In virtual reality, mapping one’s body movements to an avatar creates a sense of embodiment, where the avatar is perceived as part of one’s own body. Previous studies have indicated that introducing a spatial offset between the avatar’s movements and the user’s real movements can lead to users unconsciously adjusting their movements to align with the avatar. However, while the effects of such spatial distortion have been studied in the movement of the upper body, little is known about its influence on gait. This study aimed to assess whether altering the step length of an embodied avatar induces a more asymmetrical walking pattern. Thirty healthy participants underwent a treadmill walking experiment while wearing a head-mounted display that displayed an embodied avatar mirroring their movements from a first-person perspective. An algorithm was developed to dynamically increase one of the avatar’s step lengths in real-time, introducing gait asymmetry. The effect of step length manipulation on gait asymmetry was assessed under two conditions, where the avatar’s step length was gradually increased unilaterally for either the dominant or non-dominant leg. Results indicated that participants did not significantly adjust their gait symmetry to match the avatar’s increased step length, whether it was applied to the dominant or non-dominant leg. These findings suggest that healthy participants may be more resistant to visual feedback-induced changes provided by an embodied avatar, in contrast to the adjustments observed in upper-body movements.

Avatars can serve as virtual representations of a user’s body, replicating their movements in real time within an immersive environment. When these movements are closely synchronized with those of the user, a sense of embodiment over the avatar is often experienced. Recent research shows that introducing spatial offsets between a user’s real movements and those of their avatar can alter motor behavior, with users aligning with the distorted avatar. While previous studies have typically focused on upper-limb movements toward visual targets, it remains unclear how users respond to distortions without such targets, particularly during lower-limb tasks where postural demands may impose constraints. This study examined whether users adjust their movements to follow their distorted avatar during upper- and lower-limb reaching without visual targets. Twenty-four participants performed reaching tasks while their avatar’s movements were spatially offset. Results revealed that participants counteracted the avatar’s distortion by moving in the opposite direction in both limb tasks. These findings suggest that, in the absence of explicit visual targets, participants treat spatial distortions as execution errors and prioritize internal movement goals over minimizing sensory conflict. This suggests that adaptation depends not only on a strong embodiment but also on whether the task performance permits reducing the mismatch between the user’s and the avatar’s movements.

This perspective article explores the current challenges and future potential of artificial intelligence (AI)-assisted immersive virtual reality (VR) for clinical applications in movement-based physical rehabilitation. It highlights the limitations of current immersive VR and introduces AI as a potential solution to improve the overall VR experience, particularly in the context of physical rehabilitation. AI’s role in content creation, personalizing clinical interventions, achieving the right balance between challenge and flow (where tasks are difficult enough to stay engaging but not so hard that they cause frustration), and optimizing sensory experiences in VR environments is discussed. In addition, the article highlights how AI-driven brain-computer interfaces, motion synthesis techniques, and personalized therapeutic immersive experiences can make VR more engaging and effective in enhancing physical rehabilitation outcomes. Finally, we examine the challenges associated with integrating AI and VR systems in physical rehabilitation, including financial, ethical, and user-related barriers that must be addressed to ensure effective and responsible implementation.

Time perception in virtual reality (VR) is influenced by various factors, including avatar embodiment. This study investigates how avatar type (human vs. Godzilla) and bodily proportions (human-like vs. Godzilla-like) affect time perception during passive (waiting) and active (walking) tasks. Nineteen participants embodied four avatars, with different combinations of these features, in two virtual environments—a biomechanics lab and an urban cityscape—while their embodiment levels and perception of time were assessed. Results reveal that avatar proportions influenced the estimation of time during the waiting phase only, with human-like proportions leading to greater underestimations. These findings highlight the distinct roles of avatar characteristics in shaping user experiences in VR, particularly their influence on temporal judgment and embodiment.

Embodiment refers to the sensation of owning, controlling, and perceiving a virtual or artificial body as one’s own. This study investigates how embodying an avatar with a leg twice its normal length affects proprioception, with congruent or incongruent visuotactile stimuli. Preliminary results (n = 10) show that participants experienced embodiment with a lengthened virtual leg, regardless of congruence of stimuli. A proprioceptive drift of 31.2 cm toward the virtual foot was observed. These findings extend research on upper-body proprioception to include virtual lower-limb deformations.

