Body part-centered and full body-centered peripersonal space representations

Abstract

Dedicated neural systems represent the space surrounding the body, termed Peripersonal space (PPS), by integrating visual or auditory stimuli occurring near the body with somatosensory information. As a behavioral proxy to PPS, we measured participants' reaction time to tactile stimulation while task-irrelevant auditory or visual stimuli were presented at different distances from their body. In 7 experiments we delineated the critical distance at which auditory or visual stimuli boosted tactile processing on the hand, face, and trunk as a proxy of the PPS extension. Three main findings were obtained. First, the size of PPS varied according to the stimulated body part, being progressively bigger for the hand, then face, and largest for the trunk. Second, while approaching stimuli always modulated tactile processing in a space-dependent manner, receding stimuli did so only for the hand. Finally, the extension of PPS around the hand and the face varied according to their relative positioning and stimuli congruency, whereas the trunk PPS was constant. These results suggest that at least three body-part specific PPS representations exist, differing in extension and directional tuning. These distinct PPS representations, however, are not fully independent from each other, but referenced to the common reference frame of the trunk.

Figure 1. Experimental Setup.

Upper Left Panel; illustration of the 2-speaker setup utilized in Experiment 1, 3, 4, and 5. Auditory looming and receding sounds were simulated by modulating the intensity emitted by a near (5 cm) and a far (100 cm) speaker. Upper Right Panel: Estimate of distance (in an arbitrary unit–A.U) from 0 (near) to 100 (far) as a function of the moment when participants received a tactile stimulation, while a looming sound, originating from the far loudspeaker and illusorily moving towards the near loudspeaker, was presented. Error bars indicate +/−1 S.E.M. Lower Left Panel: illustration of the 16-speaker setup utilized in Experiment 2 and 6. Looming and receding sounds were simulated by placing participants between two arrays of 8 speakers (2 meters of longitudinal distance, and 50 cm between participant midline and each array of speakers in the horizontal plane), and modulating the intensity of the sound produced by each speaker as a function of time. Lower Right Panel: Estimate of sound distance as a function of point in space sampled for the 16-speaker setup. Participants estimated sound distance for sounds originating 1 meter in front (positive x-value) and terminating 1 meter behind (negative x-value) in light gray, and for sounds originating 2 meters in front and terminating 2 meters behind the subjects in dark gray. Error bars indicate +/−1 S.E.M. (Figure drawn by JPN and EC)

Figure 2. Peri-Trunk Space.

Upper Panel-Experiment 1; Multisensory facilitation (in ms; negative values indicate multimodal audio-tactile RT < unimodal baseline tactile RT) as a function of sound distance at the moment of tactile stimulation (D1 indicates the smallest distance, while D6 indicates the largest distance). RT facilitations due to looming sounds are illustrated in solid red, while responses to receding sounds are portrayed in dashed red. The black dashed horizontal line indicates baseline, i.e., RT to the fastest unimodal tactile stimulation, and *indicate a significant facilitation effect with respect to baseline (p < 0.05 Bonferroni-corrected Error bars indicate +/−1 S.E.M.). Lower Panel-Experiment 2; Multisensory facilitation (in ms; negative values indicate multimodal audio-tactile RT

Figure 3. Peri-Hand Space-Experiment 3.

Multisensory facilitation (in ms; negative values indicate multimodal audio-tactile RT

Figure 4. Peri-Hand and Peri-Trunk Space comparison.

Upper panel – Experiment 4. Left upper panel: Participants received a tactile stimulation either on their chest (to study the peri-trunk space) or on their right hand (to study the peri-hand space), while the right hand was placed on the chest. Right upper panel: Multisensory facilitation (in ms; negative values indicate multimodal audio-tactile RT http://my.smithmicro.com/poser-3d-animation-software.html)).

Figure 5. Peri-Face and Peri-Trunk comparison-Experiment 6.

Left upper panel: Participants received a tactile stimulation on their forehead (to study the peri-face space), while sounds (depicted by the couple of loudspeakers) approached their face (congruent audio-tactile stimulation) or their trunk (incongruent audio-tactile stimulation). Right upper panel: Multisensory facilitation (in ms; negative values indicate multimodal audio-tactile RT http://my.smithmicro.com/poser-3d-animation-software.html)).

Figure 6. Visuo-Tactile Peri-Face and Peri-Trunk representation – Experiment 7.

Mean RT difference between multisensory (visuo-tactile) and unisensory (tactile) trials as a function of the distance where a virtual ball was presented at the time of tacile stimulation. Green solid line shows the RT profile when touch was administer to the face (to study the peri-face space), while the red solid line shows the RT profile for when tactile stimulation was given on the chest (to study the peri-trunk space). The black dashed horizontal line indicates baseline (RT to tactile stimulation), * and ** represent a significant difference between multimodal and unimodal conditions at a particular sound-touch distance (respectively, p < 0.05 and p < 0.01, Bonferroni-corrected), and error bars indicate +/−1 S.E.M.

Figure 7. Left panel: Schematic representation of the relative size of peri-hand (blue), peri-face (green), and peri-trunk (red) representation (Notice the gradient in the depiction of peri-hand and peri-face representation: data was not collected for the back space for the hand and the face and hence this panel is to be taken simply as a conceptual schema.

Dashed lines represent spatial extensions that were actually probed in this study). Right panel: Estimates were obtained by fitting all data corresponding to a particular body part across studies 1–6 to a sigmoidal curve after adjusting for total space mapped and points sampled. In this manner, regardless of the total amount of space sampled (1 or 2 meters) or the total amount of distances sampled (5, 6, or 7), the central point parameter of the sigmoid function (peri-hand and peri-face) or the intersection between the linear function describing audio-tactile RTs and the lower bound of the 95% confidence interval describing tactile RTs (in the case of the peri-trunk) were extracted as a punctuate estimate of peri-hand (~45 cm), peri-face (~59 cm), and peri-trunk (~72 cm) space. (Figure drawn by JPN; the human was created with Poser 9, SmithMicro Software, lincense: XF90CRD-0001-14YT-QJF0-201X-EBL7).