LOS ANGELES (Reuters) - There really is such a thing as a "mind's eye" in which brain cells retain and recall specific visual images long after a person has gazed upon them, according to a 2-year scientific study released on Wednesday.

"Visual images can be generated in our mind's eye in the absence of actually looking at the image," said Itzhak Fried, a neurosurgeon at the University of California at Los Angeles which jointly conducted the study with California Institute of Technology.

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"Our study reveals that the same brain cells that fire when a person looks at a picture of the Mona Lisa are, in fact, the same neurons that excite when the person is asked to imagine the Mona Lisa," Fried said.

During the study researchers implanted tiny electrodes in the brains of nine epilepsy patients to locate the focus of their seizures. The electrodes were put into parts of the brain involving memory and social behavior.

Scientists recorded the activity of 276 single brain cells, known as neurons, after asking patients to imagine images of faces, household objects, spatial layouts, cars, animals, food, drawings and photos of famous people and complex patterns shown to them earlier.

The researchers found that those single neurons in certain areas of the brain selectively altered their firing rates depending on the images imagined.

Researchers found that when patients were imagining the images, the firing rate of the brain cells was nearly as high as when the subjects were actually looking at the photos.

"When you look at something, it's really vivid and when you close your eyes to imagine it the image is not so vivid," Fried said. "So we were surprised that the brain cells fired at almost the same intensity."

"Our results provide a rare opportunity to directly observe the activity of the human brain in a pure mental state without visual stimulation from the outside world," Fried said. "This activity may represent the retrieval of the picture information from memory or the maintenance of the visual image during imagination."

He continued: "It also seems possible that these neurons could be activated during storage of incoming visual inputs, and later reactivated during the ... retrieval process required for imagery."

The findings build upon previous UCLA research that found evidence that single brain cells are involved in memory and can respond selectively to a wide variety of facial expressions and emotions.

Most recently, Fried and his team found that single neurons in the human brain can differentiate between categories of visual images, ranging from animals to photos of celebrities.