Previous study [Cunthasaksiri et al., Color Res Appl 2004,-29:255-260] showed that simultaneous color contrast (SCC) of an achromatic center-chromatic surround configuration is a result of a cognitive process where color appearance of the center stimulus is determined in relation to a cortical representation of illuminant for a space or recognized visual space of illumination (RVSI) derived from the contiguous surround. Based on the same basis, we further examined the other characteristic of SCC in which it is imperceptible at relatively high stimulus luminance. It was thought that at this luminance the stimulus possibly appears as a light source where its color appearance is irrelevant to the RVSI. This is because the visual system has learned and adopted the fact that light from an illuminated surface includes the attributes of both illumination and surface reflectance whereas light from a light source is not contaminated. As a result, it could be hypothesized that SCC would not manifest in the light source mode. Observers adjusted a variable luminance achromatic stimulus under four chromatic illuminant conditions (from greenish to whitish) for imperceptible SCC threshold (LT-noSCC) and light source threshold (LT-ls). The results showed that first LT-noSCC remained under LT-ls under all illuminant conditions, implying thatno SCC occurs in light source mode. Second, neither the highest strength of SCC nor the luminance ratio (L-test/L-surround) explains LT-noSCC. It suggested that the RVSI determines LT-noSCC. (C) 2006 Wiley Periodicals, Inc.