Analysis of EPZ-6438 the rivalry index yielded a significant statistical interaction between stimulus types (rivalry/replay) and attention conditions (attended/unattended): F (1,12) = 22.7; p < 0.001. In the rivalry conditions, removing attention reduced the rivalry index by nearly a factor of four. When

attention was focused on the conflicting stimuli, the rivalry index reliably differed from zero (t [12] = 8.92; p < 10−4), and when attention was focused away, it did not (t [12] = 1.88; p > 0.05). In the replay conditions, the attended and unattended rivalry indices were comparable and both reliably different from zero (t [12] = 22.9 and t [12] = 15.8, respectively, in both cases, p < 10−6). As a complementary analysis, not dependent on finding peaks, we also computed the Pearson's r correlation coefficient between the left and right eye frequency-tagged amplitude time course ( Figure S2B). We found

strong negative correlations in the attended rivalry (r = −0.319), attended replay (r = −0.594), and unattended replay (r = −0.537) conditions, but not in the unattended rivalry condition (r = −0.078). The fact that the rivalry index in the unattended rivalry condition was not statistically SCH 900776 datasheet different from zero could not be attributed to generally weak EEG signal because the power of the tagged frequencies was actually stronger in that condition than in the unattended replay conditions, where counterphase modulation was readily detectable (Figure 3D). It is impossible, of course, to prove that the rivalry index was equal to zero in the unattended rivalry condition, but any small counterphase modulation that might have been present was likely due to some residual attention paid to the rivalry stimuli.

Post hoc subjective reports (see below) suggested that subjects were largely, but not completely, unaware of the unattended rivalry stimuli. Given the absence of a neurophysiological Tryptophan synthase signature of rivalry when attention is directed away from the conflicting stimuli, a natural next question is: What is the state of the visual system when presented with unattended, conflicting dichoptic signals? In a pilot study, we gathered post hoc subjective reports from subjects viewing the same stimuli as used during the EEG recordings (for details, see Supplemental Experimental Procedures). Subjects were very uncertain about the nature of their percepts in the unattended situation, confirming the effectiveness of the attentional manipulation, but at the same time providing very limited information about the state of the conflicting stimuli. Indeed, this uncertainty was the main reason we adopted the frequency-tagged SSVEP measure to begin with. Nevertheless, the data did suggest that perceptual alternations were greatly reduced when attention was withdrawn.