The complex interaction of two opposed active processes take place in the nervous centers of the cerebral cortex: excitation and inhibition. Excitation of certain parts of the brain inhibits the other parts. This explains why a person stops reacting to the environment if he is absorbed by some other activity. Thus, for instance, switching your attention is connected with the transition of excitation from one part of your brain to another other as well as the inhibition of abandoned parts. Excitation and inhibition can be either balanced or with one prevailing over the other; they can have various strengths and progress from one center to another and replace each other in the same center, i.e. they feature certain mobility.