Extending Peri-Personal Space in Immersive Virtual Reality: A Systematic Review

Abstract

Peri-personal space (PPS) refers to the area immediately surrounding our body where interactions with objects and others occur. Immersive virtual reality (IVR) offers a controlled and adaptable environment, enabling the precise modulation of PPS boundaries. This provides significant benefits across various fields, including enhancing spatial awareness, advancing therapeutic interventions, and improving ergonomic designs. This systematic review aims to synthesize and evaluate the existing literature on this topic through various methodologies. To achieve this, three databases, PubMed, Scopus, and Web of Science, were searched following the PRISMA framework. Twenty studies met the eligibility criteria, were assessed for quality, and were included in the review. Across all studies, IVR was utilized to provide multisensory interactions and implement methods were used to manipulate PPS boundaries. The review categorizes PPS extension methods into three main domains: tool-use extension, extension related to tool use, social interaction extension, and embodiment-related extension. The findings confirm IVR’s potential to expand PPS boundaries and offer recommendations for leveraging this technology in future research. This work highlights the importance of IVR in advancing our understanding of PPS and its practical applications across diverse contexts.

1. Introduction

The relationship between the body, mind, and the world has been debated for centuries due to the unclear boundaries between them. While Western philosophy often acknowledges René Descartes’ dualistic view of the mind–body relationship, the sciences continue to seek more materialistic confirmation evidence to explain this connection [1]. Peri-personal space (PPS), which represents the area where we can interact with or respond to objects, the environment, and other individuals, helps bridge the gap between the mind and the body with its empirically measurable structure [2]. From a neuroscientific perspective, one of the first experiments on PPS was conducted by Leinonen and Nyman [3], who discovered parietal neurons in monkeys that specifically respond to stimuli near the body. This discovery laid the foundation for pioneering research by Rizzolatti et al. [4] on the brain representing the space surrounding the body in macaque monkeys, revealing a strong link between somatosensory and visual processing within reaching distance. Additionally, Rizzolatti et al. [5] identified specific neurons in the premotor cortex that responded not only to the monkeys’ body activities but also to objects moving towards them within a defined area, which they named “peri-personal space”.

Since then, the representation of PPS has been extensively studied to uncover its properties. For instance, Spence et al. [6] demonstrated how information from different sensory modalities can interact and influence each other within the multisensory representation of PPS. The authors [7] investigated the so-called “Cross-Modal Congruency Effect” (CCE), which suggests that the compatibility between stimuli and different sensory responses (such as vision, touch, and audition) affects the speed and accuracy of responses. This experimental effect was later used in research to explore PPS representation by examining how different sensory inputs and their manipulations, such as audio tactile combinations, influence and interact within the PPS [8,9].

PPS integrates multisensory information to create a dynamic representation of the space immediately surrounding the body by means of the interaction of different brain regions, including the ventral intraparietal region, posterior parietal cortex, ventral premotor cortex, insula, and superior temporal sulcus [6,10]. Studies with monkeys have shown that PPS representation relies on the integration of multisensory information, such as tactile, visual, and proprioceptive inputs, by the premotor cortex and posterior parietal cortex to maintain its adaptability [11]. Furthermore, Fogassi et al. [12] demonstrated that the magnitude of visual receptive fields in the F4 region (premotor cortex), which is involved in defensive PPS, depends on the speed of approaching stimuli. More recently, Serino et al. [13] measured the RT to tactile stimuli with concurrent audio or visual distractions to determine the boundaries of PPS for different body parts, specifically the hand, face, and trunk. They found that extension and directional tuning of PPS varied for different body parts, but extensions were referenced from the trunk.

Studies on PPS representation raise important questions about the nature of its boundaries. What is their size? Are they fixed, or can they be modulated? Iriki et al. [14] were among the first to address these questions, showing that tool use can expand the receptive fields of neurons involved in PPS in the anterior portion of the intraparietal sulcus of monkeys, which is linked to action-related processes. Farnè and Làdavas [15] further explored this by showing that using a rake to manipulate distant objects expanded the hand-centered PPS, whereas motor actions toward far objects without a tool did not produce the same effect.

Additionally, Berti and Frassinetti [16] conducted a case study involving a patient with a right hemisphere stroke, revealing a disassociation between close and distant space. The patient demonstrated neglect in a line bisection task for close space but not for distant space when using a laser pointer. However, when the task was repeated using a stick, the neglect extended from close to distant space with tool use. Furthermore, research by Serino et al. [17] found that the long-term use of a white cane by blind individuals enhances the extension of their PPS.

An alternative perspective on PPS extension comes from Noel et al. [18], who found that the boundaries of PPS are significantly larger while walking compared to standing still, measuring approximately 65–100 cm while walking and ~166 cm while stationary. Similarly, Kuroda and Teramoto [19] demonstrated that during a bike pedaling simulation with the optic flow, participants exhibited different reaction times to tactile detection tasks, suggesting that PPS boundaries can dynamically adjust based on movement and sensory input. Another way to extend the PPS is through defensive action responses. For example, Sambo and Lannetti [20] found that individuals with higher trait anxiety scores tend to have a larger PPS. Additionally, research by de Haan et al. [21] showed that when facing an approaching threat such as a spider, reaction times are faster, and the perceived distance is greater compared to non-threatening stimuli like a butterfly. Social factors also play a role in modulating other PPS boundaries, further illustrating the dynamic nature of how we perceive and interact with our immediate surroundings. For instance, research by Teneggi et al. [22] found that participants’ PPS boundaries expanded to include a partner’s space after participating in cooperative economic games involving material gain. This indicates that social interactions can significantly influence PPS modulation. Similarly, Dell’Anna et al. [23] observed that during a jazz duet, participants responded more quickly to tactile–auditory stimuli close to them when the interaction was uncooperative. This suggests that the nature of social interaction can impact how PPS is adjusted.

