We're all familiar with personal space; the invisible bubble that encompasses our mental image of our body. Personal space expands to encompass tools (even cars) that we mentally incorporate as part of our body and may also change size according to our mood. We're less conscious of Peripersonal space; an invisible bubble that represents localized reaching distance. Any object in our peripersonal space can be reached or manipulated directly. Objects outside our peripersonal space require us to move toward them before they can be manipulated. There has been some debate as to how our brains determine the boundry of peripersonal space. A new paper (PDF format) suggests the brain doesn't determine the peripersonal space boundry purely by vision but with the help of feedback from the motor representation areas of the brain. Robots must solve these same problems and understanding how the human brain does it should give roboticists something new to think about.