In immersive VR, a self-avatar that replicates the user’s movements and is viewed from a first-person perspective can substitute the real body. If the avatar’s movements are sufficiently synchronized with the user’s actual movements, the user can experience a sense of embodiment over the avatar. Recent studies have shown that discrepancies between the movements of the avatar and those of the user can be well tolerated while maintaining high levels of embodiment. The point at which a distortion is perceived (detection threshold) and its impact on the level of embodiment has not been studied in lower limb tasks such as gait. This study aimed to identify a detection threshold of gait asymmetry by unilaterally manipulating the step length of a self-avatar, and the effect of this detection on perceived embodiment. A real-time step length distortion model was developed, and a detection threshold between actual and avatar’s gait movement was assessed on thirty healthy participants. The step length was manipulated to introduce gait asymmetry (ascending condition) or start from a large asymmetry that was gradually decreased (descending). The results showed that, on average, the avatar’s step length could be increased by up to 12% before the participants detected the distortion. Furthermore, in the descending condition, they detected increases that were above 9%. The point of detection had no effect on the sense of embodiment as participants still reported being embodied in their avatars, even when they consciously detected the step length distortion. The sense of embodiment was closely correlated with the level of distortion; as distortion increased, embodiment decreased, and vice versa. For a given distortion level, embodiment was similar whether in the ascending or descending condition. This suggests that embodiment can be achieved even when the avatar’s spatial alignment initially differs from the participants’, provided that alignment is gradually restored. These results provide valuable insights into participants’ ability to tolerate movement discrepancies in embodied avatar experiences during gait in virtual environments, with potential applications in motor training and gait rehabilitation.

Several studies have shown that users of immersive virtual reality can feel high levels of embodiment in self-avatars that have different morphological proportions than those of their actual bodies. Deformed and unrealistic morphological modifications are accepted by embodied users, underlying the adaptability of one's mental map of their body (body schema) in response to incoming sensory feedback. Before initiating a motor action, the brain uses the body schema to plan and sequence the necessary movements. Therefore, embodiment in a self-avatar with a different morphology, such as one with deformed proportions, could lead to changes in motor planning and execution. In this study, we aimed to measure the effects on movement planning and execution of embodying a self-avatar with an enlarged lower leg on one side. Thirty participants embodied an avatar without any deformations, and with an enlarged dominant or non-dominant leg, in randomized order. Two different levels of embodiment were induced, using synchronous or asynchronous visuotactile stimuli. In each condition, participants performed a gait initiation task. Their center of mass and center of pressure were measured, and the margin of stability (MoS) was computed from these values. Their perceived level of embodiment was also measured, using a validated questionnaire. Results show no significant changes on the biomechenical variables related to dynamic stability. Embodiment scores decreased with asynchronous stimuli, without impacting the measures related to stability. The body schema may not have been impacted by the larger virtual leg. However, deforming the self-avatar's morphology could have important implications when addressing individuals with impaired physical mobility by subtly influencing action execution during a rehabilitation protocol.

The impact of domain-general cognitive ‘brain’ training on improving sports performance is highly debated. This study sought to follow-up on research that showcased the benefits of perceptual-cognitive 3D-Multiple Object Tracking (3D-MOT) training in enhancing the on-field performance of soccer players. Additionally, it explored the correlation between athletes’ cognitive performance and early career success.Sixty-two males from a professional soccer academy were randomly divided into a dual-task 3D-MOT training group (n=30) and a control group (n=32). Participants underwent a 3D-MOT test, a cognitive test of attention, and small-sided games at pre- and post-training. Pre-post-test performances were compared using ANCOVAs. A Chi-squared test evaluated the association between the training regimen and early career success. A Spearman test assessed the correlation between performance on the 3D-MOT, attention test, and early career success.The dual-task 3D-MOT trained group significantly improved its performance on 3D-MOT compared to the control group (p < 0.001). However, no significant pre-post-test differences were observed between the groups in the near-transfer cognitive test and on-field performance (ps > 0.05). There were no associations between the athletes’ early career success and the training regimen, and no associations between cognitive test performances and early career success (ps > 0.05).This follow-up study failed to replicate previous findings with dual-task 3D-MOT training unable to produce near or far transfer on soccer performance. In addition, cognitive performance was not related to early career success in this study. The value of cognitive screening and training in sport is discussed.

The increasing use of immersive technologies for sexual purposes raises questions about their capacity to enhance a unique aspect of presence-Sexual Presence (SP). Investigating this phenomenon hinges on our ability to measure it accurately. This paper improves our understanding of SP by identifying potential quantitative electroencephalography variables associated with SP. Twelve heterosexual cisgender males were exposed to virtual scenarios featuring sexual content performed by a Virtual Character (VC). After viewing, participants completed a Sexual Presence questionnaire. Correlations were observed between self-reported SP and the alpha band activity in the frontal and parietal regions.