The application of the metaverse in psychological research is rapidly expanding, with virtual reality (VR) emerging as a crucial tool in psychology and cognitive science [24,25]. Traditional computer screen-based simulations often fall short in therapeutic contexts due to their limited ecological validity. In contrast, VR offers the advantage of creating immersive, realistic environments that enable the study of behaviors that are challenging to assess in conventional settings [26,27,28].

Recently, immersive virtual reality (IVR) has emerged as a powerful tool in PPS research for several important reasons. First, IVR environments enable researchers to systematically manipulate various environmental variables, such as object properties and spatial configurations. For instance, in one of the first studies to use VR to investigate PPS in patients with left hemispatial neglect, researchers noted that VR allowed them to systematically adjust the position of the virtual objects within the virtual environment relative to the location of actual objects in the real world [29]. Additionally, the IVR environment can be tailored to address specific research questions and experimental paradigms, offering a highly adaptable research setting. For instance, Bernasconi et al. [30] demonstrated that audio–tactile multisensory integration and PPS effects share common temporal and spatial processes. Given that multisensory integration is crucial to studying PPS, IVR allows researchers to simultaneously stimulate and manipulate multiple inputs, such as visual, auditory, and tactile stimuli. Moreover, dynamic and interactive IVR environments enable researchers to investigate participants’ reactions to changing stimuli, further enhancing their value as a tool for studying PPS. Additionally, as highlighted in this systematic literature review, IVR facilitates more ethical research practices by mitigating real-world dangers that participants might otherwise face. This added safety makes the research environment even more flexible and versatile for experimental design.

Despite its advantages, no systematic review currently examines methods for manipulating PPS in immersive virtual reality (IVR). Therefore, this systematic review comprehensively analyzes existing literature on IVR-based PPS manipulation, prioritizing methodologically novel and relevant studies. This focused approach facilitates a detailed synthesis of techniques, informing future research directions. Specifically, the review evaluates the feasibility of modulating PPS boundaries using IVR, categorizing methods into three types: tool-use, social interaction, and embodiment-related extensions. Finally, the review identifies limitations in the current literature and suggests avenues for future research.

This systematic review is essential because it organizes and evaluates fragmented research, advances theoretical knowledge, and offers actionable insights that could guide technological and scientific progress in IVR-related PPS research. This sounds potentially relevant across multiple domains, from healthcare to education and even entertainment.

2. Methods

2.1. Study Design

This systematic review was registered within the PROSPERO platform and conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [31,32]. Using the PRISMA checklist [32], reviewers transparently document each stage of the review process, including the rationale for the review process, the steps taken by the authors, and the findings. This approach ensures that the findings are both robust and representative of methodological advancements in the field.

To identify relevant studies, a comprehensive literature search was conducted using three primary keywords: ‘peripersonal’, ‘peri-personal’, and ‘virtual’. These keywords were carefully chosen to encompass research exploring the relationship between peripersonal space (PPS) and virtual reality, ensuring the inclusion of diverse and pertinent studies in the review. Since the term ‘peripersonal’ is inconsistently used in the literature, as both ‘peripersonal’ and ‘peri-personal’, these two variations were combined using the Boolean operator ‘OR’. To refine the search and focus on studies related to virtual reality and PPS, the keyword “virtual” was then combined with this group using the Boolean operator “AND”. This strategy ensured that the search results were both comprehensive and specific to the topic of interest.

Eligible publications were identified in May 2024 through searches in the PubMed, Scopus, and Web of Science databases without any time restrictions. The keywords used in the search are detailed in

Table 1. Demonstration of databases, search filters, and search phrases with limitations applied.

Database	Search Fields	Search Phrase	Limitations
SCOPUS	Article title, abstract, and keywords	“peripersonal” OR “peri-personal” AND “virtual”	English; article
PubMed	All Fields	((peripersonal) OR (peri-personal)) AND (virtual)	None
Web of Science	Topic (article title, abstract, and keywords)	((TS = (peripersonal)) OR TS = (peri-personal)) AND TS = (virtual)	English; article

. The search process led to the identification of 327 research papers. Initially, the dataset consisted of 327 articles, which were reduced to 145 after duplicates were removed using Microsoft Excel. Titles and abstracts were then screened for relevance, followed by a full-text assessment based on inclusion criteria, focusing on methodological rigor and relevance to the PPS extension in IVR. After this thorough screening process, 20 high-quality studies were selected for final analysis. This work was conducted by the first author of the review under the supervision of all co-authors.

2.2. Eligibility Criteria

The study selection process employed specific inclusion and exclusion criteria to ensure high methodological accuracy and rigor. Inclusion criteria: Studies were included if they were original quantitative research examining the extension of PPS representation in IVR using multisensory, behavioral, and psychophysiological measures of PPS. Only peer-reviewed empirical studies published in English were considered. Additionally, studies conducted with healthy adult participants were selected to maintain a targeted and homogeneous sample group.

Exclusion criteria: To maintain methodological focus and reliability, reviews, meta-analyses, single case studies, conference papers, and theses were excluded. Additionally, studies employing augmented, semi, or mixed realities were not considered, as the review specifically focused on research using IVR as a methodological or experimental tool. Articles investigating spaces other than PPS, such as studies exclusively addressing personal space, were also excluded.

The flow diagram outlining the study selection process is shown in Figure 1.

