Contrary to what Shermer claims, these aren’t artifacts of an oxygen-deprived brain; they are meaningful experiences full of detail and coherence, and often they appear after the brain ceases all activity.
How to understand this? Describing such an experience as fact depends on the person's ability to recall the experience once consciousness is regained. At that point, how does the person account for the timing of the experience? When did it actually happen, and how long did it last? I know from my own experiences of "normal" dreams during sleep that the sense of time (both absolute and relative) can be skewed and counter-factual -- the dreams can seem to span a period longer than the sleep lasted, and apart from knowing that a dream ends at the point where I wake up, I have no clear sense (when conscious) of how long the dream actually lasted (i.e. when it started), regardless of how vivid it may have been. Apparently this could be measured (to some extent) in terms of eye movement and brain activity during sleep.
But Chopra asserts that NDE occurs when "the brain ceases all activity". He cites a reference web site where I was able to find this "case study" of one person's NDE -- http://www.near-death.com/experiences/evidence01.html -- and sure enough, the person reports seeing instruments and hearing voices of nurses and doctors in the operating room, presumably during the period when the person's brain was registering no activity whatsoever (no cortical or brain-stem impulses, no blood-flow).
The patient cited a particular phrase she heard from a doctor or nurse during the NDE, but the article does not report confirmation from the staff that this phrase was actually uttered. The patient reports hearing the sound of the cutting instrument, though of course there's no way to compare her recollection to the actual sound of the tool. But whether or not these questions can be more carefully resolved may not be the main issue.
If the NDE is an intrinsically non-physical event (a "pure consciousness" leaving the body), what is the mechanism whereby the disembodied consciousness detects the visual and aural stimuli of the body's physical surroundings? After departing from eyes and ears, what is the basis for perception of sights and sounds? If the enabling factor is some non-physical plane (the patient's soul taps into knowledge acquired by the souls of the doctors and nurses? light and sound propagate through some non-physical or extra-dimensional medium?), we are stuck in the realm of the unverifiable. Maybe the patient simply had a vivid dream during recovery in which various details were fairly close to what really happened -- not too surprising, if the patient had been primed before the operation with a description of what would happen.
I can appreciate Shermer's reluctance to accept this sort of evidence for the existence of consciousness outside the body. At the same time, I also appreciate Chopra's frustration with what strikes him as a dogmatic refusal by Shermer to even consider any sort of evidence for such existence. I think Chopra's goal of seeking to "prove" afterlife is admirable, but can we convey to him, in a constructive way, the criteria for such evidence to be acceptable?