Colour appearance and compensation in the near periphery

Abstract

The spectral sensitivity of the visual system varies markedly between the fovea and surrounding periphery owing in part to the rapid fall in macular pigment density with eccentricity. We examined how colour appearance changes between the fovea and near periphery (8 degrees) by measuring achromatic loci and the loci of unique and binary hues. Chosen colours remained much more similar at the two locations than predicted by the change in spectral sensitivity. Compensation for white may reflect long-term gain changes within the cones that equate sensitivity for the local average stimulus in the fovea and periphery. However, adjusting only to the average stimulus cannot correct for all of the effects of a spectral sensitivity change, and predicts differences in colour percepts between the fovea and periphery that were not observed. The similarities in hue percepts at 0 and 8 degrees thus suggest that additional processes help compensate colour appearance to maintain constancy in the near periphery. We model the results of previous studies to show that similar adjustments are implied by age-related changes in lens pigment, and to show that these adjustments are consistent with previous measurements of peripheral colour appearance based on hue cancellation.

Figure 1.

(a) Achromatic settings for eight observers at the fovea (open circles) or 8° (filled triangles). Lines connect settings for each observer. Filled diamonds: predicted change in periphery assuming no compensation for macular screening and a pigment density difference of 0.15, 0.3 or 0.45. Open diamonds: predictions if only the S cones are compensated. (b) LvsM (filled circles) or SvsLM (open triangles) coordinates in the fovea or periphery.

Figure 2.

(a) Hue angles chosen by nine observers (indicated by symbols) for unique and binary hues in the fovea or periphery (hue settings on black background). Contours show settings predicted if there is no compensation for macular pigment (red), or if compensation adjusts only to the average spectral difference (blue), assuming macular pigment densities of 0.15 (dotted), 0.3 (solid) or 0.45 (dashed). (b) Settings for seven observers repeated on a grey background.

Figure 3.

Wavelengths chosen as unique or binary hues at the fovea or 8° for three observers (symbols). Lines show shifts predicted if there is no compensation for macular pigment (bold dotted line), or compensation only for the average spectral difference for pigment densities of 0.3 ± 0.15 (dotted, solid or dashed lines).

Figure 4.

Predicted settings for narrowband hues in observers at different ages who differ only in lens pigment density. Lines: settings for observers age 12 (diagonal), 32 (dotted lines), 52 (dashed lines), 72 (bold dotted line) or 92 (solid lines). Circles: average projected differences between ages 12 and 72 in unique blue, green and yellow (Schefrin & Werner 1990).

Figure 5.

Predicted hue cancellation functions for (a) red–green or (b) blue–yellow. Absolute cancellation energy is shown for the fovea (solid curves) or periphery assuming no compensation (dashed curves), gain changes in all cones (triangles) or only in S (crosses), assuming a density difference of 0.3.