The EEG is an indicator of all physiological and neuropsychological activity. The alpha rhythm was considered as a key phenomenon in research of human mentation from the discovery of EEG. Two methods are known for the estimation of cognitive deficit by the use of quantitative EEG (QEEG). The first is based on the hypothesis, that the mean values of the normal EEG from healthy volunteers can be used as reference, and deviation from the normal values of EEG parameters may suggest disease. This kind of "neurometry" was elaborated by R. E. John. The second method assesses event related (ER) transients evoked by somatosensory and mental stimuli. Quantity and localization of signals may refer to the functional state of the cortex. These reactions depend strongly on the test-paradigms. Recognition of the attention-intention cycle disclosed the physiological mechanism of ERD (event related desynchronisation) and ERS (event related synchronisation). In contrast with the classical "stimulus-reaction" model, both perception and voluntary movement are initiated by the brain itself, and not by the environment. Human behavior and conscious actions depend on the intention. QEEG analysis proved that the attention and intention localize in segregate areas of the brain. Both "static" and "dynamic" neurometric methods are able to differentiate the EEG records of demented patients from healthy controls, furthermore some dementias from each other. We conclude that with the help of sophisticated methods of QEEG analysis minimal functional deficit of the electrogenesis can be recognized, which could be helpful in the differential diagnosis Notwithstanding the EEG can not explain the evolution neither the normal or the diseased mental processes. The only "instrument" which is able to approach the human mind is the human cogitation itself with the aids of appropriate tests. The QEEG can be conclusive in the analysis of particular processes of mental activity, such as timing, state of activation, hierarchical organisation of cortical territories and mechanism of electrogenesis.