Have you ever felt a presence in the room that you just couldn't explain?
You're not alone.
It happens to endurance athletes,
polar explorers...
even the sleep-deprived.
But what causes it?
Scientists think that it arises in the brain rather than the spirit realm – it is common especially among patients with neurological conditions such as schizophrenia.
But where in the brain?
Researchers from the Federal Polytechnic School of Lausanne, in Switzerland, may have an answer.
They examined patients who reported frequently feeling a presence.
Brain scans showed that they had unusual activity at points that link different lobes of the brain.
The team guessed that perhaps a ghostly presence comes about from a glitch in the brain's ability to integrate signals from those different lobes.
To test the theory, they built a two-part robotic arm.
One part was moved by a healthy volunteer.
The second part traced that same movement on the volunteer's back.
The setup activates the parts of the brain dedicated to movement, the sense of touch, and the volunteers' perceptions of where they are in space.
By introducing a short delay between the volunteers' movement and its replication, the team was able to induce the feeling of a spooky presence in them - one even reported feeling the presence of four ghosts.
That begins to reveal a mechanism for the ineffable feeling of a presence.
Why it should happen to people in extreme conditions remains a mystery.
But it is a starting point that could help explain, and maybe even to treat, those who are troubled more frequently by the phantoms.
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