Introduction:Recent evidence has started to demonstrate that 360°VR, a type of VR that immerses a user within a 360° video, has advantages over two-dimensional (2D) video displays in the context of perceptual-cognitive evaluation and training. However, there is currently a lack of empirical evidence to explain how perceptual-cognitive strategies differ between these two paradigms when performing sports-related tasks. Thus, the objective of this study was to examine and compare the impact of different viewing conditions (e.g., 3D-360°VR and 2D video displays), on gaze behavior and head excursions in a boxing-specific anticipatory task. A secondary objective was to assess the workload associated with each viewing mode, including the level of presence experienced. Thirdly, an exploratory analysis was conducted to evaluate any potential sex differences. Methods:Thirty-two novice participants (16 females) were recruited for this study. A total of 24 single-punch sequences were randomly presented using a standalone VR headset (Pico Neo 3 Pro Eye), with two different viewing modes: 3D-360°VR and 2D. Participants were instructed to respond to the punches with appropriate motor actions, aiming to avoid punches. Gaze behavior was recorded using a Tobii eyetracker embedded in the VR headset. Workload and presence were measured with the SIM-TLX questionnaire. Fixation duration, number of fixations, saccades, search rate and head excursions (roll, pitch, yaw) were analyzed using linear mixed models. Results:The results revealed significant shorter fixation durations and more head excursions (roll, pitch) in 3D-360°VR, compared to the 2D viewing mode (ps < 0.05). The sense of presence was found to be much higher in the 3D-360°VR viewing mode (p < 0.05). No sex differences were observed. These results demonstrate that 360°VR elicited shorter fixation durations but mostly greater head excursions and immersion compared to a 2D projection in the context of a boxing-specific task. Discussion:These findings contribute to the understanding of previous evidence supporting the possible advantages of using 360°VR over 2D for perceptual-cognitive evaluation and training purposes. Further validation studies that compare behaviors and performance in 360°VR with those in the real-world will be needed.

Existing clinical scales to evaluate the severity of Friedreich's ataxia (FA) impairment rapidly reach a plateau when patients lose the ability to walk, and they rely on the examiner to quantify ataxia. Therefore, more objective tools are necessary to complement the neurological examination and distinguish subtle changes in ataxia over time. Using virtual reality (VR), we aim to develop precise tools to measure the progression of FA. The purpose of this study was to evaluate a series of 5 different upper limb tasks in VR with FA patients in order to determine how well they were tolerated, if they allow to distinguish FA patients from a control group, and if they correlate to clinical measures. Twelve FA patients and 9 healthy partici-pants underwent traditional assessment of upper limb function and of FA severity (patients only), performed the series of five VR task, and completed a subjective evaluation. Only 6 out of 12 FA patients were able to successfully complete all tasks, primarily due to the challenges associated with manipulating the controllers for patients with advanced stages of FA. Different performance metrics of the tasks were significantly different between groups and had strong correlations to a clinical assessment scale. All participants reported very low simulator sickness and a high level of virtual presence. The results of this study demonstrate the feasibility of using VR with FA patients, although adaptation of the technology may be necessary for those with more severe impairments.

In virtual reality, it is possible to simulate one's visual self-representation by mapping one's body movements to those of an avatar. Accepting the virtual body as part of one's own body creates an ownership illusion. This study aimed to assess the perception threshold between a subject's actual gait movements and those of their modulated self-avatar during treadmill walking. Preliminary results on two subjects suggest that healthy subjects can detect the mismatch, but differences may exist between subjects.

Brain-computer interfaces (BCI) have been used to control the gait of a virtual self-avatar, using motor imagery of the feet, in order to restore motor control in gait rehabilitation. The considerable training time required to use such a BCI is an obstacle to their adoption in a clinical setting. One technique used to enhance BCI control and to shorten training time is to eliminate offline calibration using a generic classifier that is pre-trained over many participants, each performing many trials. This paper investigates the performance of generic models that were derived from 2 datasets, each containing the data of 20 participants. They participated in a sequential training to control the gait of an avatar when cued to imagine a single step forward using their left or right foot, or to start walking forward. The avatar moved in response to two calibrated RLDA classifiers that used the $\mu$ PSD over the foot area of the motor cortex as features. The generic models were tested on the offline and online data of the participants. The models performed as well as models obtained from participant-specific offline data with a mean performance of 86%. The results show the possibility of designing a participant-independent, zero-training lower-limb MI-BCI.

When immersed in virtual reality, users who view their body as a co-located virtual avatar that reflects their movements, generally develop a sense of embodiment whereby they perceive the virtual body to be their own. One aspect of the sense of embodiment is the feeling of agency over the avatar, i.e., the feeling that one is producing the movements of the avatar. In contexts such as physical rehabilitation, telepresence and gaming, it may be useful to induce a strong sense of agency in users who cannot produce movements or for whom it is not practical to do so. Being able to feel agency over a walking avatar without having to produce walking movements could be especially valuable. Muscle vibrations have been shown to produce the proprioceptive perception of movements, without any movement on the part of the user. The objectives of the current study were to: 1-determine if the addition of lower-limb muscle-vibrations with gait-like patterns to a walking avatar can increase the illusory perception of walking in healthy individuals who are standing still; 2-compare the effects of the complexity of the vibration patterns and of their synchronicity on the sense of agency and on the illusory perception of walking. Thirty participants viewed a walking avatar from a first-person perspective, either without muscle vibrations or with one of four different patterns of vibrations. These five conditions were presented pairwise in a two-alternative forced choice paradigm and individually presented, after which participants answered an embodiment questionnaire. The displacement of center of pressure of the participants was measured throughout the experiment. The results show that all patterns of proprioceptive stimulation increased the sense of agency to a similar degree. However, the condition in which the proprioceptive feedback was realistic and temporally aligned with the avatar’s leg movements led to significantly larger anteroposterior sway of the center of pressure. The frequency of this sway matched the cadence of the avatar’s gait. Thus, congruent and realistic proprioceptive stimulation increases the feeling of agency, the illusory perception of walking and the motor responses of the participants when viewing a walking avatar from a first-person perspective.

