Whole brain death occurs when the ceases to function.

Whole brain death occurs when the BRAINSTEM ceases to function.

Explanation:

In the US, whole brain death = legal death. Most people consider that a person is brain dead when the cerebrum, or the higher brain, is dead because the person is not able to think or have control over his/her body, but legally you are brain dead only when the brainstem ceases to function. The brainstem is responsible for keeping you alive and performing the most basic bodily functions like breathing and keeping your heart beating. But once it fails, you are legally dead. Even if your body could still survive using a ventilator and other devices, your brain has ceased to work and you wouldn't be able to survive if you were unplugged.