How tools change the brain
Perception of arm size altered after using mechanical grabber.
Researchers have found convincing evidence that using a tool for just a few minutes can have a lasting effect on how someone perceives the size and position of their body.
A team led by Alessandro Farnè and Lucilla Cardinali of Claude Bernard University in Lyon, France, assessed the effects of using a grabber tool, similar to those used by litter collectors, on volunteers' body schema — the brain's sense of where different body parts are in space.
The volunteers each spent 10–15 minutes using the tool to pick up and replace a rectangular block. Before and after these training sessions, the volunteers were asked to grab the block, or point at it, using only their hands.
The researchers recorded all of these tasks using a high-resolution three-dimensional motion-tracking system, so that they could compare in detail the movements performed in each task. They found that after using the grabber, the volunteers approached the blocks with slightly lower acceleration and velocity, although their accuracy was not affected. "They behave like their arm is longer," says Farnè. "They aren't clumsy, but they are slower and more determined." The results are published in Current Biology1.
This implies that body schema is more plastic than was previously thought, says Charles Spence, a cognitive neuroscientist at the University of Oxford, UK, who was not involved in the work. "There are longer-term effects of using a tool or an implement on the representation of your body than anyone had imagined before," he says.
In an additional experiment, subjects were blindfolded and touched lightly at the elbow, wrist and middle fingertip. When asked to point to where they had just been touched, they regularly estimated their arms to be longer after using the tool, indicating that the tool had been incorporated into people's body schema.
Farnè points out that the effects are subtle — a difference of a few millimetres in estimated length — and not enough to cause any difficulties with everyday manual tasks.
Researchers have been trying to quantify how tools are incorporated into bodily perception for more than a decade. Using monkeys, for example, researchers have measured individual neurons that fire when an object is placed within reach. This 'receptive field' expands when the monkey is allowed to use a rake to pull objects towards itself2.
Any time a tennis player picks up a racquet, or a woodsman an axe, that person's perceived personal space expands. But the latest experiment shows that there are also changes to the longer-term perception of the body itself, says José Bermúdez, director of the philosophy–neuroscience–psychology program at Washington University in St Louis, Missouri. "It's not a surprise that tools get incorporated into the moment-by-moment body schema while they're being employed," he says. "What's interesting is that that tool use has longer-lasting effects on the subjects' perceptions of bodily proportions and dimensions."
Farnè plans further experiments on expert tool users: the sanitation workers at a Lyon hospital, who use similar grabbers to pick up litter. The researchers also hope to study the effects of tool use in people with a condition known as pathological deafferentation, which results in an impaired body schema. Such individuals must guide all bodily movements by sight alone, without the assistance of that intrinsic sense of body position and size that most people possess.
Spence suggests that the research could help to improve the functionality of prosthetic limbs. He adds that sports-science researchers have begun to take an interest in the field, hoping that the results could help professional athletes to improve their performance.
- Cardinali, L. et al. Curr. Biol. doi:10.1016/j.cub.2009.05.009 (2009).
- Maravita, A. & Iriki, A. Trends Cogn. Sci. 8, 79-86 (2004).
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