Tools are 'temporary body parts'

Participants used a 40-cm gripping tool to perform tasks

The brain represents tools as extensions to the body, according to researchers writing in Current Biology.

After the use of a grasping tool, participants asked to grasp an object with their own hands did so more slowly and sluggishly.

Blindfolded participants also overestimated the length of their tool-using arm after the exercise.

The research seems to confirm a century-old hypothesis that the brain models tools as parts of the body.

"There is a great debate in neuroscience about the representation of the body and representation of space," said Lucilla Cardinali of the National Institute of Health and Medical Research (Inserm) in France.

"There are a lot of papers about the effects of tool use, but they all focus on space - none investigated the effect on our own body," she told BBC News.

Ms Cardinali and her colleagues tested that effect by giving participants a gripping tool, similar to the tool commonly used to pick up rubbish.

They had to use the tool to pick up and replace a small block. After just a few minutes of the task, they were then asked to pick up the block using their own hand, or to simply reach out and place a finger on top of the block.

This is the first unambiguous and definitive proof that using a tool modifies the representation of our body Lucilla Cardinali Inserm

The team found that the participants' performance was noticeably different in carrying out those tasks than they had been before using the tool.

They accelerated and decelerated their movements more slowly, taking a longer time overall - which the team attributes to adjustments the participants needed to make in the absence of the tool.

Long thought

More telling, however, was an experiment performed with the participants blindfolded after the tool use.

After an experimenter touched the participants' elbow and middle fingertip, they were asked to point using the other hand to those two locations.

After a session of using the tool, the participants indicated locations further apart than before tool use: they seemed to perceive the tool-using arm as longer.

"This is the first unambiguous and definitive proof that using a tool modifies the representation of our body; previous studies suggested this but never proved it directly," Ms Cardinali said.

Patrick Haggard of the Institute of Cognitive Neuroscience at University College London, UK, said that the study contrasted with prior work that concentrated on the perception of space, rather than the body performing an action in that space.

"Previous studies had shown that using tools changes the way humans and animals perceive external space, bringing far-away objects into reach," he told BBC News.

"Neuroscientists have known for a long time that the brain's map of the body is not static: in fact, the brain needs to adjust to the changes in our body that occur with growth, ageing, and traumas such as amputation or injury.

"But this paper shows how rapid these adjustments are."