Newsweek has an excerpt from an upcoming book by neurosurgeon Eben Alexander: Proof of Heaven: A Neurosurgeon's Journey into the Afterlife. The Newsweek excerpt tells of Alexander's experience of "heaven" while in a deep coma caused by a bacterial meningitis infection:
I’m not the first person to have discovered evidence that consciousness exists beyond the body. Brief, wonderful glimpses of this realm are as old as human history.
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.
* * *
All the chief arguments against near-death experiences suggest that these experiences are the results of minimal, transient, or partial malfunctioning of the cortex. My near-death experience, however, took place not while my cortex was malfunctioning, but while it was simply off. This is clear from the severity and duration of my meningitis, and from the global cortical involvement documented by CT scans and neurological examinations.
According to current medical understanding of the brain and mind, there is absolutely no way that I could have experienced even a dim and limited consciousness during my time in the coma, much less the hyper-vivid and completely coherent odyssey I underwent.
(Extra paragraphing added.)
I'm agnostic (in the weak sense) about these experiences; just because we don't know how the brain might create delusions of this kind, that's not proof of a glorious afterlife.
On the other hand, it does seem unwise to categorically dismiss the possibility that some sort of heaven does indeed exist.