People experiencing PVS have a severely damaged cerebral cortex. This is the upper brain responsible for cognition, perception, language and purposeful movement. If there is a cut off of oxygen from the brain, it will lay waste to the cerebral hemispheres well before it damages the lower brain, or brain stem, which controls respiration, blood pressure, and the sleep-wake cycle, as well as voluntary reflexes.
The brain stem remains largely intact in PVS patients. Thus they will occasionally yawn, chew, swallow, sigh, grimace, laugh and cry. They open their eyes and seem to glance about: on closer inspection, however, their eyeballs are seen to be moving uncoordinated, random directions with no visual input to the brain. (Death & Dying).
PVS patients test our institutions about the boundary between life and death. What precisely does "death" mean? Is it primarily a biological, philosophical or religious concept? What marks the moment when somebody dies? Whatever "death" is, it ought to be permanent and irreversible.