## 2 Why the Electroencephalogram (EEG)?
### Causality of qEEG Oscillatory Patterns in Neuropsychopathology  
The above brief review of qEEG features and properties and their association with neuropsychopathology suggests the existence of _circular causality_, where, on the one hand, different pathological processes affect the qEEG pattern and, on the other hand, changes in the qEEG pattern affect pathological processes. This supposition is supported by converging empirical evidence: (a) central nervous system (CNS)-active drugs that affect known neuromediators change different features of the qEEG oscillatory pattern in a consistent and predictable manner, with a parallel reduction in symptoms [360, 361, 362, 363]; (b) specific features of the qEEG oscillatory pattern have better predictive power for medication response compared to a syndrome-based diagnosis [364, 365, 366, 367, 368, 369, 370, 371]; for example, the overall predictive accuracy in differentiating treatment responders from non-responders is 84%, with a sensitivity of 77% and a specificity of 92% [372]; (c) different features of the qEEG oscillatory pattern predict future (i) decline within the next 7 years in normal elderly people with subjective cognitive complains (no objective evidence of cognitive deficit) [259], (ii) clinical outcomes in patients in the vegetative state 6 years after brain injury [315, 327], and (iii) developments of delinquent (antisocial) behavior [373]; (d) normalization of the distorted structure of the qEEG oscillatory pattern by an exogenous magnetic field stimulation changes the subjective experience of neuropsychopathology, accompanied by a clinical decrease (>50% reduction) of symptom severity [374] (see also [375, 376]); (e) normalization of atypical qEEG oscillatory patterns through operant conditioning with neurofeedback results in sympton reduction in neuropsychopathologies such as epilepsy [377, 378], depression and anxiety [379], schizophrenia [380], addiction [381], ADHD [382, 383], sleep disorders [384], autism [385], chronic pain [386], learning difficulties [387], and dyslexia [388]; last but not least, (f) cognitive enhancement in the elderly by qEEG neurofeedback [389, 390].  
A substantial corpus of evidence supports the proposition that the successful treatment of psychiatric patients results in the normalization of the previously demonstrated qEEG abnormalities [213].  
Such circular causality is possible because the qEEG oscillatory pattern is not just a correlate of information processing, communication, integrated phenomenal experience, and the associated neuropsychopathology but is, indeed, a _constitute (substrate)_ of these very things [330, 391].  
From the above review, it is clear that the qEEG is a natural and non-invasive 'window' into the living brain and mind since only the qEEG permits direct observation of the ongoing dynamics and coordinated processes organized in the patterns of brain activity that reflect the overall architecture of information processing, behavior, and subjective experience during both healthy and pathological conditions [392].  
To make sense of the 'view' from this 'window', many different methods have been suggested. However, when processing the qEEG signal, it is essential to remember that it is a neurophysiological phenomenon that has its own peculiarities, regularities, and complex rules of organization which are _functionally relevant_[13, 393, 394, 395, 396] (for reviews, see [397, 18, 399, 398, 399]). Only when one knows these characteristics is it possible to make proper use of the qEEG as a tool and to give a more neurophysiologically adequate interpretation of the data. In connection to this, a much deeper understanding of the brain dynamics reflected in the qEEG is essential for progress in psychophysiological, cognitive, and clinical sciences.