Quick, where's your wallet? You can reach at once for your billfold--without even glancing from your computer screen--because your brain unconsciously keeps track of nearby objects. Now scientists have found out what charts this mental map. In the current issue of Science,* they report that in experiments with monkeys, bunches of neurons start to fire after an object is seen and keep on firing after it's out of sight.
For a creature to move and interact with its surroundings, it must coordinate its muscles with what it sees, hears, and touches. In humans and other primates, the command center for this neural activity resides in the ventral premotor cortex (PMv), the part of the brain that relays information from the senses to muscles. To study what happens when the eyes locate an object, Michael Graziano, a psychologist at Princeton University, and his team inserted a tiny electrode into two monkeys' PMv's. The electrode's tip touches the body of a neuron, where it records electrical signals as the neuron sends or receives messages. "When we put it over a loudspeaker, it sounds like a click," Graziano says.
The researchers then placed a plastic tube in the monkeys' field of view and listened for a response from specific PMv neurons. When the researchers turned out the lights, one group of neurons continued to fire--as if the monkey were still seeing the tube. These neurons appear to be preserving the memory of the tube's location, Graziano says. The neurons stopped firing when the researchers quietly removed the tube and turned the lights back on. Graziano adds that these groups of neurons in the PMv each remember a specific area of space relative to the body.
The finding backs up the idea that the PMv helps construct a map of our surroundings, says neuroscientist Giacomo Rizzolatti of the University di Parma in Parma, Italy. He's intrigued by the idea that a mental map is drawn with past experience, and not just with what the eyes see now.