We are extremely sensitive to “change”. The sudden changes in the environment attract our attention, and processing resources are allocated for processing these changes. This is an indispensable mechanism for adapting to the environment. In order to notice change, it is required to remember the previous environment. We continue to compare information from the environment to our memories, and as a result, when mismatches occur, they attract our attention, and we thereby become aware of them. However, the process up to the detection of the mismatch does not enter into consciousness. Thus, in order to examine this mechanism, we need measures with a high time-resolution, reflecting this preconscious information processing. Using the scalp-recorded event-related brain potentials (ERPs), I conduct research to explore the cognitive processes and to evaluate cognitive functions, such as perception, attention, and memory.