The neurophysiological orientation in psychiatry opened new leads in revealing the functional alterations in the background of pathological behavior. Conditioning in psychiatry is linked with the behavioral and the neurophysiological approaches, the conditional reflex being a behavioral phenomenon and a functioning pattern of the nervous system. Classical psychiatry and, consequently, psychopathology were based on detailed clinical descriptions. General psychopathology progressed from a descriptive to an experimental stage, in which direct descriptive observation was supplemented by instrumentally quantified registration. Mental deficiency is defined as a condition of arrested or incomplete development of mind existing before the age of eighteen years, irrespective of cause (Gregory 1961). A mental defective is a person lacking intelligence to the degree that he cannot make an average adjustment to life. "Organic brain syndrome" refers to that constellation of psychiatric symptoms which is associated with morphological, macroscopic, or microscopic alterations in the central nervous system.