2.3. Characteristics of the Selected Studies

Seven hundred and three participants were involved in the included studies, with sample sizes ranging from 15 to 119 (Mean ± Standard Deviation = 35.15 ± 24.28). Complete age and gender data were unavailable across all studies, precluding overall statistical conclusions (see supplementary file, Table S1, for demographic details). Study designs included twelve within-subject studies (four [33,34,35,36] randomized and eight [37,38,39,40,41,42,43,44] counterbalanced), five crossover studies (one randomized [45] and four counterbalanced [46,47,48,49]), and three [50,51,52] studies using 2 × 2 × 2 factorial, between-subject counterbalanced, and mixed counterbalanced designs, respectively (see supplementary file, Table S1, for further details).

3. Results

3.1. Synthesis of Evidence

The final database included 20 published studies that investigated methods for extending PPS representation in IVR. These studies were categorized into three groups: tool use-related extension, social interaction-related extension, and embodiment-related extension, with six, six, and eight studies in each group, respectively (see Table S1). Studies within each category were reviewed individually to provide detailed methodological and experimental information. This facilitated a structured analysis to identify general patterns, trends, and gaps, which were then synthesized in the discussion. An overview of the risk assessment bias is provided in Table S2.

3.2. Key Findings from Included Studies

In the first group, five out of six studies reported significant findings. The one study that found only partly effective results differed from the others in the type of tool used. According to research, training with a hammer and toy gun can be considered two types of combat-related actions that can stimulate defensive behavior, as participants were asked to use the “gun” to shoot a blob avatar [49].

In the second group, five out of six studies reported significant findings. One study found that only partly effective results differed from the others in terms of social context. According to the research, while a sense of power over others blurred the boundaries between near and far space, it did not extend the PPS [34]. In the last group, all eight studies found significant results.

Tool use-related extension:

• Processing of peripersonal and extrapersonal space using tools: evidence from visual line bisection in real and virtual environments [47]:

Gamberini et al. [47] investigated how tool usage in IVR can expand PPS. Sixteen participants interacted with objects in VR using either a joystick as a laser pointer simulator or virtual wooden sticks of four different lengths (i.e., 49.2, 78.6, 104.3, and 121.8 cm). They completed a line bisection task at four different distances (30, 60, 90, and 120 cm). The mean difference between the observed midpoint and the correct midpoint was calculated for each device to compare the bias in each condition. In the base conditions, participants exhibited a leftward shift from the true midpoint in PPS (at 30 and 60 cm) and a rightward shift in extra-personal space (EPS at 90 and 120 cm). The results indicated that using virtual sticks led to a leftward shift in both PPS and EPS. With the laser pointer, a shift from left to right was observed between 60 and 90 cm.

These findings indicate that virtual tool use can effectively expand the PPS, aligning with results from real-world experiments. This suggests that interacting with tools in a virtual environment can alter our perception of PPS, extending it to cover areas that the tools can reach. Additionally, these results suggest the adaptability of our spatial representation of PPS.

2.

Extending the body to virtual tools using a robotic surgical interface: evidence from the crossmodal congruency task [51]:

Sengül et al. [51] investigated how using a robotic surgical interface in an IVR environment affects the perception of PPS (PPS). Nineteen participants used a robotic arm as a golf club simulator to perform visuo-tactile CCE tasks in IVR. Their ability to integrate this tool into their body schema was assessed under two conditions: with the clubs in a crossed position and in a straight position). Moreover, in Study 1, participants actively (by themselves) crossed or uncrossed the tools every four trials. In Study 2, the tools were crossed by the experimenter (passive condition) after only 240 trials, with participants receiving only visual feedback about the change. During both studies, participants were asked to respond to vibrotactile stimuli using a foot pedal while ignoring visual distractors. The results demonstrated that participants could manipulate the robotic arm with increasing accuracy and experienced an expansion of their PPS, perceiving the space around the robotic arm as part of their reaching space. PPS remapping occurred in both active and passive conditions, with a more pronounced effect observed in the active condition. This finding suggests that when virtual tools are effectively embodied, they can significantly expand PPS.

3.

Reshaping the peripersonal space in virtual reality [49]:

Petrizzo et al. [49] investigated how different tools and motor training affect the expansion of PPS in an IVR. Participants performed a visuotactile detection task before and after using various tools to interact with virtual objects, which was used to measure changes in PPS boundaries.

There were two training sessions: “push and pull”, where 41 participants needed to move marbles into a target space, and “hummer and toy-gun”, where 32 participants were required to hit a blob avatar. The results showed that using a shovel to pull objects towards oneself significantly extended PPS. However, other tool interactions, such as hammering or shooting an avatar, did not result in significant changes in PPS. These results highlight that the type of tool and the nature of the interaction play critical roles in the dynamic changes of PPS in virtual environments.

4.

The remapping of peripersonal space in a real but not in a virtual environment [37]:

Ferroni et al. [37] investigated the plasticity of PPS following real and virtual motor training sessions. Twenty-two participants performed a visuo-tactile PPS task to measure their individual PPS boundaries before and after training sessions. They were required to respond to tactile stimuli while ignoring visual distractors.

In both training sessions, they were asked to position 50 colored objects from one location to another and then reposition them. Tactile stimuli of 10 ms were delivered at five different temporal delays from the onset of the looming and receding visual stimuli (after 2165, 1732, 1299, 866, and 433 ms), corresponding to five different distances from the body (D1–D5, ranging from 37.12 to 167.03 cm from the participant, in 32.5 cm intervals). PPS boundaries were calculated as points of subjective equality (PSE) using the Spearman–Karber (SK) method, which described visuo-tactile RTs as a function of visuo-tactile distance. The results showed a significant extension of PPS following real-world training but not after virtual training.

