Sensations of a non-existent person nearby might arise from the brain's failure to integrate different body signals.
Olaf Blanke of the Swiss Federal Institute of Technology in Lausanne and his colleagues found that people who reported these feelings often had injuries in the frontoparietal cortex, which integrates different types of sensory and motor information.
The team recreated this effect in healthy volunteers who were asked to move a robot in front of them using their finger. A second robot behind them mimicked the movements by touching their back. The participants were blindfolded and wore headphones so that they could not see or hear the robots moving.
When the rear robot moved immediately, most participants felt that they were touching themselves on the back with their own finger, even though their arms were stretched forward. But when there was a half-second delay, they felt that the touch was coming from someone, or something, else. The illusions were caused by a mismatch between the expected and actual sensory information, the authors say.
Curr. Biol. http://doi.org/wzb (2014)