Abstract
Uniform backgrounds appear lighter or darker when elements containing luminance gradients move across them, a phenomenon first presented by Ko Nakamura at the 2010 Illusion Contest in Japan. We measured the apparent lightness of the background with a configuration where the grey background was overlaid with moving square patches of vertically oriented luminance gradient. For black-to-grey gradients, the background appeared lighter when the black edges were leading than when they were trailing. For white-to-grey gradients, the background appeared darker when the white edges were leading than when they were trailing. For white-to-black gradients, the background appeared darker with a white edge leading and lighter with a dark edge leading, but the effects were weaker. These results demonstrate that lightness contrast can be modulated by the direction of motion of the inducing patterns. The smooth gradient is essential, because the effect disappeared when the black-to-white gradient was replaced with the binary black and white pattern. We speculate that asymmetry in the processing of a temporal gradient with increasing and decreasing contrast, as proposed to explain the “Rotating Snakes” illusion (Murakami, Kitaoka, & Ashida, 2006, Vision Research, 46, 2421–2431), might be the basis for this effect.
Highlights
1 Introduction Perception of lightness has been extensively studied in vision science (e.g. Gilchrist, 2006; Kingdom, 2011), but the stimuli used are mostly static and the interaction between lightness and motion has been relatively neglected
Questions arise as to how this illusion occurs: is the effect polarity specific; does the effect involve lightening of an area, darkening of the other area, or both; is the crucial effect enhanced lightness contrast at the leading edges, enhanced lightness assimilation at the trailing edges, or both; and how is the lightness gradient itself crucial, apart from the asymmetry? We addressed these questions by measuring the perceived background lightness in two experiments
The baseline was defined as the averaged matching luminance for the white to black (W-B) stimuli at speed 0 deg/s, on the assumption that it should not have biases of either motion or overall luminance
Summary
Perception of lightness has been extensively studied in vision science (e.g. Gilchrist, 2006; Kingdom, 2011), but the stimuli used are mostly static and the interaction between lightness and motion has been relatively neglected. Perception of lightness has been extensively studied in vision science Arrays of oval patches of horizontal luminance gradient are placed on a white background (see Figure 1). When the patches in the top and the bottom halves of the display move in opposite directions, the background lightnesses appear to be different: lighter when the black parts are leading, and darker when the black parts are trailing. This is a striking demonstration that motion can directly affect lightness perception, but the underlying cause is yet to be investigated. Intend to restrict ideas either to phenomenology or underlying mechanisms at this stage
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