Modulation of auditory peripersonal space by approaching and receding sources moving in discrete trajectories

The region of space immediately surrounding the body, where we can interact with the outside world using our limbs, is commonly known as the peripersonal space (PPS). The representation of this region in the brain is dynamic and can be influenced by kinematic properties of the stimulus such as speed and direction. It has been shown that visual or auditory approaching objects expand the observer's PPS, supporting the hypothesis of an anticipation mechanism that prioritizes approaching stimuli to better react to potential threats. However, the response to receding stimuli is not so well understood. Here, we studied the auditory PPS for approaching and receding sound sources that traveled at different speeds. In each trial, the source was presented at a fixed distance and listeners judged whether it was within arm's reach. In the next trial, the source could be presented either closer or farther, creating the impression of a discrete approaching or receding trajectory. We found that reachability judgments were affected by the previous positions of the sound source. Approaching sources were judged more reachable at the same position than receding ones while non-defined trajectories (control) displayed intermediate values, suggesting an expansion of the auditory PPS for approaching sounds. Interestingly, receding sources were judged less reachable than controls after accounting for speed, suggesting compression of the auditory PPS. This result is consistent with the idea that reachability judgments arise from an impact prediction mechanism. The implications of this idea for our understanding of PPS are discussed.