## 2 Why the Electroencephalogram (EEG)?
### qEEG and Neuropsychopathology  
The literature indicates that there is a greater proportion of abnormal EEGs in individuals with psychopathology: (a) up to 68% of qEEGs in psychiatric patients display evidence of pathophysiology, and these results have additional utility beyond simply ruling out 'organic brain lesions' [214; 215]; (b) up to 73% of nonepileptic adults have qEEG epileptiform discharges (EDs) [216] that are attributable to underlying brain abnormalities (traumatic, vascular, tumor, metabolic), medications, and psychiatric disorders (see, for example, [217]); (c) the mean prevalence of interictal qEEG abnormalities in psychogenic nonepileptic seizures is estimated to be 26% [218; 219; 220; 221; 222; 223; 224; 225]; (d) up to 30% of panic attack patients have demonstrable qEEG abnormalities, especially in atypical presentations of panic attacks, and the incidence of abnormal qEEG findings in mood disorders reaches 40% [227]; (e) up to 78% of antisocial and criminal populations have underlying qEEG abnormalities [228] that are more prevalent in subjects with violent crimes, repeated violence, and motiveless crimes; (f) up to 76% of children with reading disabilities but without severe disorders of behavior have EEG abnormalities [229], and (g) 69% of youngsters with behavior disorders with a predominance of aggressiveness have EEG deviations [230]. Additionally, there is evidence that abnormal EEGs are associated with the following clinical conditions: negative histories (13%), severe head injury or neuropsychiatric disorder (46%), psychopathic personality (88%), and family history of seizures (62%) [231]. Basic mechanisms of cerebral rhythmic activities in norm and pathology are described in detail in Steriade et al. [212]. This emphasizes that the presence of qEEG abnormalities should be inferred as '_electrographic markers_' of underlying brain _dysfunction_ and is suggestive of the potential usefulness of qEEGs in clinical practice. Indeed, more recent research shows that certain neuropsychological, such as attention deficit hyperactivity disorder (ADHD), specific learning disabilities, schizophrenia, obsessive-compulsive disorder (OCD), borderline personality disorder (BPD), depression, suicidal ideation, anxiety disorders, traumatic brain injury (TBI), mild cognitive impairment (MCI), Alzheimer's disease (AD), and other disorders are associated with specific qEEG patterns and that these spontaneous electric potentials provide reliable markers of brain function and dysfunction [232; 233; 234; 235; 236; 237; 238; 239; 240; 241; 242; 243; 244; 245; 246] (for reviews, see [199; 213; 247]). Given that patients with different disorders display abnormal and distinct qEEG-profiles, it is not surprising that they can be differentially classified utilizing qEEG-variables [248]. For example, qEEG utility in discrimination/differentiation between affective disorders and schizophrenia [249], between Alzheimer and non-Alzheimer dementias [199; 250], between sub-types of dementia [251], between depression and dementia [199], between schizophrenia and unipolar and bipolar depression [199], and between panic disorder and depression [199] have been demonstrated. For sensitivity and specificity values of qEEG-based detection/discrimination of patients with specific disorders, see Table 1.  
It is argued that the levels of specificity found in qEEG studies are often higher than those found in routinely used clinical tests, such as mammograms, cervical screenings, and brain scans such as CT or single photon emission computed tomography (SPECT) [199; 262; 263]. Even within the same disorder, qEEGs may be beneficial in identifying the cause of the abnormal behavior. For example, Kropotov distinguishes five reasons for the neuro-physiology of ADHD, stating that \"[...] mentioned dysfunctions are associated with specific patterns in spontaneous and evoked electrical potentials, recorded from the head by multiple surface electrodes\" ([264] (p. 74; see also [3; 265]).