## 2 Why the Electroencephalogram (EEG)?
### qEEG Historicism  
An adult human qEEG is characterized by 'historicism'--the information about primate phylogeny, pre- and post-natal maturation (individual development), and early life events (utero characteristics and early life stress). Indeed, phylogenetically (_phylogenesis_--the evolutionary development and diversification of a species or group of organisms), the proportion of power of qEEG oscillations changes as a function of primate phylogeny [5; 22; 28]. Likewise, ontogenetically (_ontogenesis_--physical and psychological development of an individual organism from inception to maturity), qEEGs undergo significant transformation as a function of _pre_-natal (in utero) development (maternal stress exposure, anxiety, and depression during pregnancy are considered in utero adverse experiences and have been associated with future health problems [29; 30; 31; 32]; this is so because intrauterine life events have a much greater impact on epigenetic profiles than stressful exposures during adult life due to heightened brain plasticity that is adversely affected by exposure to environmental insults [33]) [34] as well as a function of _post_-natal maturation (_maturation_ refers to the timely appearance or unfolding of brain structures, events, and processes that are the result of the interaction between genes and the environment; brain maturation can be _delayed_, _equal_, or _accelerated_ when compared to chronological age) [35; 36; 37; 38]. It seems that ontogenetic differences mirror those of phylogenetic differences in the cause of brain development, where there is a gradual increase in qEEG complexity and change in the qEEG oscillations' composition and proportions [17; 39; 40; 41]. Why is this relevant? The qEEG has been found to have a high prognostic value for identifying the functional level of 'brain maturity' [42; 43]. The knowledge of typical qEEG oscillatory patterns for a given phylogenesis/ontogenesis stage gives one the ability to assess the level of qEEG maturation or regression, which often accompanies the development of neuropsychopathology [44]. For example, a person with an immature qEEG is more easily swayed by external influences and has a lower threshold for aggressive and/or antisocial behaviors [45]. Additionally, early life stress (ELS) has been associated with abnormalities in the qEEG of adults and is also paralleled by a range of adverse outcomes in adults, such as personality dimensions, increased vulnerability to substance abuse, depression, anxiety, psychosis, and post-traumatic stress disorder (PTSD) [46; 47; 48]. Indeed, ELS such as protein energy malnutrition in the first year of life, extreme social and cognitive deprivation as a result of institutional care, physical or emotional neglect, and low socioeconomic status are all associated with abnormal qEEG characteristics on one hand and with developmental lag or deviation, persistent specific cognitive and behavioral deficits in adulthood, and accelerated cognitive decline [49; 50; 51; 52; 53; 54] on another hand. Further, childhood traumas (including childhood sexual abuse) are also associated with adult qEEG deviations in parallel with cognitive dysfunction [55; 56]. It seems that changes in catecholamine levels following a traumatic event can impede brain regional development, which, in turn, can compromise later cognitive functioning and emotional regulation and leave a person susceptible to stress later on in life. Additionally, traumatic brain injury may also be reflected in qEEG deviations that correspond to complaints of cognitive symptoms that can persist anywhere from 11 [57], 22 [58], or even 27 [59] years post-injury, characterizing persistent post-concussive syndrome. This brief overview suggests that the qEEG contains information that is a historical consequence of individual development, ELS, and significant life events.