Maxwell's spot measurements in changing white light spectra

Abstract

The macular pigment covering the fovea maximally absorbs at 460 nm and minimally beyond approximately 540 nm (Bone et al, 1992). Consequently, foveal cones continuously receive less short-wavelength light than cones in surrounding areas, yet in normal circumstances we do not perceive a darker or colored spot in the central visual field. The difference in spectral transmission can be made visible by looking through a dichroic filter to a white surface or by alternately looking at a blue and yellow field. The resulting spot appearance, which fades with time, is called the Maxwell spot. Here, we show that the phenomenon can also be demonstrated by alternately looking at two light fields of similar white appearance, but differing in spectral composition (E1 and E2). When changing from E1 to E2, the color appearance of the spot is complementary to that when changing from E2 to E1. We measured the color appearance of both these spots, using successive haploscopic color matching. The Maxwell spots were generated by exposing the left eye for 6 seconds to E1 and E2 alternated at 0.5 Hz. A synchronized audio signal indicated which of the two Maxwell spots was to be matched. The observers used a computer mouse to navigate through (a*,b*) color space to match the color in the right eye to the perceived color of the Maxwell spot in the left eye, while keeping both eyes at equal (constant) luminance. The spectral distributions E1 and E2 and the color matches were produced by a multi-primary LED light source. Our results indicate that the Maxwell spots are due to differential local chromatic adaptation, i.e. independent sensitivity adjustments of the three cone types. We conclude that even small differences in light appearance may already elicit the Maxwell spot. Meeting abstract presented at VSS 2017