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Christopher S Wu, Nimesh Bhikhu Patel, Daniel R Coates; Spatial Summation of Chromatic Contrast in the Parvocellular Pathway. Invest. Ophthalmol. Vis. Sci. 2022;63(7):2242 – F0450.
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© ARVO (1962-2015); The Authors (2016-present)
Perimetry is a widely used tool for assessing visual function. Conventional tests measure locational sensitivity to luminance contrast. While the spatial summation characteristics of luminance are well documented, the summation of color is poorly understood. As the visual mechanisms responsible for achromatic and chromatic contrasts are thought to differ, stimuli in chromatic spaces may offer diagnostic benefits. The aim of this study was to determine the spatial summation characteristics of chromatic contrast mechanisms.
Contrast detection thresholds were measured as a function of size and eccentricity in four color normal subjects. Stimuli were solid circular patches of either luminance or chromatic contrast increments presented for 300 ms on a background averaging 10 cd/m2. Cone contrasts were calculated in DKL color space by modulating along the achromatic magnocellular (L+M) or chromatic parvocellular (L-M) cardinal axes. Stimuli varied between 0.25° to 4.0° in diameter, and were presented in four principal oblique meridians at 5° intervals within the central 40° of visual field. In the chromatic paradigms, luminance noise was added to non-test positions. Thresholds were obtained using interleaved staircases, with subjects responding with the perceived meridian of presentation.
Achromatic thresholds plotted on a log-log scale were fit with a two-line model. The average slope for smaller stimuli sizes was -1.02 (complete summation) which became shallower, averaging -0.32 (partial summation) beyond a critical area (Ricco’s Area). This critical area increased with retinal eccentricity and was similar for locations at a given eccentricity. In the chromatic paradigms, a single linear regression model was used, as a bilinear fit poorly represented the data. In the +(L-M) direction, the average slope was -0.67, with an average r2 of 0.92. In the -(L-M) direction, the average slope was -0.76, with an average r2 of 0.86.
While achromatic curves followed the expected two-line model, the distinct linearity of chromatic summation suggests different underlying detection mechanisms. Complete spatial summation was absent for tasks that isolated the chromatic parvocellular mechanism for the range of stimulus sizes tested. The linearity of chromatic spatial summation and lack of critical area may provide improved consistency over achromatic tests, as relative sensitivity is unaffected by stimulus area.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.
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