Capturing the dynamics of peripersonal space by integrating expectancy effects and sound propagation properties

•Measuring expectation helps to interpret audiotactile integration behavioral impact.•Tactile RTs follow a logarithmic decrease due to audiotactile integration.•Logarithmically distributed auditory distances provide more pertinent RTs measures.•PPS is better characterized and quantified with these enhancements. Humans perceive near space and far space differently. Peripersonal space (PPS), i.e. the space directly surrounding the body, is often studied using paradigms based on audiotactile integration. In these paradigms, reaction time (RT) to a tactile stimulus is measured in the presence of a concurrent auditory looming stimulus. We propose here to refine the experimental procedure by disentangling behavioral contributions of the targeted audiotactile integration mechanisms from expectancy effects. To this aim, we added to the protocol a baseline with a fixed sound distance. Furthermore, in order to improve the relevance of the audiotactile integration measures, we took into account sound propagation properties and assessed RTs for logarithmically spaced auditory distances. Expectation contributed significantly to overall behavioral responses. Subtracting it isolated the audiotactile effect due to the stimulus proximity. This revealed that audiotactile integration effects have to be tested on a logarithmic scale of distances, and that they follow a linear variation on this scale. The current method allows cleaner and more pertinent sampling measures for evaluating audiotactile integration phenomena linked to PPS. Furthermore, most of the existing methods propose a sigmoid fitting, which rests on the intuitive framework that PPS is an in-or-out zone. Our results suggest that behavioral effects follow a logarithmic decrease, thus a response graduated in space. The proposed protocol design and method of analysis contribute to sharpen the experimental investigation of the factors influencing and modifying multisensory integration phenomena in the space surrounding the body.