, abstract), it remained unknown whether binocular rivalry could occur at all when attention was strongly diverted. In previous attempts to address this question, subjects

were allowed to attend to the stimuli during brief intervals interspersed with longer periods of unattended rivalry (Cavanagh and Holcombe, 2006, J. Vis., abstract; He et al., 2007, J. Vis., abstract). Under these limited sampling conditions, it was inferred that perceptual alternation significantly slowed down or even stabilized, which might suggest that rivalry did not occur during the intervals between attentional sampling. The interpretation of this work, however, is limited by the fact that attention was applied to the stimuli immediately before each perceptual report. Our experiment measured the status of rivalry when attention was continuously diverted from the competing stimuli. Our conclusions may seem at odds with a previous fMRI study (Lee et al., selleckchem 2007) that observed a propagating wave of binocular rivalry resolution even when attention was diverted. The nature of the dynamic change in V1 BOLD signal measured in that study was ambiguous, however, and could have arisen from other neural events besides the resolution of rivalry. Indeed, using similar stimuli, a more recent fMRI study from the same group failed to find evidence of interocular suppression with dichoptic stimuli when attention was diverted (Moradi and Heeger, 2009). Instead, the find more authors suggested

that the V1 BOLD signal change Sclareol observed in Lee et al. (2007) likely reflected cross-orientation suppression rather than binocular rivalry. The ability of the frequency-tagging method to identify signals originating from each eye makes our results less susceptible to such alternative explanations. In addition, we found that unattended conflicting dichoptic stimuli produced large amplitudes at intermodulation frequencies, and that such frequencies are more consistent with the neural state produced by binocular fusion than the state produced by patchwise rivalry. The simplest interpretation of these results is that, without attention, rivalry ceases, and the two eyes’ signals locally

combine in the visual cortex. One possible mechanism that may contribute to this fusion-like neural state is that orientation tuning may broaden without attention (Saproo and Serences, 2010), with the consequence being that cells become more permissive in the orientation domain. In sum, binocular rivalry, seemingly automatic, is in fact highly dependent on attention. Rivalry ceases when observers are not attending to and interrogating the information at a location. These results argue for a specific role of visual attention in binocular rivalry that is consistent with its general role in object perception. Attention may be required to bind features together into an object (Treisman, 1998), perhaps by biasing and resolving competition in neurons between features that belong to different objects.