This article in Newsweek about a neurosurgeon named Eben Alexander who had a near-death experience, states that he is convinced that heaven actually exists, and that science is wrong about the origin of consciousness and what happens after death. His argument: "But as far as I know, no one before me has ever traveled to this dimension (a) while their cortex was completely shut down, and (b) while their body was under minute medical observation, as mine was for the full seven days of my coma."
Nowhere, however, does he seem to consider that, perhaps, his hallucination could have happened in the brief interval of the cortex shutting down or the interval of him waking up. Even if he experienced days away in paradise doesn't mean that the actual experience lasted at all during the time that his cortex was confirmed to be inactive. Since this possibility took me a whole 10 seconds to think of (and there could be others) it is clear he is not being critical about, what is most likely, a very moving experience for him. I do not begrudge him this experience, and would love to have one myself (if it didn't involved the significant risk of actually dying in the process), but I would be loathe to make universal metaphysical claims from this experience.
A nice response to this, as always, comes from Sam Harris. Here Sam Harris says:
Alexander asserts that the cessation of cortical activity was “clear from the severity and duration of my meningitis, and from the global cortical involvement documented by CT scans and neurological examinations....The problem, however, is that “CT scans and neurological examinations” can’t determine neuronal inactivity—in the cortex or anywhere else. And Alexander makes no reference to functional data that might have been acquired by fMRI, PET, or EEG—nor does he seem to realize that only this sort of evidence could support his case....whatever structural damage appeared on CT could not have been “global.”...Coma is not associated with the complete cessation of cortical activity, in any case. And to my knowledge, almost no one thinks that consciousness is purely a matter of cortical activity.
That about sums it up!