Binocular rivalry is an experimental setup where different images are projected to each eye. We do not consciously see both images but an alternative awareness of one and then the other. From paper sited below (Roeber, Veser, Schroger, O’Shea):
One sees one of the images for a few moments, referred to as the dominant image, while the other is completely invisible, suppressed. Then, after a brief period of transition, when both or parts of the two images are seen together, the other image becomes dominant and the first becomes suppressed. The images continue to alternate in visual consciousness randomly as long as one bothers to look at them. Binocular rivalry is an important phenomenon for researching the neural correlates of consciousness because visual consciousness changes without any change in the physical stimulation.
The cause of the rivalry have been proposed by two theories. It could be the top-down result of attention; or, it may be a bottom-up mechanism involving reciprocal inhibition and adaptation.
The behavioural, fMRI, and EEG evidence is consistent with attention’s being required for rivalry to occur. But Paffen et al. proposed an intriguing alternative hypothesis, at least for their behavioural results. They proposed that:
Attention is not required for rivalry to occur,
Attention increases the underlying neural activity of each of the representations of the rival stimuli that compete in the low-level rivalry mechanism; this is similar to increasing the contrast of the rival stimuli, and
This increase in activity leads to greater adaptation, leading to faster alternations.
….We decided to test Paffen et al.’s explanation of attention’s effects on rivalry by measuring ERPs. ERPs are changes in electrical activity of the brain that follow some event, measured from electrodes placed on the scalp. ERPs have temporal resolution in the order of milliseconds. The typical form of the ERP when the event is the sudden appearance of a specific visual object or feature includes a positive component peaking about 100 ms after the event, the P1, and a negative component about 170 ms after the event, the N1. …If attention affects binocular rivalry by boosting neural responses to the rival stimuli, then attending to rival stimuli should increase ERPs from a change to a rival stimulus compared to when attention is on something else. If adaptation affects binocular rivalry and attention is accompanied by increasing adaptation, as proposed by Paffen et al., then attending to rival stimuli should decrease ERPs from a change to a rival stimulus. We found the latter: Attending to the rival stimuli decreases the size of the N1 compared with when attention is on something else.
In particular when attention was on the rival grating images and subjects had to report changes in the orientation the ERP (N1 160-210 ms) was smaller than when attention was on a fixation target distant from the grating images.
To explain this paradoxical effect of attention, we propose that rivalry occurs in the attend-to-fixation condition (we found an ERP signature of rivalry in the form of a sustained negativity from 210–300 ms) but that the mechanism processing the stimulus changes is more adapted in the attend-to-grating condition than in the attend-to-fixation condition. This is consistent with the theory that adaptation gives rise to changes of visual consciousness during binocular rivalry.
For my interest, this is a further separation of attention from consciousness. Although they are found together most of the time – they do appear to be separate processes.
Roeber, U., Veser, S., Schröger, E., & O’Shea, R. (2011). On the Role of Attention in Binocular Rivalry: Electrophysiological Evidence PLoS ONE, 6 (7) DOI: 10.1371/journal.pone.0022612