Oc occasions, people report feeling that there is "someone" or "something" present, even when they know they are alone. It happens to mountain climbers, solo sailors and others. In a recent article titled "The Sensed Presence Effect" in Volume 301 number 4 April 2010 of "Scientific American," author Michael Shermer suggest four possible explanations.
"1) The hallucination may be an extension of the normal sensed presence of real people and is, perhaps triggered by isolation. 2) During oxygen deprivation, sleep deprivation or exhaustion, the rational cortical control over emotion shuts down, as in the fight-or-flight response, enabling inner voices and imaginary companions to arise. 3) The body schema, or our physical sense of self-believed to be located primarily in the temporal lobe of the left-hemisphere-is the image of the body that the brain has constructed. If for any reason your brain is tricked into thinking that there is another you, it constructs a plausible explanation that the other you is actually another person-a sensed presence-nearby. 4) The mind schema, or our psychological sense of self, coordinating the many independent neural networks that simultaneously work away at problems in daily living so that we feel like a single mind."
I have previously written about the "sense of presence" and "sense of self" and its possible application to abduction reports, in posts dated 22 Oct 2009 and 27 Oct 2009.