The scientists said a brain region activated when people were asked to perform mathematical calculations in an experimental setting is similarly activated when they use numbers -- or even imprecise quantitative terms, such as "more than" -- in everyday conversation, they said.
"These nerve cells are not firing chaotically," neurology Professor Josef Parvizi said. "They're very specialized, active only when the subject starts thinking about numbers. When the subject is reminiscing, laughing or talking, they're not activated."
Thus it was possible to know, simply by consulting the electronic record of participants' brain activity, whether they were engaged in quantitative thought during non-experimental conditions, the researchers said.
"We're now able to eavesdrop on the brain in real life," Parvizi said.
The researchers said the finding could lead to "mind-reading" applications that, for example, would allow a patient rendered mute by a stroke to communicate via passive thinking.
But a darker outcome could see chip implants that spy on or even control people's thoughts, some scientists said.
"This is exciting, and a little scary," Henry Greely of the Stanford Center for Biomedical Ethics said.
Greely did not take part in the study but said he is familiar with its contents and was "very impressed" by the findings.
"It demonstrates, first, that we can see when someone's dealing with numbers and, second, that we may conceivably someday be able to manipulate the brain to affect how someone deals with numbers."
However, any fears of impending mind control are at the very least premature, Greely said. "Practically speaking, it's not the simplest thing in the world to go around implanting electrodes in people's brains. It will not be done tomorrow, or easily, or surreptitiously."
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