## 2 Why the Electroencephalogram (EEG)?
### qEEG and Disorders of Consciousness  
A vegetative state (VS), recently re-termed as 'unresponsive wakefulness syndrome' (UWS) [316], and MCS belong to the so-called disorders of consciousness or DoCs [317]. While, by convention, VS/UWS patients are unresponsive to their external and internal environments and are thus unconscious [318], patients in MCS show some level of overt awareness and fluctuating ability to follow commands non-reflexively [319] (see also [320]). The factual simplicity of the qEEG assessment, its portability and adaptability for longitudinal protocols, and its relatively low cost have opened up a wide area of qEEG investigations in the recent decade--these assessments aim to study the pathophysiology of DoCs as well as look for prognostication markers for the recovery of consciousness in DoC patients [321, 322]. Already, the simple description of standard EEGs (guided by accurate qualitative scales) has shown a robust correlation of such patterns with both the level of consciousness impairment (VS/UWS or MCS) and the degree of short-term consciousness recovery [323]. These studies reveal that the _overall_ electrical activity of the brain is _differentially_ impaired in patients that fall under different DoCs and that it may be related to the degree of recovery, as follows from the group-analyses [321]. The implementation of more complex numerical computations of the EEG signal--qEEG analyses--has contributed in a much more nuanced way to the evaluation of DoC patients [322], leading to a better understanding of the neural _constituents_ of consciousness' impairment [166]. For example, studies on qEEG oscillations have demonstrated that patients in VS/UWS have a considerably reduced repertoire of local qEEG oscillations compared to those in MCS or a fully conscious state [272]. Additionally, unawareness in patients with VS/UWS was associated with an altered composition of qEEG oscillations and their proportions compared with a full consciousness state [272, 275]. These results confirmed previous observations that loss of consciousness is associated with altered oscillatory contents of the qEEG [324, 325, 326]. In agreement with these findings, it has been proposed that the degree of reduction in the dynamic correlates of neuronal networks' complexity measured by the qEEG may be useful for distinguishing patients with different levels of consciousness impairment (VS/UWS vs. MCS) or even as a prognostic measure [327, 328, 321, 326, 327, 165, 328]. Indeed, evaluation of qEEG spatial-temporal patterns (which reflect functionally connected neuronal assemblies and their dynamics over time) [327, 329, 166, 329] in DoC patients demonstrated that neuronal assemblies become considerably smaller, with shortened life-spans, and they became highly unstable and functionally disconnected (desynchronized) in patients in VS/UWS [166]. In contrast, fluctuating (minimal) awareness in patients who are in MCS is paralleled by partial restoration of qEEG functional integrity, whose parameters approach those of the levels found in healthy, fully conscious participants [166]. These studies lead to the conclusion that consciousness is likely to vanish in the presence of many very small, extremely short-lived, and highly unstable neuronal assemblies that perform their operations completely independently of one another (functional disconnection) and, thus, are not capable of supporting any coherent content to be experienced subjectively. Importantly, it has been documented that the observed impairment in the brain's functional integrity in DoC patients is independent of brain damage etiology and, thus, reflects functional (and potentially reversible) damage, as opposed to irreversible structural neuronal loss [273]. As a whole, these findings are in keeping with a recent study [331], where it was shown that, in contrast to MCS, the VS/UWS brain is characterized by small, disconnected networks that do not contribute to higher integrative processes [332].