5.

Embodiment of supernumerary robotic limbs in virtual reality [33]:

Arai et al. [33] investigated the effects of projecting an extra limb in an IVR environment on PPS expansion. Sixteen participants first completed a CCE task without prior training, where they were required to identify the position (up or down) of tactile stimuli while ignoring visual distractors. This task was used to measure changes in PPS, and participants also completed an embodiment questionnaire. Next, they were equipped with multiple robotic limb systems and performed a ball-touch task using virtual robotic arms while receiving corresponding tactile feedback on their feet.

Afterward, participants completed another CCE task and an embodiment questionnaire to assess changes in PPS. The results showed a significant expansion of PPS with cross-modal adaptation effects increasing after training with the extra limb. These results suggest that the brain incorporates new body parts into the PPS, demonstrating the flexibility and adaptability of PPS in response to the addition of artificial limbs to the virtual environment.

6.

A behavioral experiment in virtual reality to verify the role of action function in space coding [35]:

Gamberini et al. [35] investigated the influence of tool functionality on the perception and categorization of PPS within an IVR environment. Thirty-eight participants were tasked with bisecting lines presented at either a reachable or unreachable distance, respectively 60 cm and 120 cm, using two tools: a laser pointer and a laser cutter. While the laser cutter caused the line to break into two, the laser pointer did not show any physical consequence to the action.

The results showed a significant link between the distance and the tool used. At the unreachable distance, participants’ errors were significantly closer to the actual center in the line bisection task when using the laser cutter compared to the laser pointer. On the other hand, this effect was not observed at a reachable distance. The findings of this study suggest that the capability to produce physical consequences in the environment, even at a distance, can lead to the re-categorization of that space as PPS.

Social interaction-related extension:

• The effect of facial expressions on peripersonal and interpersonal spaces [42]:

Ruggiero et al. [42] investigated how the emotional valence of facial expressions impacts PPS and interpersonal space (IPS) in an IVR. Seventeen participants had their reachability distance (peri-personal space) measured using a reaction time (RT) task, and their comfort distance (IPS) assessed using the IPS task. In the IPS task, participants were asked to indicate their comfort level with avatars displaying different emotional expressions (happy, angry, and neutral) and at varying distances (0.5, 1.5 m). This was carried out under two conditions: when participants either approached the virtual confederates (passive) or were approached by them (active). In the RT task, participants had to press a button as quickly as possible when the virtual avatar changed its facial expression at two different distances and in two conditions.

The results showed an increase in the comfort distance with angry faces compared to happy faces in both conditions. However, for the reachability distance, this effect was significant only with the passive approach condition. The findings suggest that negative emotional stimuli, such as anger, can expand both PPS and IPS, especially when individuals cannot control the approaching movement. These results highlight the defensive function of PPS.

2.

Near or far? It depends on my impression: moral information and spatial behavior in virtual interactions [38]:

Iachini et al. [38] investigated how moral information about virtual confederates influences spatial behavior in virtual interactions. Thirty-eight students participated in the experiment. The task included four conditions to measure the comfort distance and reachability distance: passive comfort distance, active comfort distance, passive reachability distance, and active reachability distance. In the passive approach condition, participants stood still and observed a virtual avatar walking toward them. They were required to stop the avatar by pressing the button, at which point the stimuli disappeared.

In the active approach condition, the virtual avatar remained stationary while participants walked toward it. They were instructed to stop and press a button when they felt the distance was appropriate. After pressing the button, the virtual stimuli disappeared, and participants returned to their starting position. The virtual confederates were presented with varying moral character descriptions (positive, negative, or neutral). Participants were then asked to press the button when they felt uncomfortable to measure comfort distance or to reach the virtual confederate to measure reachability distance. The results showed that participants maintained larger distances from confederates described with immoral traits.

Additionally, female participants showed a greater distance from virtual confederates, especially from male confederates. The approach condition also had a significant effect on distance: participants kept a larger distance in the passive condition compared to the active condition. These findings suggest that moral information significantly influences how people regulate IPS and PPS.

3.

Space for power: feeling powerful over others’ behavior affects peri-personal space representation [34]:

Bertoni et al. [34] investigated how feelings of social power influence the representation of PPS using IVR. A total of 119 participants completed a multisensory interaction task to evaluate their PPS representation. The task included three types of trials: bimodal visuo-tactile, unimodal tactile, and catch trials (without tactile stimulation). Participants viewed a white corridor with a virtual actress positioned 1.5 m away and a virtual cube. The cube approached within 0.75 m in one second, and tactile stimulation was delivered at six different distances (D1–D6). Participants responded to tactile stimuli by pressing a mouse button while ignoring visual stimuli presented in the VR environment. The task was conducted in two scenarios: social and non-social.

In the social scenario, individuals with a higher sense of power exhibited less distinct differentiation between near and far space compared to those with a lower sense of power. This effect was also observed in the non-social context, but only when participants recalled their sense of power before the task. The findings suggest that the perception of social power influences the multisensory representation of space, blurring the boundaries between one’s own PPS and the space of others.

4.

