Change blindness and time to consciousness.

Detection of changes in a visual scene can be substantially delayed when the original and the modified image are separated by a brief screen flicker. We used this phenomenon of "change blindness" to find when the brain detects the mismatch in relation to when the observer reports it, and whether changes in identity and position are processed similarly. Event-related brain potentials (ERPs) recorded while the subjects searched for the change in alternating series of images showed that the epoch during which they indicated detection was characterized by a marked positivity from 300 to 700 ms. Analysis of data from image presentations preceding the subjects' response revealed a similar but smaller ERP positivity one (identity) or even two (position) epochs before detection. As each epoch lasted 1500 ms, the brain may register a change as early as 3000 ms before the observer.