Response from de weerd, desimone and ungerleider.

1 summarize our results well and the comparison made with the results of De Weerd et al. 2 is interesting, provocative and worthy of experimental testing. They don't mention that, like De Weerd et al., we also find that cortical measures determine the speed of construction of illusory brightness. We showed that our observed flicker fusion frequencies for 'Craik–O'Brien–Cornsweet effect' (COCE) gratings of different stripe widths could most simply be understood if one assumed a constant cortical propagation velocity, calculated as either 155 or 205 mm.s –1 depending on whether V1 or V2 was involved. Other authors have reported that illusory brightness and actual brightness perception also take time and may involve a filling-in process 3,4. At the same time we feel that it might be premature to accept that a filling-in process happens per se. Our results could accommodate versions of the 'interpretive' hypothesis of Campbell and Robson 5. Under this hypothesis , the brain 'recognizes' (in some way) that it can't see the low-spatial-frequency components of a low-contrast square wave and so it interprets all objects having higher spatial-frequency components consistent with a low-contrast square wave as square waves. Our findings could support versions of the interpretive hypothesis where the time required for the recognition process depends on the spatial frequency. This could be a natural consequence of the wavelet-like transform performed in early visual processing, where information about progressively lower spatial frequencies is processed over progressively greater cortical distances. So, for example, the rate-limiting process might be the speed at which the brain recognizes that low spatial frequencies in the image have unreliably low contrasts , this recognition being processed over longer cortical distances for broader gratings. This example is perhaps a little contrived but it illustrates that we have not demonstrated that filling-in occurs in a literal sense. One thing is clear: the construction of the illusory brightness in COCE gratings continues at contrasts well above the threshold for seeing the fundamental spatial frequency of the equivalent square wave, and this finding runs contrary to the original interpretive hypothesis. Several authors have made related observations 6,7 , however, Moulden and Kingdom 8 have proposed versions of the interpretive hypothesis that might account for the brightness induction. The diffusion model of Cohen and Grossberg 9 discussed by Pessoa and Neumann provides an intriguing , testable, formal framework for discussion of filling-in effects. In short, more experiments are …