The two women were being evaluated for epilepsy surgery at University Hospital in Geneva, where doctors implanted dozens of electrodes into their brains to pinpoint the abnormal tissue causing the seizures and to identify adjacent areas involved in language, hearing or other essential functions that should be avoided in the surgery. As each electrode was activated, stimulating a different patch of brain tissue, the patient was asked to say what she was experiencing.

Dr. Olaf Blanke, a neurologist at the Ecole Polytechnique Federale de Lausanne in Switzerland who carried out the procedures, said that the women had normal psychiatric histories and that they were stunned by the bizarre nature of their experiences.

The Sept. 21 issue of Nature magazine includes an account by Blanke and his colleagues of the woman who sensed a shadow person behind her. They described the out-of-body experiences in the February 2004 issue of the journal Brain.

There is nothing mystical about these ghostly experiences, said Peter Brugger, a neuroscientist at University Hospital in Zurich, who was not involved in the experiments but is an expert on phantom limbs, the sensation of still feeling a limb that has been amputated, and other mind-bending phenomena.

"The research shows that the self can be detached from the body and can live a phantom existence on its own, as in an out-of-body experience, or it can be felt outside of personal space, as in a sense of a presence," Brugger said.

Scientists have gained new understanding of these odd bodily sensations as they have learned more about how the brain works, Blanke said. For example, researchers have discovered that some areas of the brain combine information from several senses. Vision, hearing and touch are initially processed in the primary sensory regions. But then they flow together, like tributaries into a river, to create the wholeness of a person's perceptions. A dog is visually recognized far more quickly if it is simultaneously accompanied by the sound of its bark.

These multisensory processing regions also build up perceptions of the body as it moves through the world, Blanke said. Sensors in the skin provide information about pressure, pain, heat, cold and similar sensations. Sensors in the joints, tendons and bones tell the brain where the body is positioned in space. Sensors in the ears track the sense of balance. And sensors in the internal organs, including the heart, liver and intestines, provide a readout of a person's emotional state.

Real-time information from the body, the space around the body and the subjective feelings from the body are also represented in multisensory regions, Blanke said. And if an electric current directly simulates these regions, as in the cases of the two women he studied, the integrity of the sense of body can be altered.

As an example, Blanke described the case of a 22-year-old student who had electrodes implanted into the left side of her brain in 2004.

"We were checking language areas," Blanke said, when the woman turned her head to the right. That made no sense, he said, because the electrode was nowhere near the area involved in the control of movement. Instead, the current was stimulating a multisensory area called the angular gyrus.

Blanke applied the current again. Again, the woman turned her head to the right. "Why are you doing this?" he asked.

The woman replied that she had a weird sensation that another person was lying beneath her on the bed. The figure, she said, felt like a "shadow" that did not speak or move; it was young, more like a man than a woman, and it wanted to interfere with her.

When Blanke turned off the current, the woman stopped looking to the right, and said the strange presence had gone away. Each time he reapplied the current, she once again turned her head to try to see the shadow figure.

When the woman sat up, leaned forward and hugged her knees, she said that she felt as if the shadow man was also sitting and that he was clasping her in his arms. She said it felt unpleasant. When she held a card in her right hand, she reported that the shadow figure tried to take it from her. "He doesn't want me to read," she said.

Because the presence closely mimicked the patient's body posture and position, Blanke concluded that the patient was experiencing an unusual perception of her own body, as a double. But for reasons that scientists have not been able to explain, he said, she did not recognize that it was her own body she was sensing.

The feeling of a shadowy presence can occur without electrical stimulation to the brain, Brugger said. It has been described by people who undergo sensory deprivation, as in mountaineers trekking at high altitude or sailors crossing the ocean alone, and by people who have suffered minor strokes or other disruptions in blood flow to the brain.

Six years ago, another of Blanke's patients underwent brain stimulation to a different multisensory area, the angular gyrus, which blends vision with the body sense. The patient experienced a complete out-of-body experience.

When the current flowed, she said: "I am at the ceiling. I am looking down at my legs."

When the current ceased, she said: "I'm back on the table now. What happened?"

Further applications of the current returned the woman to the ceiling, causing her to feel as if she were outside of her body, floating, her legs dangling below her. When she closed her eyes, she had the sensation of doing sit-ups, with her upper body approaching her legs.

Because the woman's felt position in space and her actual position in space did not match, her mind cast about for the best way to turn her confusion into a coherent experience, Blanke said. She concluded that she must be floating up and away while looking downward.

Some schizophrenics, Blanke said, experience paranoid delusions and the sense that someone is following them. They also sometimes confuse their own actions with the actions of other people. While the cause of these symptoms is not known, he said, multisensory processing areas may be involved.

When otherwise normal people experience bodily delusions, Blanke said, they are often flummoxed. The felt sensation of the body is so seamless, so familiar, that people do not realize it is a creation of the brain, even when something goes wrong and the brain is perturbed.

Yet the sense of body integrity is rather easily duped, Blanke said.

And while it may be tempting to invoke the supernatural

when this body sense goes awry, he said the true explanation is a very natural one, the brain's attempt to make sense of conflicting information.