Red-green chromatic discrimination with variegated and homogeneous stimuli

Chromatic discrimination thresholds were measured under conditions which yielded fine and degraded discrimination steps. Discrimination was assessed by identification of the location of one of four homogeneous equiluminant stimuli arranged in a square or with pseudoisochromatic (PIC) figures using the stimulus design of Regan, Reffin and Mollon (Vis Res 1994; 34: 1279-1299). Stimuli were presented on CRT monitors and specified in units of cone trolands. They were viewed within a surround metameric to the equal energy spectrum. L troland threshold versus retinal illuminance (TVR) functions were measured by four-alternative spatial forced-choice staircase procedures for (1) a four 1 x 1 degree equiluminant stimuli arranged in a square and (2) 'C' shaped pseudoisochromatic figures in which the observer had to identify the gap location. The 'C' was constructed of spatially discrete patches of varying size and luminance to ensure that the observer's responses depended on chromatic signals. The TVR functions appeared V-shaped and were similar for the two paradigms. The minimum occurred near the L excitation of equal-energy white. The PIC stimuli yielded poorer discrimination with the TVR function being displaced by approximately 0.5 log unit. Discrimination for stimuli degraded by luminance and spatial noise presented within an achromatic appearing surround is sharpest near the chromaticity metameric to the equal energy spectrum.