Contrast Sensitivity (CS): Color CS is Decreased More than Luminance CS at Low Light Levels

Abstract

Purpose: It is well-known that spatial & temporal vision decrease with decreasing luminance but less is known about the impact of luminance on color vision. Our purpose was to compare the effect of luminance on chromatic (color) & achromatic (luminance) CS.

Methods: Luminance CS was measured with the Pelli-Robson (PR) chart which has large letters which decrease in contrast in 0.15 log steps (0.05/letter). Color CS was measured with the cone contrast test (CCT; Innova Systems, Inc.) which uses a Netbook computer & rapid staircase to present colored letters on a grey background which decrease in cone contrast to measure red (L), green (M) and blue (S) cone contrast thresholds. PR & CCT were measured at photopic (85 cd/m²), low photopic (5.4 cd/m²) & mesopic (0.7 cd/m²) levels by having subjects view monocularly through neutral density filters. 16 color vision normals (CVN) & 14 hereditary color vision deficients (CVD) were tested.

Results: In CVN color (L & M cone) and luminance log CS were equal at high luminance (1.95 log CS) but decreased linearly with log luminance with color CS decreasing more rapidly (3.5X) vs. luminance CS (2.5X) such that color CS was 0.2 log units (1.6X) lower than luminance CS at the mesopic level (p<0.001). A linear decrease with luminance also prevailed for L & M cone CVD but both luminance & color CS decreased more rapidly in CVD for both affected & unaffected cones (L for protans, M for deutans). Suprisingly at the lowest light level achromatic luminance CS was 0.2 log units lower for CVD than for CVN (p<0.01). L & M CVD also showed a more rapid decrease in S cone CS with luminance (3.1X) vs. CVN (1.7X; p<0.01).

Conclusions: Foveal color CS deceases more rapidly than luminance CS with decreasing light level in CVN & CVD. It is plausible that color-sensitive parvo cells mediate both color & luminance CS. The more rapid decline in color CS with decreasing luminance may reflect the selective nature of cone CS (e.g., red on grey) which stimulates fewer neurons at threshold (in this example mainly R+/G- opponent cells). The lack of redundant stimulation at low luminance yields decreased CS, more pronounced in CVD. These results indicate that cone CS may be more sensitive for detection of acquired CVD early in eye disease. The finding that low luminance achromatic CS can be decreased in hereditary CVD may have occupational significance.

Keywords: 471 color vision • 478 contrast sensitivity • 468 clinical research methodology