In the wee hours of the morning on August 8, 1983, while I was traveling along a lonely rural highway approaching Haigler, Neb., a large craft with bright lights overtook me and forced me to the side of the road. Alien beings exited the craft and abducted me for 90 minutes, after which time I found myself back on the road with no memory of what transpired inside the ship. I can prove that this happened because I recounted it to a film crew shortly afterward.

When alien abductees recount to me their stories, I do not deny that they had a real experience. But thanks to recent research by Harvard University psychologists Richard J. McNally and Susan A. Clancy, we now know that some fantasies are indistinguishable from reality, and they can be just as traumatic. In a 2004 paper in Psychological Science entitled "Psychophysiological Responding during Script-Driven Imagery in People Reporting Abduction by Space Aliens," McNally, Clancy and their colleagues report the results of a study of claimed abductees. The researchers measured heart rate, skin conductance and electromyographic responses in a muscle that lifted the eyebrow--called the left lateral (outer) frontalis--of the study participants as they relived their experiences through script-driven imagery. "Relative to control participants," the authors concluded, "abductees exhibited greater psychophysiological reactivity to abduction and stressful scripts than to positive and neutral scripts." In fact, the abductees' responses were comparable to those of post-traumatic stress disorder (PTSD) patients who had listened to scripts of their actual traumatic experiences.

In 1983, in rural Nebraska, I was abducted by aliens.

The abduction study was initiated as a control in a larger investigation of memories of sexual abuse. In his book Remembering Trauma (Harvard University Press, 2003), McNally tracks the history of the recovered memory movement of the 1990s, in which some people, while attempting to recover lost memories of childhood sexual molestation (usually through hypnosis and guided imagery), instead created false memories of abuse that never happened. "The fact that people who believe they have been abducted by space aliens respond like PTSD patients to audiotaped scripts describing their alleged abductions," McNally explains, "underscores the power of belief to drive a physiology consistent with actual traumatic experience." The vividness of a traumatic memory cannot be taken as evidence of its authenticity.

The most likely explanation for alien abductions is sleep paralysis and hypnopompic (on awakening) hallucinations. Temporary paralysis is often accompanied by visual and auditory hallucinations and sexual fantasies, all of which are interpreted within the context of pop culture's fascination with UFOs and aliens. McNally found that abductees "were much more prone to exhibit false recall and false recognition in the lab than were control subjects," and they scored significantly higher than normal on a questionnaire measuring "absorption," a trait related to fantasy proneness that also predicts false recall.

My abduction experience was triggered by sleep deprivation and physical exhaustion. I had just ridden a bicycle 83 straight hours and 1,259 miles in the opening days of the 3,100-mile nonstop transcontinental Race Across America. I was sleepily weaving down the road when my support motor home flashed its high beams and pulled alongside, and my crew entreated me to take a sleep break. At that moment a distant memory of the 1960s television series The Invaders was inculcated into my waking dream. In the series, alien beings were taking over the earth by replicating actual people but, inexplicably, retained a stiff little finger. Suddenly the members of my support team were transmogrified into aliens. I stared intensely at their fingers and grilled them on both technical and personal matters.

After my 90-minute sleep break, the experience represented nothing more than a bizarre hallucination, which I recounted to ABC's Wide World of Sports television crew filming the race. But at the time the experience was real, and that's the point. The human capacity for self-delusion is boundless, and the effects of belief are overpowering. Thanks to science we have learned to tell the difference between fantasy and reality.