Peripersonal and interpersonal space in virtual and real environments: effects of gender and age [39]:

Iachini et al. [39] investigated the relationship between environmental space (PPS) and IPS in both virtual and real environments. The study involved forty participants whose reachability (PPS) and comfort (IPS) distances were measured in response to virtual avatars (female child, male child, young adult woman, young adult man, old woman, and old man) and real people while standing still (passive approach) or approaching them (active approach). They were asked to press a button when they felt uncomfortable in both approaching conditions. The results showed that both PPS and IPS were similarly sensitive to social aspects (gender and age) to regulate the self-other space. Specifically, accessibility and comfort distances decreased for female avatars, increased for male avatars, and were greater for adult avatars compared to children, especially in the passive condition. These findings suggest a shared motor basis between PPS and IPS and highlight the influence of social features on defensive function of space perception in IVR environments.

5.

Defensive functions provoke similar psychophysiological reactions in reaching and comfort spaces [43]:

Ruggiero et al. [43] investigated how visual and tactile stimuli affected PPS boundaries in the virtual environment to better understand the defensive functions within PPS. The experiment consisted of six blocks, each involving different facial expressions (positive, negative, and neutral) and included both comfort distance and reaching distance tasks. In the comfort distance task, twenty-four participants were asked to press a button when they felt uncomfortable with the proximity of the confederate’s presence.

In the reaching distance task, participants pressed the button when they felt they could reach the virtual confederate. The distance between participants and the confederate was recorded when they pressed the button. The results revealed that participants maintained a greater distance when the confederate displayed angry expressions, both in terms of PPS and IPS, compared to when the confederate had happy or neutral expressions. Additionally, a gender effect was observed, with participants maintaining larger distances from male confederates than from female ones. These findings suggest that PPS can dynamically expand in response to threats and social interactions, enhancing defensive capabilities.

6.

Sharpening of peripersonal space during the COVID-19 pandemic [52]:

Serino et al. [52] investigated the effects of COVID-19 social distancing measures on PPS in IVR. Forty-four participants completed the visuo-tactile stimuli task featuring approaching avatar faces in a pre-pandemic cohort (June–July 2018), pre-lockdown cohort (10 February−10 March 2020), and post-lockdown cohort (10 June−25 July 2020). They completed visuo-tactile, unisensory (Tactile only), and visual-only trials. Each trial began with virtual faces appearing to the left, right, or center of the participants’ visual field, moving approximately 9 m to ≈0.30 m from the participant.

Participants were instructed to respond to the tactile stimulus on their faces while ignoring the looming visual stimulus, which consisted of gender-matched avatar faces with neutral expressions in VR. The tactile stimulation was delivered at five different time delays (D5 = 0.5 s; D4 = 1 s; D3 = 1.5 s; D2 = 2 s; D1 = 2.5 s), corresponding to five distances from the participants (D1 ≈ 45 cm, the nearest point; D2 ≈ 80 cm; D3 ≈ 115 cm; D4 ≈ 150 cm; D5 ≈ 185 cm, the farthest point). The results revealed a significant reduction in PPS following the lockdown. Participants exhibited faster reaction times to tactile stimuli when avatars were closest, indicating a smaller PPS. This reduction in PPS size is interpreted as an implicit protective behavior that developed due to social distancing, leading to increased separation between personal and social spaces in response to the perceived threat of the pandemic.

Embodiment-related extension:

• Movement of environmental threats modifies the relevance of the defensive eye-blink in a spatially tuned manner [44]:

Somervail et al. [44] investigated the concept of defensive peri-personal space (DPPS) by examining how the environmental threats in IVR influence the magnitude of the hand–blink reflex, which, in turn, affects the boundaries of PPS. The study involved forty participants who were subjected to virtual arrows flying towards them from different directions while their eye–blink reflex was measured in real time.

In experiment 1, participants interacted with a single tower while their hand was placed in one of three positions: near, middle, and far. In experiment 2, the setup was expanded to include three towers (right, center, and left), and participants placed their hands in one of two positions: either with the left hand aligned with their eyes and the left tower or with the right hand aligned with their eyes and the right tower.

Additionally, there were two conditions in the experiment: arrow-congruent and arrow-incongruent. In the arrow-congruent condition, arrows were launched from the tower on the same side as the stimulated hand, while in the arrow-incongruent condition arrows were launched from the opposite side. The results showed that the presence of moving threats led to an expansion of the DPPS. Participants exhibited stronger and more spatially tuned eye–blink reflexes in response to the approaching threats. This expansion of DPPS was directionally adjusted, with a greater expansion occurring in the direction from which the threat was approaching. This study suggests that the brain continuously adapts DPPS to optimize defensive reactions based on environmental dynamics.

2.

The impact of embodiment and avatar sizing on personal space in immersive virtual environments [46]:

Buck et al. [46] investigated how embodiment and avatar sizing influence PPS within IVR. Forty-eight participants interacted within virtual environments, where their avatars had varying arm lengths (normal, shorter, and longer) and experienced levels of embodiment (low, medium, and high). The embodiment was manipulated by introducing latency of the self-avatar’s movements, slowing down the avatar, and altering the similarity of the avatar to the participant, such as using a humanoid or realistic appearance and matching or mismatching race and gender.

The effect of embodiment on PPS was measured by a tactile stimuli reaction given at different distance levels (0.75 m, 1 m, 1.25 m, 1.45 m, or 1.85 m) while a virtual avatar approached from different directions. These directions corresponded to doorways positioned at specific angles around the participant (0°, 45°, 90°, −45°, and −90°). In the second experiment, PPS was measured using different arm dimensions. Participants were asked to react to yellow cubes appearing at different heights to get used to their avatar’s arm length. Following this familiarization, the same task from the first experiment was applied, but participants repeated the block task after every five trials as a reminder of their arm’s length.

