[joberdorf (OP)]

The "bright light" of near death experiences could simply arise from the way our photoreceptors function. 

When we think of a typical neuron, its resting state occurs when its plasma membrane is hyperpolarized and the secretion of neurotransmitters is at a minimum.  Loss of oxygen to the brain will program neurons to this resting state to conserve their energy, and consequently their ability to survive the state of anoxia.  In extreme cases this protective mechanism can lead to a coma.

Vertebrate photoreceptors are unusual in that their resting/hyperpolarized state occurs while they are exposed to light.   It is only when photoreceptors are deprived of light that their plasma membranes depolarize to initiate the release of neurotransmitters to downstream neurons connecting them to the brain.  In this sense vertebrates are described as seeing darkness rather than light.  Since both the "dark current" resulting from membrane depolarization and consequent neurosecretion are energy consuming; the protective response to any loss of oxygen will force the photoreceptor into a hyperpolarized/inactive state, which is precisely the state recognized by the brain as light.

In cases where photoreceptor shutdown momentarily precedes loss of consciousness, the person will sense the bright light of a near death experience..

[dlorde]

It could also be due to spontaneous activity further up the visual pathways or in the visual processing areas. Strong bursts of activity have been observed in these areas, and those associated with awareness, prior to death (see Nature: Brain Metrics)