Don't take that hammer for granted. Using tools may seem like second nature, but only a few animals can master the coordination and mental sophistication required. So how did primates learn to use tools in the first place? A new study in monkeys suggests that the brain's trick is to treat tools as just another body part.
Primates, with their four flexible fingers and opposable thumbs, have a highly evolved ability to grasp and manipulate objects. Previous research has shown that many of these actions are controlled by an area of the brain called F5. As the hand opens and closes to grasp an object, neurons in area F5 fire in a predictable sequence. In the parlance of neuroscientists, the neurons are "coded" to control the hand movements. When a primate learns to use a tool, its brain must code neurons not only to move the hand but also to make the tool manipulate an object, a much more cognitively complex task.
To investigate how the brain performs this sleight of hand, a team led by neuroscientist Giacomo Rizzolatti of the University of Parma in Italy recorded brain activity in two macaque monkeys. Each was trained for 6 to 8 months to grasp items of food with pliers. The team documented the activity of 113 neurons in F5 and in a brain area called F1, which has also been implicated in the manipulation of objects. The researchers first established the brain's firing sequence when the monkeys grasped only with their hands. The experiment was then repeated while the monkeys used normal pliers that required first opening the hand and then closing it to grasp the food. The same neurons fired in the same order. Remarkably, the same neurons also fired, in the same order, when the monkeys used "reverse pliers" that required them to close their fingers first and then open them to take the food, the team reports online today in the Proceedings of the National Academy of Sciences.
Rizzolatti and his co-workers conclude that when learning to use a tool, the pattern of neuronal activity is somehow transferred from the hand to the tool, "as if the tool were the hand of the monkey and its tips were the monkey's fingers." As for how the same neurons could affect both the opening and the closing of the hand, the team speculates that they may be connected with other sets of neurons that more directly control these movements. The authors also point out that area F5 is rich in so-called mirror neurons, a type of nerve cell discovered earlier by Rizzolatti that fires both when a primate performs an action and when it observes another individual doing the same thing (ScienceNOW, 13 July 2007). Mirror neurons in F5, the authors suggest, may be involved in this transfer process as a monkey learns how to use a tool by watching others.
The findings "fairly clearly show that monkey tool use involves the incorporation of tools into the body schema, literally as extensions of the body," says Dietrich Stout, an archaeologist specializing in tool use at University College London. Scott Frey, a neuroscientist at the University of Oregon, Eugene, says that in humans, this ability to represent tools in the brain, combined with a capacity for innovation, "was no doubt a fundamental step in the development of technology."