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Effect of stimulus size on chromatic discrimination

Version 2 2025-02-04, 20:12
Version 1 2025-02-04, 20:12
Posted on 2025-02-04 - 20:12
Colour discrimination requires observers to compare the spectral power distributions of two discriminanda. This paper reports the results of psychophysical experiments where discrimination thresholds were measured in different directions from points in the MacLeod-Boynton chromaticity diagram under conditions of constant adaptation to a metamer of D65. The data were fitted with ellipses. Several areas in the diagram were probed using a forced-choice procedure: the observers were presented with a brief 150 ms disc divided in four sectors and the task was to detect one sector that differed from the other three. The diameter of the test disc varied from 32 min to 2.4 deg in diameter. Ellipses of the largest tested size (2.4 deg diameter) were predominantly oriented so that their longer axis was aligned with the line connecting the chromaticity of D65 (adaptation point) and the centre of the ellipse. Along such lines, colorimetric purity varies, and the orientation of the ellipses reflects reduced sensitivity to saturation differences compared to hue differences. With decreasing test size, the ellipses change their orientation so that their longer axis is rotated towards the tritan line and the detection of changes in chromaticity is dependent mostly only on the activity of long- and middle--wave cones. The changes in orientation and shape depend, however, on the location of the ellipse in the diagram. (i) Ellipses centred on the tritan line through D65 with low S-cone input are circular for large stimuli, but they are elongated and become aligned with the tritan line for smaller ones; ellipses with high S-cone input change their orientation from being aligned with tritan line for the largest size to being slightly rotated away from the line with decreasing stimulus size. (ii) Ellipses with their centres on the line connecting unique blue and unique yellow with low S-cone input do not change their orientation, but they are not aligned exactly with the colorimetric purity line. (iii) Ellipses centred on the horizontal line through D65 are elongated along this line for larger sizes and become more circular with decreasing stimulus size.

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Journal of the Optical Society of America A

AUTHORS (2)

Marina Danilova
John Mollon

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