 4(a) and 4(b)) were, in the order performed:
Starting from the extreme green position, while moving from green to red, report the first color where greenness is no longer perceived (GR1).
Starting from the extreme red position, while moving from red to green, report the first color where redness is no longer perceived (GR2).
Starting from the extreme green position, while moving from green to red, report the first color where redness is first perceived (GR3).
Starting from the extreme red position, while moving from red to green, report the first color where greenness is first perceived (GR4).
While performing these tasks, subjects were not explicitly asked to report simultaneous presence of redness and greenness in a single color. Using a variation of the method of adjustment, subjects were instructed to move back and forth along the continuum, which typically involved multiple iterations converging to a color consistent with the goal of one of the hue boundary tasks. Depending on the task, that color would mark the location on the red–green continuum before which (or after which) redness (or greenness) is perceived. Thus, in each response subjects were asked to report either presence or absence of a single color quale (either redness or greenness). Subjects were never given any instructions about whether the color zones should, or should not, overlap, nor did they have any way to compare their responses across different trials or tasks.