Chromatic Intensities with Achromatic Surrounds

Abstract

Brightness changes induced in red or green tests by an achromatic surround were studied by a nulling method with a centre — surround spatial configuration. The amplitude of the cancelling modulation was found to increase linearly with the contrast of the inducing field when its mean luminance was constant. The quotient between the amplitudes of the cancelling and the inducing modulations increased with increasing test luminance and decreased with increasing mean luminance of the surround. For a given contrast, the amplitude of the cancelling modulation increased with the amplitude of the inducing modulation, except when the mean luminance of the surround was close to the test luminance, where an abnormally high value was found. With the nulling method, the values of the cancelling modulation were practically the same for the green and the red tests. Nevertheless, with a heterochromatic brightness-matching method, the luminance changes needed to match the test brightness depended on test chromaticity. At equal surround luminance, the greatest changes were for the green test and the smallest for the blue test, the red test being at an intermediate level. The results are consistent with previous studies and were fit by the model recently developed by B Spehar, J DeBonet, and Q Zaidi. In this model the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround with local luminance gain controls in the test and surround fields.