## 2 Why the Electroencephalogram (EEG)?
### qEEG and Neuropsychopathology
Additionally, qEEGs can play a unique role when it comes to dealing with ambiguous or edge cases in clinical practice. It may help to identify/differentiate:  
* Electrical changes that precede the clinical onset of a seizure by tens of seconds to minutes--the _early detection of a seizure_. It has been shown that patients go through a preictal transition for approximately 0.5 to 1 h before a seizure occurs [266]. On average, the prediction rate is ~81% and has an average warning time of 63 min [267];
* Whether a given seizure is _epileptic_ or _nonepileptic in origin_: For example, there are groups of disorders that produce symptoms similar to an epileptic seizure: (a) cardiac arrhythmias causing syncope, episodes caused by cerebrovascular disease, movement disorders, and unusual manifestations of sleep disorders; (b) events of psychiatric origin (often referred to as psychogenic nonepileptic seizures (PNES)) [268];
* _Subclinical seizures_: Some seizures recorded during prolonged EEG monitoring may be asymptomatic or'subclinical';
* Whether the cognitive impairments and behavioral problems in question are due to emotional, psychological, or social factors or because of brain dysfunctions or sensory deficits with quantitatively demonstrable abnormalities in brain electrical activity;
* Whether the hyperactive sensation-seeking behavior (typical for ADHD and mania) is due to _hypervigilance_ or _vigilance autostabilization behavior_, which is a compensatory behavioral pattern to counter regulate a hypovigilance state, and whether withdrawal \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Dysfunction/Disorder** & **Sensitivity** & **Specificity** & **References** \\ \hline Cerebrovascular disease & \textgreater{}80\% & & [199] \\ ADHD/ADD vs. normal children & 90\% & 94\% & [237] \\ ADD vs. DLD & 97\% & 84.2\% & [252] \\ LD vs. normal children & 72\% & 80\% & [253] \\ Depression & 72–93\% & 75–88\% & [254] \\ Panic disorder & 71\% & 84\% & [255] \\ Dementia & 91.9\% & 92.2\% & [256] \\ AD & 71–81\% & & [257; 258] \\ Declining to MCI & 95\% & 94.1\% & [259] \\ Converting to AD & 96.3\% & 94.1\% & [259] \\ Alcohol and drug abuse predicting relapse & 61\% & 85\% & [260] \\ mTBI vs. sTBI & 95.5\% & 97.4\% & [238; 261] \\ \hline \hline \end{tabular}  
* ADHD/ADD = attention deficit disorders with or without hyperactivity; DLD = developmental learning disorders; LD = learning disorders; AD = Alzheimer’s disease; MCI = minimal cognitive impairment; m(s) TBI = mild(severe) traumatic brain injury.  
\end{table} 
Table 1: qEEG-based detection/discrimination of patients with specific dysfunction/disorder. behavior (typical for depression) is due to _hypovigilance_ or the result of a compensatory behavioral pattern that counter-regulates hypervigilance [269,270];
* Between a degenerative disorder such as AD and _pseudodementia_ due to psychiatric illness [271];
* Between normal and abnormal maturational patterns, such as _brain maturation lag_ (characterized by a pattern of qEEG that is typical for younger age) and _brain maturational deviation_ (characterized by a pattern of qEEG that is not normal at any age) [253];
* Between _presence_ or _absence of consciousness_ in minimally and unresponsive patients [272,273,274,275,276]. In this context, different spatial-temporal qEEG patterns may reflect different underlying mechanisms/functions/symptoms; this hints at the existence of several clinical sub-types within a given diagnostic group that are not recognized by the current diagnostic systems [276]. Distinct aspects of pathophysiologic mechanisms may be elucidated depending on which qEEG oscillations or their combinations are altered in the qEEG oscillatory pattern of any given neuropsychopathology. It seems that neuropsychopathology manifests through the considerable reorganization of the composition of qEEG oscillations and their ratios over a broad frequency range of 0.5-30 Hz, which constitutes the dynamic repertoires of qEEG states.