In both experiments, after participants acclimated to their avatar’s arm length, their tactile reaction times were measured. Finally, they completed an embodiment questionnaire. The results showed that lower levels of embodiment were associated with larger PPS. This suggests that when the perception of near space is less accurate, as in conditions of low embodiment, PPS extends. On the other hand, changes in avatar arm length did not have significant effects on PPS. These results highlight the role of embodiment in the perception and regulation of PPS in IVR.

3.

Disconnected hand avatar can be integrated into the peripersonal space [41]:

Mine and Yokosawa [41] explored whether PPS can extend to include objects that are disconnected from the body in an IVR environment. Twenty-seven participants used either a virtual disconnected hand avatar or a laser pointer to bisect lines of varying lengths (4, 8, 16, and 32 cm), presented within PPS at 30 cm and within EPS at 120 cm. The study aimed to determine if and how the perception of PPS could be influenced using tools or avatars that are not physically connected to the participant’s body.

These results revealed a significant shift in line bisection bias from left to right as the distance of line presentation increased when participants used the laser pointer. However, no similar change was observed when participants used the virtual hand avatar. The findings suggest that the virtual hand was integrated into the participants’ PPS, even though it was disconnected from their bodies in the IVR. To summarize, the study indicates that PPS representation can extend to include body-like objects presented in EPS, highlighting the flexibility of PPS boundaries in IVR settings.

4.

Immersive virtual reality reveals that visuo-proprioceptive discrepancy enlarges the hand-centered peripersonal space [45]:

Fossataro et al. [45] investigated the impact of visuo-proprioceptive conflicts in hand-centered PPS within an IVR environment. Twenty-two participants were asked to respond to the tactile (T) stimuli while ignoring the visual (V) stimuli. Participants’ hands were presented in the virtual environment, either in congruent (matching the real hand’s position) or incongruent (opposite side) conditions. The study aimed to understand how mismatches between visual and proprioceptive information in IVR influence the perception and processing of stimuli within hand-centered PPS.

The researchers measured the visual enhancement of touch (VET) effect by comparing response time in conditions where tactile (T) stimuli were presented alone or in combination with visual (VT) stimuli, with the visual (V) stimuli positioned either near or V far from the real hand. The results showed that participants in the incongruent condition, where the virtual hand was displayed on the opposite side, showed faster responses to tactile stimuli when visual stimuli were near the real hand. This outcome confirmed the classical VET effect, where visual stimuli enhance tactile perception when they are close to the body. The misalignment of sensory information, where the virtual and real hands were incongruent, led to an expansion of the hand-centered PPS. These findings suggest that visuo-proprioceptive discrepancies can effectively modulate and extend the boundaries of PPS.

5.

Remote hand: hand-centered peripersonal space transfers to a disconnected hand avatar [36]:

Mine and Yokosawa [36] investigated the expansion of PPS using a remote-controlled hand avatar in a virtual environment. Twenty-seven participants completed a task where they received tactile stimuli while visual stimuli were simultaneously presented at three different distances (120 cm, 80 cm, and 30 cm) from their bodies in VR. They were instructed to press a button whenever they received a tactile stimulus while ignoring the visual stimulus.

The reaction task was performed with the hand in two positions: near (30 cm) or far (90 cm). The results showed faster reaction times when the hand was in the position (120 cm). This suggests that participants’ sense of PPS extended to the area around the remote hand.

6.

Peripersonal space as the space of the bodily self [50]:

Noel et al. [50] investigated the full-body illusion (FBI) within an IVR environment, focusing on the expansion of PPS through synchronized visuo-tactile stimulation. Thirty-four participants experienced touch on their real bodies while simultaneously observing the same touch on a virtual body located 200 cm away. The touch was either synchronized or asynchronized to induce the illusion of ownership over a virtual body. To measure the boundaries of PPS, visuo-tactile stimulation was alternated with audio–tactile trials. Participants were positioned between two sets of speakers, with four speakers in front and four behind them.

Participants were asked to respond as fast as possible to vibro-tactile stimuli on their chest (Experiment 1) or back (Experiment 2) while ignoring looming sounds with six different delays (190 ms to 1.14 s). Their reaction times were measured. The results showed that PPS expanded towards the virtual body in synchronized conditions. Specifically, the initial boundary of PPS, which ranged between 60 and 75 cm in front of the participants, enlarged to 75 and 90 cm in front. This indicates that participants’ PPS shifted from the physical body to the observed virtual body. However, this effect was not observed in desynchronized trails. Therefore, this study highlights how synchronized sensory experiences can influence the perception of PPS in the virtual environment.

7.

Adaptation to delayed visual feedback of the body movement extends multisensory peripersonal space [48]:

Mine and Yokosawa [48] investigated the effects of delayed visual feedback on the multisensory representation of PPS within an IVR environment. Twenty participants enrolled in tasks where their hand movements were either delayed by approximately 440 ms or synchronized. They completed three tasks, unisensory, adaptation, and multisensory, under each condition. The unisensory task, used as a baseline, required participants to react to tactile stimuli while the white ball, positioned 300 cm away, simultaneously disappeared. In contrast, the adaptation task requires participants to reach the moving cubes. In the multisensory tasks, as the visual stimuli (a white ball) approached the participants, tactile stimuli were delivered through a controller in their right hand at five different distances (185 cm, 155 cm, 125 cm, 95 cm, and 65 cm), and their reaction times were measured. The results revealed that delayed visual feedback significantly extended the multisensory PPS compared to the no-delay condition. The extension of PPS was particularly significant when the approaching visual stimuli were closer to the participant (65 cm and 95 cm) compared to further distances. These results suggest that delays in visual feedback prompt earlier preparation for defensive actions, thereby expanding the boundaries of PPS.

8.

