It is well known that each part of the brain has a different function. In the early days of neuroscience, knowledge about localization of brain function was accumulated through observations of patients suffering brain disorders. Cerebrovascular disorders or injuries occurring in a specific part of the brain often cause a particular brain function abnormality. This reveals that a specific part of the brain is necessary for fulfilling the function. Broca and Welnicke identified the speech center based on such an approach. Observations of patients with brain disorders played important roles in understanding the localization of brain function. However, a methodological limitation existed owing to the uncontrollability of the location of the disorders. Penfield performed direct electric stimulations to a patient’s brain during a surgical treatment of epilepsy and recorded the evoked responses. Various mental activities arose according to the positions of stimulation, which provided a brief map of the localized brain function. Because transcranial electric stimulations of the brain cause pain, mapping of brain function using electric stimulation could not be performed practically except for patients under craniotomy.