Audio-tactile integration at rest and during hand movement: a behavioural study

Introduction The Peri Personal Space (PPS)refers to the space immediately surrounding the body which representation is supported by specific neural systems of that support an individual’s ability to interact with the environment.PPS has been studied by measuring the influence of moving auditory stimuli on individual’s tactile reactivity. This approach has revealed that looming stimuli speed up tactile reactivity as they approach the body unlike receding stimuli. Notably, literature has mainly studied these effects passively, whereas in real-world settings, individuals interact actively with environmental stimuli. Our study aims to investigate how hand movements may influence multisensory (audio-tactile) interaction, specifically examining whether observed effects are confined to the moved body part or extend to others as well. Furthermore, we aim to draw conclusions by accounting for expectancy effects on tactile reactivity. Methods 24 healthy subjects were required to respond to a tactile stimulus delivered on their right index finger or trunk at one of 5 different temporal delays from the onset of looming/receding sounds, by pressing a button with their left hand. Participants were asked to stay still in half of the trials and moving their right hand with a straight trajectory in front of them during the sound in the other half. Tactile stimulation was applied to either the right hand or the trunk. They also underwent the same conditions without sound to control for expectancy effect. Statistical analysis included a to a 2 (Body part: hand vs trunk) x 2 (Movement: motor vs static) x 2 (Sound direction: looming vs receding) x 5 (Sound distance: D1 to D5) within-subjects ANOVA for unisensory-corrected RTs. Finally, individual RTs across the five sound distances were fitted to a sigmoidal model to extract the inflection point. Results Results show no main effects (all ps > 0.05). However, a significant Body part x Sound distance interaction, F(2.525,55.539) = 3.263, p = 0.035, η_p^2 = 0.129, and a Movement x Sound direction × Sound distance interaction were found, F(2.749,60,474) = 4.505 p = 0.008, partial η_p^2 = 0.170. Bonferroni post-hoc comparisons showed that RTs did not differ between motor and static conditions across the distances both for the looming and receding sounds, all ps >0.05. However, post-hoc tests revealed that the only statistical differences occurred within the static looming condition between far (D1) and close (D5) (t = - 4.298, p < 0.01) and between D2 and D5 (t = - 4.196, p < 0.01). Moreover, while fitting RTs with a sigmoidal function, only the static looming conditions showed a good fit with R2 ≥ 0.50, while the static receding and the moving either looming or receding conditions got a badfit to the sigmoid. Discussion This study aimed to investigate the influence of dynamic sounds (looming and receding) on tactile reactivity (audio-tactile integration) to delineate the boundaries of peripersonal spaces for different body parts (hand and trunk) under static conditions and during goal-directed hand movements. By accounting for the effects of temporal expectations, we found that looming sounds boosted tactile reactivity as the sound approached the participant, but only in static conditions. Notably, the inflection point for this effect was farther for the trunk compared to the hand confirming previous literature about a larger PPS of the trunk. However, this effect disappeared when participants moved their hand, suggesting that movement execution eliminates any facilitatory effect of looming sounds over tactile processing, regardless of whether the tactile stimulus is delivered over the moving hand or the static trunk.