While unattended stimuli still evoked negative afterimages, we found that without attention the competitor had no effect on afterimage strength, and this was true for both the large competitor and the small competitor (Figure 8C; Figure S5B). Fits with the afterimage functions revealed a similar pattern of

effects across all observers: for none DAPT supplier did afterimage strength differ across conditions (Figure 8D; Figure S5B). This is consistent with the model predictions: the response gain reduction brought about by the small competitor is the byproduct of attentional modulation of normalization, and without attention, the gain change consists of only a contrast gain shift—just like what we observed with the large suppressor. These results suggest that the type of modulation of awareness through rivalry hinges critically on attention. Without attention, the suppression

of competing stimuli is substantially weakened at high contrasts. We propose a computation model, under the normalization framework, whereby attention plays a pivotal role in modulating competition for visual awareness. Previous studies have reported that, without attention, rivalry is weakened or altogether abolished in visual area V1 (Zhang et al., 2011; Watanabe et al., 2011) and in other, extrastriate cortical areas (Lee et al., 2007). The model proposed by us can accommodate the results PLX3397 purchase from these studies, because in this model attentional modulation is a driving force behind the suppression of awareness typically observed under rivalry. The present results, however, do not compel us to conclude that rivalry suppression simply does not occur at all without attention. Rather, the model proposes

that the interaction between attention and awareness is more nuanced, with Phosphatidylinositol diacylglycerol-lyase the magnitude of suppression relying on a variety of factors that include stimulus size, attentional state, and contrast of the competing stimuli. It is possible, for instance, that previous failures to find evidence for suppression without attention were working in a high-contrast regimen where suppression may not reveal itself when under the influence of contrast gain modulation. While the effects of binocular rivalry suppression have been observed throughout the visual hierarchy (Tong et al., 2006), the results from our experiments hint at a very early cortical locus for the effects suppression, due to the small size (1.5°) of the probe stimulus used in our study. Under the normalization framework, reductions in the response gain of a stimulus would occur only if probe stimuli were large enough to encompass not only the excitatory field, but the inhibitory field as well. Otherwise, we would observe no difference between competitor sizes.