Widening the body to rubber hands and tools: what's the difference?

The brain represents the body in different ways (e.g., perceptual, motor) for different purposes (recognising oneself, acting in space). Several concepts and even more numerous labels (e.g., body image, body schema) have historically been proposed to define these representations in operational terms. Recent evidence of embodiment of external objects has added complexity to an already quite intricate picture. In particular, because of their perceptual and motor effects, both rubber hands and tools can be conceived as embodied, that is, represented in the brain as if they were parts of one's own body. But are there any limits to what we can embody? What constraints lay upon embodiment? And are they similar both for motor embodiment (i.e. integration within the body schema) and for perceptual embodiment (i.e. integration within the body image)? Here, we consider the implications emerging from the different, and up-to-now relatively separate research domains of tool use and rubber hand illusion for understanding the rules of embodiment. In particular, we compare what the embodiment of tools and prostheses may or may not have in common. We conclude that in both cases, although for different reasons and with different constraints, embodiment is only partial