Dr. Jack Gallant, a neuroscientist at the University of California in Berkeley, said his team has figured out how to use functional magnetic resonance imaging (fMRI) to tell what someone is looking at based on brain activity.

A report, published online in the journal Nature, said it is the first step to being able to see the contents of someone's visual experiences.

"When the deck of cards, or photographs, has about 120 images, we can do better than 90 percent correct," Gallant said.

He said the next step is to interpret what someone is seeing without having the subject select from known images.

The research team said a device that can read out the brain's activity could be used to assess damage from strokes, the effect of drug treatments or to help diagnose conditions such as dementia.