Neuronal mechanisms for illusory brightness perception in humans

Biological visual systems are extraordinarily capable of recovering the shape and brightness of objects from sparse and fragmentary information. Using functional magnetic imaging, we show that two associative areas of the dorsal pathway--in the caudal region of the intrapariatal sulcus and in the lateral occipital sulcus--respond specifically to the Craik-O'Brien-Cornsweet illusion generated by high-pass filtered edges. Other visual areas, including primary visual cortex, also respond strongly to the retinotopic location of the edge, but these areas respond equally well to a line of matched contrast and detectability, rather than specifically to the brightness illusion. The reconstruction of surface and/or its brightness seems to be achieved by associative areas from the information about visual features provided by the primary visual cortices, even where there is no physical difference in luminance.