Dynamic sounds capture the boundaries of peripersonal space representation in humans

Abstract

Background: We physically interact with external stimuli when they occur within a limited space immediately surrounding the body, i.e., Peripersonal Space (PPS). In the primate brain, specific fronto-parietal areas are responsible for the multisensory representation of PPS, by integrating tactile, visual and auditory information occurring on and near the body. Dynamic stimuli are particularly relevant for PPS representation, as they might refer to potential harms approaching the body. However, behavioural tasks for studying PPS representation with moving stimuli are lacking. Here we propose a new dynamic audio-tactile interaction task in order to assess the extension of PPS in a more functionally and ecologically valid condition.

Methodology/principal findings: Participants vocally responded to a tactile stimulus administered at the hand at different delays from the onset of task-irrelevant dynamic sounds which gave the impression of a sound source either approaching or receding from the subject's hand. Results showed that a moving auditory stimulus speeded up the processing of a tactile stimulus at the hand as long as it was perceived at a limited distance from the hand, that is within the boundaries of PPS representation. The audio-tactile interaction effect was stronger when sounds were approaching compared to when sounds were receding.

Conclusion/significance: This study provides a new method to dynamically assess pps representation: The function describing the relationship between tactile processing and the position of sounds in space can be used to estimate the location of PPS boundaries, along a spatial continuum between far and near space, in a valuable and ecologically significant way.

Figure 1. Experimental setup.

Panel A. Procedure. Subjects received a tactile stimulus at their hand while task-irrelevant sounds either approached to or receded from the hand. Tactile stimuli were delivered at different temporal delays from sound onset (from T1 to T5), so that they were processed when sounds were perceived at a different distance from the hand. Panel B. Sound localization experiment results. The graph shows subject’s mean responses indicating the perceived position of sound in space when they receive a tactile stimulus at different temporal delays from sound onset, from T1 to T5. Filled line refers to IN sound condition, hatched line refers to OUT sound condition. Error bars denote S.E.M. A repeated measure ANOVA with Sound (IN, OUT) and Temporal Delay (from T1 to T5) confirmed that IN and OUT were perceived as an approaching and receding auditory stimuli, respectively, as clearly shown by the significant two-way interaction (F(4,24) = 304.30, p<0.00001).

Figure 2. Effects of IN and OUT sounds on tactile processing.

Mean RTs (and S.E.M.) to the tactile target at different temporal delays (from T0 to T6) for IN (filled line) and OUT (hatched line) sounds. The shaded region indicates the duration of the sounds.

Figure 3. Best fitting function for the relationship between sound position in space and tactile processing.

Data from a paradigmatic subject are reported. Figure 3 plots mean RTs (and S.E.M.) at different times of tactile stimulus delivery and the best fitting sigmoidal functions for IN (filled line) and OUT (hatched line) sounds.