Loss of chromatic response to monochromatic increments on intense achromatic pedestal backgrounds

Abstract

The slope of the t.v.i. (log threshold vs log background illuminance) function for 2° monochromatic increments on achromatic backgrounds often exceeds unity above 4 log trolands (td) (Drum & Sternheim 1990). At such illuminances, the chromatic appearance of increments presented 0.6 log unit above threshold fades with increasing background intensity despite increasing excitation purity. We here report further evidence that achromatic backgrounds can produce response saturation in color-opponent pathways. We used a constant stimulus procedure simultaneously to measure increment threshold and to assess stimulus color above threshold at background illuminances from 3.6 to 5.1 log td and at test illuminances up to 0.8 log unit above threshold. The test stimulus was a foveal, 50 msec, 1° diameter field superposed on a steady, 1° diameter, achromatic (5128K) pedestal. A concentric, 15° diameter, achromatic surround one log unit dimmer than the background eliminated detection of stray light from the test flash. Test chromaticities (450, 550 and 650 nm and achromatic), test illuminances (5 levels in 0.2 log unit steps) and blank trials (one-sixth of all trials) were presented in a mixed order from trial to trial. On each trial in which the observer detected a flash he scaled the relative percentages of red, yellow, green, or blue, and the overall percentage of white in the field. Our results show that when pedestal background illuminance is increased from 3.6 to 5.1 log td, hue sensations are reduced relative to white. Red hue responses at 450 nm and the entire hue response at 550 nm are almost completely suppressed. Only the yellow hue response component of the 650 nm stimulus appears to be unaffected. These findings confirm that achromatic pedestal backgrounds can modify hue signals in chromatic pathways.