The distinguishing feature of the persistent vegetative state is an irregular but cyclic state of circadian sleep and wake cycles, unaccompanied by any detectable expression of self-awareness (The Multi-Society Task Force on PVS Part 1 1500). Along with maintaining autonomous functions, such as cardiovascular and renal functions, patients in a persistent vegetative state may be aroused by certain stimuli, opening their eyes if they are closed, changing their facial expressions, or even moving their limbs. Furthermore, they can grind their teeth, swallow, smile, shed tears, grunt, moan, or scream without any reason. Their heads and eyes can follow a moving object or move towards a loud sound (Zeman 796). Yet, these responses have been observed in patients in whom careful study has shown no evidence of awareness. Consistent with a persistent vegetative state is a lack of sustained visual pursuit. Although they may move their eyes, patients in a vegetative state neither fixate on a visual object nor track a moving target with their eyes (The Multi-Society Task Force on PVS Part 1 1500-1501). "A circumstance where somebody could have his eyes open and not see, and not look, and not track, and not use his eyes for the purpose of gathering information about their environment...is hard for many people to understand," said Dr. Stell (personal interview). Positron-emission tomographic (PET scan) studies have shown an average of a fifty percent decrease in metabolic activity in the cerebral cortex and cerebellum of patients in a persistent vegetative state (The Multi-Society Task Force on PVS Part 1 1506). Thus, although these patients may exhibit behavior that appears to be the result of conscious thought and reasoning, these behaviors are merely reflexive and do not indicate awareness.