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Julia Harris, Bahador Bahrami, Geraint Rees; Invisible context modulates conscious perception. Journal of Vision 2009;9(8):350. doi: 10.1167/9.8.350.
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There is now considerable evidence that invisible stimuli can undergo cortical processing and influence behaviour. However, whether such unconscious processing can causally influence conscious perception is less well understood. Many visual illusions rely on the influence of a spatially distinct context on perception of a central element. For example, in Simultaneous Brightness Contrast, the perceived brightness of a central disk is influenced by the luminance of a spatially distinct surround, such that a disk on a dark background is perceived as brighter than the same disk on a light background. Here, we rendered the background invisible and examined whether people could still perceive illusory or real brightness differences in such a central disk. This was achieved by Continuous Flash Suppression, where a Simultaneous Brightness Contrast stimulus was presented to one eye while the other eye was stimulated by a brightly coloured flashing pattern with a central hole. This effectively rendered the background of the Simultaneous Brightness Contrast stimulus invisible, while allowing the observer to view the central disk through the ‘hole’ in the Continuous Flash Suppression stimulus. In initial experiments, observers continued to report that disks presented on a now invisible dark background appeared brighter than disks of identical luminance presented on a now invisible light background. Our results suggest that, for at least some classes of contextual visual illusion, invisible context can nevertheless modulate conscious visual perception.
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