Their study is published in journal Neuron.
The study concludes that hypnosis induces a disconnection of motor commands from normal voluntary processes under the influence of brain circuits involved in executive control and self-imagery, Yann Cojan, PhD, of the Neuroscience Center and Medical School at the University of Geneva, tells WebMD in an email.
The researchers used fMRI to scan brains of 12 people who were tested on hand movement before and after hypnosis for left hand paralysis.
Despite the paralysis, neurons in the motor cortex region were still firing away in preparation for the task, Cojan tells WebMD.
He says his team confirmed "subjective reports of hypnotic phenomenon" and also that "functional connectivity is a very important process in the brain" that hypnosis is capable of modifying.
What was surprising was that the cortex appeared to be ignoring parts of the brain with which it normally communicates in controlling movement, the researchers say.
Hypnosis produced changes in areas involved in attention, and also modified connections between the brain's motor cortex and other regions, Cojan tells WebMD.
Besides the 12 who were hypnotized, the researchers also scanned the brains of six people who had not been hypnotized but who had been told to feign hand paralysis for testing. They comprised the comparison group.
"These results suggest that hypnosis may enhance self-monitoring processes to allow internal representations generated by the suggestion to guide behavior but does not act through direct motor inhibition," Cojan says in a news release. "These findings make an important new step toward establishing neurobiological foundations for the striking impact of hypnosis on the brain and behavior."
In the study participants, messages weren't sent through normal brain channels, so when hypnotized subjects were told to move their left hands, they couldn't, Cojan says.
Hypnosis, the researchers found, induces a disconnect in normal voluntary processes involved in planning to move a body part. "Hypnosis is a very old tool in many medical domains but it is still unclear how it works," Cojan says.