understood. Impaired consciousness is associated with brain damage or injury due to trauma, degenerative diseases, infections of the central nervous system, or metabolic disorders (Giacino et al., 2013). Notable impaired states of consciousness are the vegetative state (VS) and the minimally conscious state (MCS). The vegetative state is characterized by brainstem autonomic and hypothalamic function, the appearance of sleep-wake cycles, and the absence of the awareness of self or the environment (Giacino et al., 2013). Patients in a vegetative state are also unable to respond purposefully to noxious stimuli, express or understand language. However, they may have cranial nerve and spinal reflexes, such as swallowing, blinking, and knee-jerk reflexes (Shea et al., 2013; Laureys et al., 2002). The vegetative state is usually the consequence of a coma that lasts for longer than a month and can be categorized into either a persistent vegetative state, or permanent vegetative state. A coma due to a traumatic brain injury that persists for over 12 months or a coma due to a non-traumatic brain injury that persists for over 3 months is considered a permanent vegetative state. A coma that lasts more than a month is considered a persistent vegetative state (Georgiopoulos et al., 2010). Patients in a permanent vegetative state are less likely to recover compared to those in a persistent vegetative state. During wake cycles, the eyes open and move but do not fixate on a particular target and the limbs may move meaninglessly. Painful and noxious stimuli can elicit reflexes and physiological progresses such as eye opening, increase in respiratory rate, heart rate, and blood pressure (Laureys et al., 2002). The preservation of pupillary, corneal, oculocephalic, and gag reflexes also allow vegetative patients to display spontaneous movements such as chewing, teeth-grinding, swallowing, moaning, and smiling (Laureys et al., 2002). Compared to healthy individuals, the resting brain metabolism of patients in a vegetative state is 50-60% lower (Laureys et al., 2002). PET studies and mapping analysis have revealed that the brain regions associated with metabolic impairment are the prefrontal, premotor, and parietal-temporal cortices, and the posterior cingulate region (Laureys et al., 2002). These cortices are involved in attention, working memory, episodic memory, language, and conscious perception while the posterior cingulate region is involved in conscious waking (Laureys et al., 2002). Conversely, the metabolism of the brain stem, basal forebrain, and posterior hypothalamus are relatively preserve