Expansion of space for visuotactile interaction during visually induced self-motion [40]:

Kuroda and Teramoto [40] investigated how visually induced self-motion influences PPS through a visuotactile interaction task in IVR. Twenty participants were exposed to large-field (LF) and small-field (SF) visual motion simulations, which created a forward self-motion effect by displaying white dots on the walls moving at different speeds (1.5 m/s for slow and 6.0 m/s for fast). They were required to detect tactile stimuli on their chest while observing a visual probe approaching from various distances (1.2 m, 2.4 m, 3.6 m, 4.8 m, and 6.0 m). The researchers calculated the visual facilitation effect by comparing the reaction times for tactile discrimination between the LF and the SF. The results showed that, regardless of the self-motion speed, participants exhibited faster reaction times and perceived greater distance in the LF condition compared to the SF condition. This suggests that visual self-motion in IVR can significantly expand PPS in the direction of motion.

4. Discussion

This review systematically summarized studies investigating the extension of PPS boundaries in IVR. While the literature has extensively examined PPS boundaries in real-world settings, IVR offers unique advantages that can address the limitations of traditional experimental setups. Specifically, IVR provides controlled environments by manipulating and maintaining consistent variables such as distance, timing, and sensory inputs. It can integrate multisensory inputs, such as visual, tactile, and auditory, either synchronously or asynchronously, allowing for a more precise investigation of how these inputs affect PPS [53].

Moreover, IVR offers a more flexible and controlled experimental setting, as it allows for adjustments in conditions such as distance, timing, and stimulus size while isolating the experiment from potential confounding variables. This approach is more accessible and cost-effective compared to traditional real-world experiments [54]. In addition, IVR enables the exploration of diverse interactions, such as longer, extra, and disconnected limbs, as well as the examination of threatening conditions without the risk of physical harm [55,56]. The consistent environment provided by IVR also enhances the reproducibility of experimental tasks.

We reviewed six studies under the “tool use related extension” category, which investigated how virtual tool usage influences the boundaries of PPS. The results indicate that using a virtual tool to extend the PPS in IVR is as effective as a real-world experimental setup. Additionally, these studies provided valuable insights for comparing real-world experimental settings and diversifying them with IVR. For example, Gamberini et al. [47] compared the line bisection task in both real and virtual environments, finding that using a laser pointer in IVR can extend the PPS to the EPS.

Similarly, Sengul et al. [51] demonstrated that comparable to real-world experiments, the representation of PPS can be extended using virtual tools, such as a robotic surgical interface, provided that the virtual tool is effectively embodied. Their study also provides evidence that virtual tools can integrate multisensory information similarly to physical tools, influencing the boundaries of PPS, as exemplified by the tactile feedback from the robotic surgery interface.

On the other hand, Ferroni et al. [37] found that while real tool use extends the PPS, virtual tools did not show significant results. They suggested that factors such as familiarity with virtual tool use, the quality of immersive virtual reality, and the level of embodiment might explain these findings. In contrast, Arai et al. [33] applied pre-training to enhance familiarity and examined the level of embodiment, discovering that extra robotic limp in IVR can extend PPS. Another important factor to consider is the type of tool and its function. Petrizzo et al. [49] highlighted that while pulling and pushing actions with virtual tools extend the PPS, actions like hitting with a hammer or using a toy gun do not. $\underset{\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots }{\mathrm{Inizio}\mathrm{modulo}}$

Moreover, Gamberini et al. [35] support this finding, showing that PPS can extend to EPS with virtual tools, but only when the virtual tool involves an action function. On the other hand, virtual tools are not the only means of extending the PPS. In the category of “social interaction-related extension”, we reviewed six studies examining how different social interactions influence PPS boundaries in IVR. The results indicated that interactions with virtual avatars significantly affect PPS extension. Ruggiero et al. [42] demonstrated that virtual avatars displaying threatening facial expressions (e.g., anger) can extend both IPS and PPS more than avatars with neutral or happy expressions. This effect is particularly pronounced when participants lack control over approaching conditions.

Similarly, Iachini et al. [38] showed that in passive approaching conditions, PPS extends with immorally described virtual avatars. The same study also proved that gender plays a role in these interactions; female participants maintained a greater distance from male virtual avatars, thereby extending the PPS. Moreover, Iachini et al. [39] found that, while gender continues to affect PPS, the age of virtual avatars also influences it. Participants extended their PPS more when interacting with adult avatars compared to child avatars. The study suggested that the regulation of PPS is influenced by the feeling of safety and control over others’ behavior, serving as a predictive defensive mechanism.

Another study by Ruggiero et al. [43] supports these findings with similar results. This research investigated the defensive function of PPS by employing social interaction with both male and female virtual confederates displaying positive, negative, and neutral facial expressions. Moreover, Bertoni et al. [34] showed that a sense of power over others, whether in social or non-social contexts, influences the PPS and blurs the boundaries between individuals and virtual confederates.

On the other hand, from a different perspective, Serino et al. [52] found that restrictions imposed during COVID-19 in Switzerland led to an extension of PPS post-pandemic. Participants showed faster reaction time to virtual confederates in contamination distance. This suggests that all studies included in this category highlight the defensive function of PPS in a social context. Similarly, the next category also supports this claim. We reviewed eight studies under the “embodiment-related extension” category.

The studies investigated how different embodiment-related factors influence the boundaries of PPS in IVR. The results showed that different aspects of embodied cognition significantly affect PPS extension. For example, Buck et al. [46] manipulated the level of embodiment by altering physical similarity, introducing delayed visual feedback, and slowing down the avatar in IVR to measure the PPS boundaries. They found that the level of embodiment has an inverse relationship with the size of PPS, regulating the predictability of the environment as a defensive function.

