Advances in neuroscience implicate reentrant signaling as the predominant form of communication between brain areas. This principle was used in a series of masking experiments that defy explanation by feed-forward theories. The masking occurs when a brief display of target plus mask is continued with the mask alone. Two masking processes were found: an early process affected by physical factors such as adapting luminance and a later process affected by attentional factors such as set size. This later process is called masking by object substitution, because it occurs whenever there is a mismatch between the reentrant visual representation and the ongoing lower level activity. Iterative reentrant processing was formalized in a computational model that provides an excellent fit to the data. The model provides a more comprehensive account of all forms of visual masking than do the long-held feed-forward views based on inhibitory contour interactions. From the time a stimulus first enters the eye to the time a percept emerges into consciousness, the initial stimulus has been coded at several levels in the visual system. One of the main goals in studying visual information processing is to specify the representations at each level and the temporal sequence between

We propose a computational theory of consciousness and model data from three experiments in visual perception. The central idea of our theory is that the contents of consciousness correspond to temporally stable states in an interconnected network of specialized computational modules. Each module incorporates a relaxation search that is concerned with achieving semantically well-formed states. We claim that being an attractor of the relaxation search is a necessary condition for awareness. We show that the model provides sensible explanations for the results of three experiments, and makes testable predictions. The first experiment (Marcel, 1980) found that masked, ambiguous prime words facilitate lexical decision for targets related to either prime meaning, whereas consciously perceived primes facilitate only the meaning that is consistent with prior context. The second experiment (Fehrer and Raab, 1962) found that subjects can make detection responses in constant time to simple visual stimuli regardless of whether they are consciously perceived or masked by metacontrast and not consciously perceived. The third experiment (Levy and Pashler, 1996) found that visual word recognition accuracy is lower than baseline when an earlier speeded response was incorrect, and higher than baseline when the early response was correct, consistent with a causal relationship between conscious perception and subsequent processing.

Although once doubted, a consensus has emerged from the literature that visual sensitivity can be heightened locally with an appropriate precue. Experiments with partially and totally valid precues suggest an increase in sensitivity of less than one-half log-unit at the precued position, as compared with other positions. New experiments with non-informative precues demonstrate that most of this small enhancement is not due to focal attention. Sensitivity can be heightened at eight positions simultaneously, just as much as when a single position is precued. Sensitivities produced by single, totally valid precues and single, non-informative precues were similar. Thus there seems to be no capacity limit for the effect of precues on visual sensitivity.

G. Francis and F. Hermens (2002) used computer simulations to claim that many current models of metacontrast masking can account for the findings of V. Di Lollo, J. T. Enns, and R. A. Rensink (2000). They also claimed that notions of reentrant processing are not necessary because all of V. Di Lollo et al. 's data can be explained by feed-forward models. The authors show that G. Francis and F. Hermens's claims are vitiated by inappropriate modeling of attention and by ignoring important aspects of V. Di Lollo et al. 's results. We note with interest Francis and Hermens's (2002) article, which purports to show that the findings reported by Di Lollo, Enns, and Rensink (2000) can be explained by other models of metacontrast masking. To buttress their claim, Francis and Hermens reported computer simulations showing that some of our results can be modeled by the theories of Bridgeman (1978), Francis (2000) and Weisstein (1968). This claim has a good deal of surface appeal because it is parsimonious. It argues that our results can be explained without recourse to the new concept of object substitution. Parsimony, however, is achieved at the cost of inappropriate modeling of attention and modeling an incomplete portion of our masking data. Here, we reiterate our original claim that reentrant modeling is necessary for explaining our findings. We do so by showing that a plausible case for the sufficiency of feed-forward processes has not been made by Francis and Hermens. Modeling of Attention Modeling of the effects of attention in Di Lollo et al. 's (2000) study was based on the large literature on set-size effects in visual perception (Duncan & Humphreys, 1989; Eriksen,1995;