Assuming that decapitation occurs in a “civilized” way and that it doesn’t take a number of blows or cuts to the neck in order to sever the head from the body – which, in practice, is not always a correct assumption: does the brain remain conscious for a few seconds after a clean and quick decapitation? There are many historical reports of decapitated heads showing facial movements or even the attempt to speak right after decapitation. It’s not clear what to make of this, since facial movements can just as well be spasms.
However, experiments with rats have shown brain activity after decapitation. Sure, there’s no way to be sure that this is true for the human brain as well – since “further scientific observation of human decapitation is unlikely”, in the words of Alan Bellows. Still, the rat experiments are suggestive:
[R]esearchers connected an EEG machine to the brains of rats, decapitated them and recorded the electrical activity in the brain after the event. [They] found that for about four seconds after being separated from the body, the rats’ brains continued to generate electrical activity between the 13 to 100-Hertz frequency band, which is associated with consciousness and cognition, defined as “a mental process that includes thinking”. (source)
The circulatory system delivers oxygen to the brain so that it can carry out its functions. When suddenly deprived of oxygen or blood after a clean and quick decapitation – or after several severe blows to the neck with a knife, sword or axe, before full decapitation – the brain’s function deteriorates rapidly, but perhaps not instantaneously. This would imply that individuals, after suffering a clean and quick decapitation, can still think, perceive, feel and suffer pain and anguish during a few horrible seconds after which the brain, which itself receives no trauma during decapitation, stops functioning because blood loss causes unconsciousness and death.