CHICAGO — The Halle Berry fan club is expanding one brain cell at a time. By eavesdropping on the activity of single neurons in the human brain, scientists have figured out which brain cells go wild for superstars such as the popular actress. And the newest research shows that people can activate those cells selectively.

“This study is the first demonstration of humans’ ability to control the activity of single neurons,” the researchers wrote in a summary of their study. The results, presented October 19 at the Society for Neuroscience’s annual meeting by Moran Cerf of the California Institute of Technology in Pasadena, may help researchers understand how each cell in the brain sees and responds to the world.

“This type of work gives us some clues about what’s going on in the brain,” comments Christoph Weidemann of the University of Pennsylvania, who studies how the brain processes information. “It’s quite an amazing feat for the brain to make sense of its input and reliably recognize people and objects.”

The new study was conducted on people with epilepsy. Doctors had implanted electrodes in these patients’ brains to track where seizures originate. The researchers used these same electrodes to eavesdrop on the activity of single brain cells in a part of the brain called the medial temporal lobe, which is important for “memory, attention, perception — the things that we care about the most,” Cerf said in his presentation.

Before the experiment began, Cerf and his colleagues showed volunteers familiar images of people, objects or places, chosen on the basis of extensive interviews about the patients’ preferences. Pictures included images of Bill Clinton, Michael Jackson and Venus Williams, among others. “We hoped to locate neurons in their brains that respond selectively to one of those concepts,” Cerf said in his talk.

In each patient, the researchers found about five neurons that fired when the patient looked at an image of a certain person or object. A person might have, for example, a Halle Barry neuron, a Marilyn Monroe neuron, an Eiffel tower neuron, a Michael Jackson neuron and a spider neuron.

Once these neurons were identified, the researchers wanted to know if the patients could control them by thinking about that certain person or object. To do this, Cerf and his colleagues hooked up the neuron-sensing electrodes to a computer that then displayed images representative of the person’s thought. When a patient’s Marilyn Monroe neuron became active, the screen would show an image of Marilyn Monroe.

To see how well the patients could control these single neurons, researchers set up what they call the “fade” experiment, which is like a competition between two different neurons. One version of the experiment involved a neuron that responded to Goonies’ star Josh Brolin and another neuron that responded to Marilyn Monroe. Initially, the person was shown a hybrid image of these two stars overlaid on each other. When the person was told to think of Josh Brolin, the electrodes would record that neuron’s activity and send the data to the computer, causing the Monroe image to fade and the Brolin image to get brighter. The experiment was finished when the picture was completely Brolin or Monroe, or 10 seconds had elapsed. Ten patients underwent this test and successfully directed the pictures between 60 and 90 percent of the time, the researchers found. As the testing went on, the patients became better at controlling the neurons.

Scientists are still far from being able to see people’s innermost thoughts, Weidemann says. “When people talk about mind reading, there’s a tendency to think of 1984 and all the negative aspects of it,” he says. “The goal here is to understand cognitive processes.”

A better understanding of how the brain encodes information may be useful for building machines that can be controlled directly from people’s brains. Such devices may ultimately help people who are unable to communicate, such as people who have tetraplegia.

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