Full body action remapping of peripersonal space: the case of walking

The space immediately surrounding the body, i.e. peripersonal space (PPS), is represented by populations of multisensory neurons, from a network of premotor and parietal areas, which integrate tactile stimuli from the body's surface with visual or auditory stimuli presented within a limited distance from the body. Here we show that PPS boundaries extend while walking. We used an audio-tactile interaction task to identify the location in space where looming sounds affect reaction time to tactile stimuli on the chest, taken as a proxy of the PPS boundary. The task was administered while participants either stood still or walked on a treadmill. In addition, in two separate experiments, subjects either received or not additional visual inputs, i.e. optic flow, implying a translation congruent with the direction of their walking. Results revealed that when participants were standing still, sounds boosted tactile processing when located within 65-100 cm from the participants' body, but not at farther distances. Instead, when participants were walking PPS expands as reflected in boosted tactile processing at ~1.66 m. This was found despite the fact the spatial relationship between the participant's body and the sound's source did not vary between the Standing and the Walking condition. This expansion effect on PPS boundaries due to walking was the same with or without optic flow, suggesting that kinematics and proprioceptive cues, rather than visual cues, are critical in triggering the effect. These results are the first to demonstrate an adaptation of the chest's PPS representation due to whole body motion and are compatible with the view that PPS constitutes a dynamic sensory-motor interface between the individual and the environment.