|MadSci Network: Neuroscience|
I apologize that I took so long to answer your question.The answer is that if a person can perceive sensation or think after decapitation, these perceptions or thoughts only persist for about 5-30 seconds. This estimate is based on two pieces of evidence. First, deprivation of oxygen to the brain for about 10 seconds causes a loss of consciousness in humans, at which point there is no conscious perception. Second, studies in rats have investigated the persistence of electrical brain activity after decapitation using an electroencephalogram (EEG). These studies were originally performed to determine whether decapitation was a humane method of killing the animal without anesthetic. These techniques are occasionally necessary to avoid the effects of anesthetics on the body tissue being studied. By 30 seconds after decapitation, there was no electrical activity, indicating complete brain death. However, up to 15 seconds following decapitation, it was found that electrical activity similar to that in the waking brain or during the rapid-eye movement (REM) phase of sleep was present. It is somewhat difficult to interpret this activity. The EEG signal in waking or REM sleep is characterized by different regions of the brain working asynchronously (out of phase)and at higher frequencies, whereas deep sleep is characterized by synchronous (in phase) and lower frequency activity throughout the brain. However, when neurons are injured, which would likely occur following decaptitation, they often respond with a higher frequency discharge of electrical activity. It is possible that this is the response observed in the post-decapitation EEG. Concerning perception and thought during the 15 seconds in which waking-like EEG activity is present, I can suggest a grisly experiment that could attempt to answer the question. In normal animals, sensory activation such as a light or sound will cause a noticeable change in the EEG activity over the appropriate portion of the skull. So if the animal could be decapitated while recording EEG responses to sensory stimuli, it might be able to address whether the animals can still sense for a few seconds.
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