The minimal neural mechanisms that are jointly sufficient for consciousness (conscious percept, thought, memory, etc.) to occur, under constant background conditions, are called the neural correlates of consciousness (NCC) . The background conditions are enabling factors that must be present for the NCC to be able to function (e.g., the heart must beat and the lung must exchange air to supply the brain with oxygenated blood, the blood levels of sugar and other blood constituents must be in normal ranges, the ascending reticular activating system (ARAS) must be functioning, etc.), but they are not the NCC.
The ARAS is the neural network in the core of the brain extending from the rostral pons to both thalami and the basal forebrain, and from there it sends extensive axonal projections to the cerebral cortex, including the consciousness neural network (which is the anatomical part of the NCC). The ARAS is responsible for the alertness of consciousness, but it is not responsible for the content of consciousness. So, it is not part of the NCC proper. However, there can be no consciousness if there is no stimulation from the ARAS, even if the consciousness neural network is intact and able to function, such as in the case of extensive brainstem hemorrhage, infarct, and trauma. And the level of conscious alertness depends on the activation from the ARAS – the more activation, the higher level of alertness. This results in various conscious level’s states, such as (from lower to higher level of alertness) coma, stupor, drowsiness, normal alertness, and heightened alertness . So, the answer to question #2 – why this oldest part (the brainstem) is pivotal in maintaining consciousness – is that it is the way the consciousness neural process has evolved to be dependent on the ARAS for its level of function (i.e. its alertness).
On the contrary, if the ARAS is working normally but the consciousness neural process is not, there can be alertness without, with minimal, or with some awareness. The degree of impairment of awareness depends on how much the consciousness neural process is dysfunctional. For example, in the case of damages to the consciousness neural process from diffuse cerebral hypoxia, extensive bilateral cerebral infarcts, or diffuse cerebral cortical injury, the results can be various abnormal conscious awareness’s states that range in severity of abnormal conscious awareness, depending on how much the consciousness neural process is impaired, such as (from mild to severe) acute confusional state, akinetic mutism, minimally conscious state (MCS), and vegetative state (VS). In the latter two categories, which are severe conditions, the patients can open eyes and have some reflex responses, such as blinking, chewing, and yawning, but show no (in VS) or minimal (in MCS) signs of conscious awareness of self and the environment (by clinical testing or by special investigations such as EEG, evoked potentials, and fMRI) .
Regarding the neural circuits of consciousness, as far as one can conclude from the evidence at present, it seems very likely that the Default Mode Network or Resting State Network , which includes the medial prefrontal cortex, posterior cingulate cortex, precuneus, hippocampal and parahippocampal cortex, retrosplenial cortex, and some parietal and temporal cortices, is the network of the consciousness neural process when the mind is in the state of passive internal mentation, not performing cognitively demanding tasks, and not attending to outside stimuli. This network overlaps with the network that functions for access consciousness (consciousness of episodic stimuli, not consciousness of continuous stimuli as in the former case). At present, evidence shows that the network of Global Workspace theory proposed by Baars  or the network of Global Neuronal Workspace hypothesis of Dehaene , which includes the cortico-thalamic (C-T) core and a network of neurons with long-range axons densely distributed in the prefrontal, fronto-parietal, parieto-temporal, and cingulate cortices, is the network of access consciousness. So, the complete neural network of consciousness that functions to be conscious of continuous events and episodic events is likely to be some form of combination of these networks, such as the neural network in the extended theory of global workspace of consciousness proposed by Song .
The complete neural circuit of consciousness, whatever it is finally found to be composed of, and the way it functions will constitute the complete NCC. The complete NCC is necessary and sufficient for consciousness to occur. Even if we do not have the complete NCC now, the above current models that we have are fairly well supported by experiments. I hope this answers question #1.
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