## 2 Why the Electroencephalogram (EEG)?  
MEG, fMRI, and PET are expensive, non-portable, partially invasive, and are usually associated with high stress due to noise, space confinement, and the need to be motionless. Additionally, fMRI and PET scans provide indirect measures of brain activity, with poor temporal resolution. Further, many types of mental activities, brain disorders, and malfunctions of the brain cannot be registered using fMRI since its effect on the level of oxygenated blood is low [5]. In contrast to these neuroimaging techniques, EEG is, at the _same time_, the cheapest, fastest, and most portable technique that measures neuronal activity directly and non-invasively. EEG does not elicit feelings of calustrophobia, does not require overt cooperative behavior from the person, has a temporal resolution adequate to mental and cognitive processes, and may distinguish between different temporal scales of information processing inherent to mental and cognitive processes.  
EEG is a summation of electric voltage fields produced by dendritic and postsynaptic currents of many cortical neurons firing in non-random partial synchrony [6; 7; 8]. The aggregate of these electric voltage fields can be detected by electrodes on the scalp. The brainstem and thalamus serve as subcortical generators to synchronize populations of neocortical neurons in both normal and abnormal conditions, thus influencing the EEG. It seems that the activity of subcortical structures can be 'visible' in the EEG either indirectly through their effects on cortical activity or--in contrast to popular belief--more directly via deep sources. See explanations given in [9] (p. 7): \"While local field potentials indeed fall off rapidly within the brain, far less attenuation is observed when recording across skull and scalp. The reason is that the lower conductivity of the skull (compared to the brain and scalp) attenuates superficial sources more strongly than deep ones, thus acting like a spatial low-pass filter. This property causes strong blurring and attenuation of the focal superficial fields but has less of an effect on the more diffuse (\"low spatial frequency\") fields from deeper sources [...] Recent evidence suggests that a considerably larger range of brain structures, layers, and cell types than previously thought can contribute to spontaneous EEG phenomena\".  
A quantitative electroencephalogram (qEEG) is a mathematically and algorithmically processed digitally recorded EEG that extracts information invisible to 'naked' eye inspections of the signal. For the rest of the paper, we will mostly refer to qEEGs as the majority of studies are performed using qEEGs.  
After decades of studies, it is becoming clear that the qEEG is closely related to brain dynamics, with millisecond temporal resolution, functional properties, and global states of brain functioning, information processing, and cognitive activity [10; 11; 12; 13; 14; 15; 16; 17]. The interaction of large populations of neurons gives rise to rhythmic electrical events in the brain, which can be observed at several temporal scales--qEEG oscillations. They are the basis of many different behavioral patterns and sensory mechanisms (for a review, see [18]). Indeed, a large body of evidence [19; 20; 21; 22; 23; 24; 25; 26; 27] has demonstrated that qEEG oscillations constitute a mechanism by which the brain can actively regulate changes in a state in selected neuronal networks to cause qualitative transitions between modes of information processing [20]. Thus, different qEEG oscillatory patterns are indicative of different information-processing states.  
The qEEG has a number of important features that make it especially useful in clinical practice. In the following sections, we present a brief review of these features.