Similarly, Noel et al. [50] used either synchronized or asynchronous tactile feedback to cause an illusion of embodiment with the virtual body with a 200 cm distance in IVR. Reaction time measurements to audio–tactile stimuli revealed that participants’ PPS extended toward the virtual body. Moreover, Mine and Yokosawa [48] showed that delayed visual feedback of approaching stimuli in IVR can significantly expand PPS as a defensive function. Furthermore, Fossataro et al. [45] found that misalignment of sensory input led to the expansion of hand-centered PPS. They applied visuo-proprioceptive conflict to a visuo-tactile stimuli reaction time task to measure the magnitude of the VET effect as an indicator of PPS boundaries.

Another study investigating hand-centered PPS from a different perspective was conducted by Mine and Yokosawa [36]. They measured visuotactile reaction time with a remote hand avatar in IVR from different distances and found that PPS space can extend to a certain distance even without a direct body connection. In addition, Mine and Yokosawa [41] proved that disconnected hand avatars can be integrated into PPS from a distance using a line bisection task. Somervail et al. [44] approached hand-centered DPPS from a different perspective by measuring the magnitude of the hand blink reflex as an indicator of DPPS boundaries. Their results showed that DPPS expands in response to a threatening moving visual stimulus and that extension is direction-oriented based on the source of the treats. Lastly, Kuroda and Teramoto [40] revealed that visually induced self-motion illusion in IVR, regardless of the speed of the motion illusion, can extend PPS. Overall, studies in this category highlight the impact of multisensory conflict on PPS and its defensive function.

Overall, this review addressed how IVR offers a transformative platform for investigating PPS and its extension through diverse methods. Unlike traditional real-world settings, IVR provides controlled and flexible experimental conditions, enabling precise manipulation of variables such as distance, timing, sensory inputs, and virtual tools. The reviewed studies confirm the potential of IVR to simulate realistic multisensory environments, offering unique advantages for exploring PPS extension related to tool use, social interactions, and embodiment.

The findings reveal that virtual tools effectively extend PPS boundaries, provided they are sufficiently embodied and integrated with multisensory feedback. Similarly, social interactions in IVR, particularly those involving virtual avatars displaying emotional expressions or engaging in passive approaches, highlight the defensive function of PPS as a mechanism to regulate safety and control. Furthermore, embodiment-related studies underscore the role of multisensory conflicts and delayed feedback in dynamically modulating PPS, emphasizing how virtual bodies or avatars influence spatial awareness.

Together, these insights illustrate the potential of IVR not only to enhance our understanding of PPS but also to offer practical applications in fields such as rehabilitation, ergonomics, and social interaction research. By leveraging the flexibility and precision of IVR, future studies can further explore the cognitive and defensive functions of PPS, paving the way for innovative applications and experimental paradigms.

5. Conclusions

5.1. Practical Implications and Future Research

Understanding PPS and extending it can enhance user experience and engagement in IVR. For example, designing a virtual environment that accounts for the dynamic nature of PPS may lead to more effective educational simulations and therapeutic interventions for individuals with social anxiety or autism spectrum disorder, particularly in improving their social skills [57,58]. Future research should continue exploring PPS extension across different populations and aim to generalize these findings across different cultural and social contexts. Longitudinal studies can offer deeper insights into how spatial behaviors evolve over time with repeated interactions in virtual settings. By integrating these insights, researchers can better understand the mechanisms underlying PPS expansion in social contexts. This knowledge can be used to enhance theoretical frameworks and practical applications.

5.2. Limitations

Despite promising findings, several limitations in the literature on PPS expansion in IVR settings should be acknowledged. First, the degree of immersion and quality of the virtual environment can significantly impact outcomes [59]. Variations in hardware and software capabilities, such as resolution, latency, and tracking accuracy, affect how realistically the virtual environment simulates real-world conditions and influences PPS measurements [60]. Additionally, low frame rates due to hardware and software limitations can cause motion sickness [61].

Second, the demographic diversity in these studies is often limited. Many studies focus on young adults, which may not accurately represent the broader population. Future research should include diverse age groups, cultural backgrounds, and individuals with specific conditions (e.g., anxiety or sensory processing disorders) to assess the generalizability of the findings.

Third, the use of behavioral indicators in studies can introduce subjective bias. Incorporating objective physiological measures, such as neuroimaging and physiological monitoring, may provide more robust data on how PPS is modulated in IVR settings.

Finally, although VR offers safer simulations of threatening conditions, e.g., [58], the psychological effects on participants must be monitored to avoid potential long-term negative impacts. Ethical concerns about inducing states such as anxiety or discomfort in VR experiments need to be addressed. Addressing these limitations in future research will enhance the validity and reliability of findings regarding PPS expansion in IVR, contributing to a more accurate and comprehensive understanding and application.

To summarize, this article systematically reviewed the literature on extending the boundaries of PPS in IVR. The controlled and flexible experimental environment provided by IVR offers unique insights into PPS expansion. The review discussed how factors such as tool use, social interactions, and embodiment influence PPS boundaries in detail. Notably, while all studies indicated a significant extension, only two out of twenty studies found partly statistically significant PPS extension [34,49]. The findings highlight the defensive function of PPS and its expansion during threat perception and social interactions, indicating that PPS is a dynamic and extensible structure that can be effectively investigated in IVR settings.

Increasing the number of studies on proprioception could offer a deeper understanding of PPS extension. Additionally, future research should explore how these findings can be applied in practical areas such as education, therapeutic applications, or the improvement of IVR experimental setups.