It’s likely that yes, the brain remains conscious for a few seconds after a head leaves its body.
When the guillotine was commonly used in revolutionary France, there were numerous reports of decapitated heads moving for up to 30 seconds, blinking eyes and showing expression. But scientists studying decapitated rats’ heads think the brain only functions for about four seconds after being severed.
There is a wave of brain activity about 40 seconds after decapitation in rats and in humans, though that researchers think might be the real line between life and death, when the neurons in the brain lose their electrical charge and completely stop functioning.
There have been accounts of people surviving what’s called internal decapitations, where the bones of the skull separate from the spine. In those cases, the neck has to be rebuilt to keep the head aloft.
The following has been re-published from the DISCOVERY website :
Ever since the guillotine was introduced in France in the 18th century as a method of capital punishment, questions have arisen about the finality of decapitation. Does the brain continue to function even after it’s been severed from the body? In one grisly incident during the French Revolution, a woman named Charlotte Corday was beheaded for assassinating a well-liked journalist and politician. Her decapitated head is said to have responded to being slapped by the executioner after the execution, with her eyes giving him a very “dirty look.” Accounts of the execution of Mary Queen of Scots in February 1587 in England claimed that he lips moved for nearly 15 minutes after her head was cut off by an axe.
These stories and others like them have led to the belief that a person can remain aware after decapitation. Defining exactly when brain death occurs can be difficult even under circumstances less “stressful” than executions. If it were easy to define, medicine and law would not require formal pronouncements of death.
The concept of brain death first was introduced late in the 1950s. As organ donation became more widespread, some physicians and medical ethicists became less comfortable with using a standard definition. Physicians and policymakers continued to refine the guidelines for determining death because determining brain death now is important in medical decision making, not just for organ donation considerations. The use of advanced technology also has made it more difficult, as medical devices can keep vital organs and functions working even when the brain no longer is calling the shots.
Basically, brain death occurs when the entire brain ceases to function [source: JAMA]. That means a person does not respond to any sort of stimulus; the definition includes the pupils’ response to light, the gag reflex and lack of breathing effort if physicians remove a ventilator.
As for tales of reactions following decapitation, physicians say they most likely were reflexive muscle contractions rather than conscious movements. Without oxygen, consciousness is lost and the brain quickly dies.