In addition, PVS patients often open their eyes in response to feeding, which has to be done by others; they are capable of swallowing, whereas patients in a coma subsist with their eyes closed.Cerebral cortical function (e.g. communication, thinking, purposeful movement, etc.) is lost while brainstem functions (e.g. breathing, maintaining circulation and hemodynamic stability, etc.) are preserved. Non-cognitive upper brainstem functions such as eye-opening, occasional vocalizations (e.g. crying, laughing), maintaining normal sleep patterns, and spontaneous non-purposeful movements often remain intact. PVS patients eyes might be in a relatively fixed position, or track moving objects, or move in a disconjugate (i.e., completely unsynchronized) manner. They may experience sleep-wake cycles, or be in a state of chronic wakefulness. They may exhibit some behaviors that can be construed as arising from partial consciousness, such as grinding their teeth, swallowing, smiling, shedding tears, grunting, moaning, or screaming without any apparent external stimulus. Individuals in PVS are seldom on any life-sustaining equipment other than a feeding tube because the brainstem, the center of vegetative functions (such as heart rate and rhythm, respiration, and gastrointestinal activity) is relatively intact. Recovery
Many people emerge spontaneously from a vegetative state within a few weeks. The chances of recovery depend on the extent of injury to the brain and the patients age – younger patients having a better chance of recovery than older patients.
In recent years, a number of studies have demonstrated an important role for functional neuroimaging in the identification of residual cognitive function in persistent vegetative state; this technology is providing new insights into cerebral activity in patients with severe brain damage. Such studies, when successful, may be particularly useful where there is concern about the accuracy of the diagnosis and the possibility that residual cognitive function has remained undetected. Diagnostic experiments
Researchers have begun to use functional neuroimaging studies to study implicit cognitive processing in patients with a clinical diagnosis of persistent vegetative state. Activations in response to sensory stimuli with positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and electrophysiological methods can provide information on the presence, degree, and location of any residual brain function. However, use of these techniques in people with severe brain damage is methodologically, clinically, and theoretically complex and needs careful quantitative analysis and interpretation. For example, PET studies have shown the identification of residual cognitive function in persistent vegetative state. That is, an external stimulation, such as a painful stimulus, still activates "primary" sensory cortices in these patients but these areas are functionally disconnected from "higher order" associative areas needed for awareness. These results show that parts of the cortex are indeed still functioning in "vegetative" patients.In addition, other PET studies have revealed preserved and consistent responses in predicted regions of auditory cortex in response to intelligible speech stimuli.