The effects of color temperature and illuminance on the color-dependent Fraser–Wilcox illusion

The color-dependent Fraser–Wilcox illusion is an illusion in which still images appear to move, and it has been reported that the direction of the illusory motion reverses depending on the illuminance level. Furthermore, preliminary observations suggest that the color temperature of the illumination also affects this reversal phenomenon of the illusion. In the present study, in addition to illuminance, we investigated how the color temperature affects the illusory motion using two types of the color-dependent Fraser–Wilcox illusions: the red-purple type and the red-blue type. The direction and intensity of the illusory motion were rated at illuminance levels of 10, 20, 40, 80, 160, 320, 640, and 1280 lx under color temperature conditions of 3000 K and 6700 K. The results showed that the illusory motion in one direction predominated under high color temperature conditions, while the illusory motion in the opposite direction predominated under low color temperature conditions. Consistent with the previous study, we also observed that the motion occurred in the former direction under brighter conditions and in the latter direction under darker conditions. Although the red-purple type was on average more dominant than the red-blue type in the former direction of motion, both types showed the same switching of illusory motion direction under high and low illumination conditions.