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Po-Jang Hsieh, Peter U. Tse; Illusory color mixing upon perceptual filling-in does not result in ‘forbidden colors’ and reveals cortical processing. Journal of Vision 2006;6(6):239. doi: 10.1167/6.6.239.
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Crane and Piantanida (1983) reported that when presenting stabilized bipartite colored fields in which no clear cues about the foreground and background exist, some observers see novel ‘forbidden’ mixtures of the two colors. The perceived colors were deemed to be ‘forbidden’ because they seemed to violate Hering's laws of color opponency, resulting in reddish greens and yellowish blues. Surprisingly, this radical claim has never been empirically tested. To test this, we took advantage of the fact that a retinally stabilized object readily undergoes perceptual fading. It is commonly believed that the color of the apparently vanished object is filled in with the color of the background because the features of the filled-in area are determined by features located outside the stabilized boundary. Here we use visual stimuli composed of spatially alternating stripes of two different colors to investigate the characteristics of color mixing during perceptual filling-in, and to determine whether ‘forbidden colors’ really occur. Our results show that (1) the filled-in color is not solely determined by the background color, but can be the mixture of the background and the foreground color; (2) apparent color mixing can occur even when the two colors are presented to different eyes, implying that color mixing during filling-in is in part a cortical phenomenon; (3) Our data also show that the ‘forbidden colors’ reported by Crane and Piantanida (1983) do not exist. Our results show that perceived colors during perceptual filling-in are not ‘forbidden colors’, but rather intermediate colors.
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