Scientific American currently has a piece on near-death experiences. They say the answer to such phenomena might be found in new research being done on abnormal functioning of dopamine and oxygen flow. In the article, they mention a thing known as Cotard Syndrome. You might remember that Caden Cotard, the lead character in Synecdoche, New York, was named Caden Cotard.

Recently, a host of studies has revealed potential underpinnings for all the elements of such experiences.

[...]

For instance, the feeling of being dead is not limited to near-death experiences—patients with Cotard or "walking corpse" syndrome hold the delusional belief that they are deceased. This disorder has occurred following trauma, such as during advanced stages of typhoid and multiple sclerosis, and has been linked with brain regions such as the parietal cortex and the prefrontal cortex—"the parietal cortex is typically involved in attentional processes, and the prefrontal cortex is involved in delusions observed in psychiatric conditions such as schizophrenia," Mobbs explains. Although the mechanism behind the syndrome remains unknown, one possible explanation is that patients are trying to make sense of the strange experiences they are having. (Source)

They include a link to a review of a book, The Tell-Tale Brain: a Neuroscientist's Quest for What Makes Us Human, and in that review they mention Capgras Syndrome. "Capgras" happens to be one name in the address list of an apartment building Caden visits. Capgras Syndrome is "when a person believes those around him have been replaced by imposters."