Animals on our planet possess a marvelous adapta­tion mechanism which helps them to study their environment and accumulate knowledge throughout their lives. This mechanism partly depends on the functions of the sense organs. Their characteristic feature is that they soon become ‘accustomed’ to stimuli of continuous action and stop responding to them, but react at the same time very actively to all new stimuli.

This is probably a phenomenon with which everyone is familiar. If you go into premises from outside and smell a pungent or offensive odor, within a few minutes the odour ceases to irritate you. Your nose becomes accustomed to the odor and stops sending messages about it to the brain. But, if you leave the room for a short time and then come back again, you are immediately aware of the odor again.

Owing to this feature of the sense organs, the brain always receives information on all the new events occurring in the environment. Besides, every new stimulus evokes the orientation reflex, which helps the organism to prepare for any surprise. If a new stimulus, inessential to the animal, is followed by some important events, a conditioned reflex is formed, and the new stimulus becomes a signal announcing the coming of an important event.