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Steven K. Shevell, Dingcai Cao; Chromatic assimilation measured by temporal nulling: Interaction between the l and s pathways. Journal of Vision 2005;5(8):94. doi: 10.1167/5.8.94.
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© ARVO (1962-2015); The Authors (2016-present)
BACKGROUND & PURPOSE The perceived color of an object depends on the context of other lights in view. This study considered neural processes that mediate chromatic assimilation, which is the shift in appearance toward nearby light. Assimilation was measured separately in the l and s chromatic pathways, and then jointly with an inducing light causing assimilation in both l and s. This tested whether assimilation resulted from independent l and s responses. METHODS The spatial frequency and chromaticity of context were chosen to optimize assimilation. Instead of directly measuring the appearance shift caused by context, light that induced assimilation was modulated in time, thereby causing a time-varying shift in test-area appearance. The magnitude of induced assimilation was quantified by the amplitude of temporally varying nulling light presented in the test area in counter-phase to the induced assimilation. The observer's criterion was a temporally steady test area; no color judgment was required. RESULTS Experiment 1 verified the temporally varying inducing light caused assimilation rather than contrast in the test area. Experiment 2 quantified the separate magnitude of assimilation in the l or s pathway alone. These measurements were compared to the assimilation expected from optical factors (including chromatic aberration) and showed that a neural process contributed to chromatic assimilation. Experiment 3 used an inducing chromaticity that caused assimilation in both l and s simultaneously. An asymmetric interaction was found: assimilation in s depended on inducing stimulation in the l direction but assimilation in l did not depend on s. CONCLUSIONS Chromatic assimilation cannot be accounted for by optical factors affecting the retinal image. The neural process of assimilation is characterized by a specific interaction between the l and s pathways, which implies assimilation cannot be understood by considering each pathway independently.
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