Properties of the peripersonal space in behaving humans

Abstract

Humans are equipped with many systems to help protect us from bodily harm. One of them is the peripersonal space that tries to help us avoid collisions or minimize the impact of collisions with external objects. This small network in the brain monitors the spaces immediately surrounding individual body-parts such as the face, hands, torso, legs and engages defensive responses such as moving the body-part away from the direction of impact, or closing the eyes in the event that impact is expected on the face. The interesting thing is that the protective zones around the body parts of independent of each other, and the defensive responses they trigger are also specific to protecting that body part. This defensive mechanism was the focus of this thesis. Initially discovered in primates, research is now being conducted in humans to gain an understanding of it. In the first study of the thesis, we show that humans are not only capable of anticipating where they are to expect an approaching object to make contact, but they are also capable of extracting exactly when the contact is to occur. We show that anticipating physical contact from a moving object facilitates the activity of the PPS mechanism. That is, both the action of the peripersonal space network and the anticipation of touch together help us prepare and respond to an object that is likely to come in contact with us. In other studies we looked at the properties of the peripersonal space. When the speed of a looming object increases, the defensive PPS around the body-part also increases, so as to be able to trigger the defensive response effectively sooner. This result is directly in line with what was observed in the neuronal activity of monkeys. Neurons tended to respond sooner when the object approaching the monkey loomed faster. We also show that the peripersonal space network integrates information from multiple senses, such as vision and touch to form its responses. That is, when you are able to both see an object approach you and feel its initial touch you are likely to detect it sooner than if you only saw it or felt its touch. This thesis looked at the effect of our action capability on our ability to estimate the locations of approaching objects. We showed that the better your action capability, the better you are estimating the location of a looming object. We compared video game players with non-video game players in a virtual reality task where they had to prevent a looming ball from making contact with an object standing next to them. We then asked the participants to estimate the location of the ball when it changed colour. We found that video game players were not only more effective at stopping the ball, they were also more accurate at estimating the location of the ball. Non-video game players, were slower at stopping the ball and also consistently judged the ball to be closer to them than it actually was. We see that when we need to interact with a looming object, our ability to effectively interact with it determines our judgement of its location. The results of this thesis demonstrate the effects of the mechanisms that our brain is equipped with to maintain a safety margin around our body.