Evaluating sexual preferences is a difficult task. Past researchrelied mostly on penile plethysmography (PPG). Even though this technique is the standard protocol used in most currentforensic settings, its usage showed mixed results. One way to improve PPG is the addition of other psychophysiological measures such as electroencephalography (EEG). However, EEG generates significant amount of data that hinders classification. Machine learning (ML) is nowadays an excellent tool to identify most discriminating variables and for classification. Therefore, it is proposed to use ML selection and extraction methods for dimensionality reduction and then to classify sexual preferences. Evidence from this proof of concept shows that using EEG and PPG together leads to better classification (85.6%) than using EEG (82.2%) or PPG individually (74.4%). The Random Forest (RF) classifier combined with the Principal Component Analysis (PCA) extraction method achieves a slightly higher general performance rate. This increase in performances opens the door for using more reliable biometric measures in the assessment of deviant sexual preferences.

Brain–computer interfaces (BCIs) have been used to control the gait of a virtual self-avatar with a proposed application in the field of gait rehabilitation. Some limitations of existing systems are: (a) some systems use mental imagery (MI) of movements other than gait; (b) most systems allow the user to take single steps or to walk but do not allow both; (c) most function in a single BCI mode (cue-paced or self-paced). Objective . The objective of this study was to develop a high performance multi-modal BCI to control single steps and forward walking of an immersive virtual reality avatar. Approach . This system used MI of these actions, in cue-paced and self-paced modes. Twenty healthy participants participated in this study, which was comprised of four sessions across four different days. They were cued to imagine a single step forward with their right or left foot, or to imagine walking forward. They were instructed to reach a target by using the MI of multiple steps (self-paced switch-control mode) or by maintaining MI of forward walking (continuous-control mode). The movement of the avatar was controlled by two calibrated regularized linear discriminate analysis classifiers that used the µ power spectral density over the foot area of the motor cortex as a feature. The classifiers were retrained after every session. For a subset of the trials, positive modified feedback (MDF) was presented to half of the participants, where the avatar moved correctly regardless of the classification of the participants’ MI. The performance of the BCI was computed on each day, using different control modes. Main results . All participants were able to operate the BCI. Their average offline performance, after retraining the classifiers was 86.0 ± 6.1%, showing that the recalibration of the classifiers enhanced the offline performance of the BCI ( p < 0.01). The average online performance was 85.9 ± 8.4% showing that MDF enhanced BCI performance ( p = 0.001). The average performance was 83% at self-paced switch control and 92% at continuous control mode. Significance . This study reports on a first BCI to use motor imagery of the lower limbs in order to control the gait of an avatar with different control modes and different control commands (single steps or forward walking). BCI performance is increased in a novel way by combining three different performance enhancement techniques, resulting in a single high performance and multi-modal BCI system. This study also showed that the improvements due to the effects of MDF lasted for more than one session.

Virtual reality (VR) enables the user to perceive body ownership towards a virtual body. This illusion is induced through first-person perspective (1PP) and synchronous movement with the real body. Previous studies have shown that pronounced differences between the real and the virtual body lead to changes in the user's behavior. It has also been shown that modifying the body image can affect the user's movements. Nevertheless, the state of the art does not refer to the kinetic and kinematic impacts of one virtual lower limb deformation. Therefore, this paper presents a methodology exploring the impact of a self-avatar with an asymmetrical lower body (one limb longer or larger than the other) on the dynamic characteristics of the user during a gait initiation task.

Synchronicity between user’s movements and that of their self-avatar leads to a subjective illusion of embodiment over the virtual avatar. Tendon vibrations can be used to produce a perception of movement, in the absence of any voluntary movements. Such stimulations have been combined with virtual reality to induce the ownership illusion. In this study we investigate the ability of using a virtual self-avatar combined with tendon vibrations to give standing users the impression of walking physically. The vibrations, applied on 24 participants, had different levels of complexity and congruency with the avatar’s movements. Overall, results suggest that the pattern of vibrations is not crucial to produce ownership illusion.