To answer questions about the earliest time and place of correlates of awareness in the human brain, we used an approach pioneered by Kaernbach, Schröger, Jacobsen, and Roeber (
1999). They measured event-related potentials (ERPs) in two conditions that differed only in awareness, exploiting the phenomenon of binocular rivalry (e.g., Blake,
2001; Dutour,
1760, translated by O'Shea,
1999; Wheatstone,
1838). In both conditions, observers viewed a left-oblique grating with one eye and an identical right-oblique grating with the other. We call this
rivalry stimulation. Because of binocular rivalry, sometimes only the left-oblique grating was visible to an observer while the right-oblique grating was completely invisible, and sometimes it was the opposite. Kaernbach et al. had each observer press keys to signal which of the two gratings he or she was perceiving. Every now and then, one grating was changed, say the left-oblique grating, to give it the same orientation as the other grating. We call this
fusion stimulation. When the change in orientation happened while the observer was perceiving the left-oblique grating (say), he or she became aware of the change in orientation. Kaernbach et al. defined awareness as the observer's pressing a key to denote that the orientation changed. We call this sort of change a
percept-incompatible transition. When the change happened while the observer was seeing the right-oblique grating, he or she did not become aware of the change in orientation. Kaernbach et al. defined lack of awareness as the observer's not pressing a key. In fact, under these circumstances, some observers noticed a very subtle change in the appearance of the visible stimulus, such as a slight enhancement in its contrast. For our purposes, we will treat this as lack of awareness. We call this sort of change a
percept-compatible transition. Because percept-incompatible and percept-compatible transitions are physically identical, any difference in their neural response characteristics is a correlate of visual awareness. By comparing ERPs from these two transitions, Kaernbach et al. found activity about 200 ms after the transition that was correlated with awareness. Similar research excluding differences in the effect of percept-compatibility between flickering (16.7 Hz), and steady stimulus presentations found an even earlier time for a neural correlate of awareness at about 100 ms (Roeber & Schröger,
2004). This time agrees with that estimated using different approaches to manipulate awareness, such as visual masking (Koivisto et al.,
2006) and multistable images (Kornmeier & Bach,
2005,
2006; Pitts, Nerger, & Davis,
2007).