Prismatic adaptation (PA) is a non-invasive technique used in the rehabilitation of patients with unilateral spatial neglect (USN), a prevalent and complex sensorimotor disorder that occurs post-stroke. USN is characterized by deficits in attention and awareness toward the side of space opposite to the brain damage (Vallar et al., 2003). Given the clinical significance of USN, it is essential to explore the efficacy of innovative therapeutic interventions such as prism adaptation (PA). Introduced by Rossetti et al. (1998), PA has emerged as a promising bottom-up approach to alleviate USN symptoms. This intervention entails wearing prism goggles that laterally shift the visual field, coupled with the use of a device such as a horizontal board with target dots (Schintu et al., 2014) or a box (Magnani et al., 2011), for pointing movements. Patients are instructed to point toward targets using their right index finger. PA temporarily alters sensorimotor mapping by shifting vision laterally (Rossetti et al., 1998) while participants are engaged in a pointing task toward visual targets. Initially, movements typically exhibit errors in the direction of the prismatic shift, but these errors diminish after repeated attempts, indicating adaptation. Following prism removal, aftereffects (AEs) often manifest as errors in the opposite direction (Redding and Wallace, 2002). Factors influencing AE include the degree of lateral shift (Gammeri et al., 2020) and awareness of the visual shift. Interestingly, the alteration of motor movements experienced during PA persists after the removal of the prisms, with sensorimotor and visuospatial effects lasting at least 35 min (Schintu et al., 2014).

Two main neural mechanisms have been proposed to account for PA effects: recalibration and realignment (Redding and Wallace, 2006). Recalibration is a compensatory mechanism that modifies motor commands while individuals carry actions such as reaching objects; it involves a rapid adjustment of the motor plan to minimize terminal errors. Realignment, on the other hand, is a slower, spontaneous process that can reconfigure the sensory maps perturbed by prism shift, resulting in a correction of motor strategy (Prablanc et al., 2020).

Several studies have reported the beneficial effects of PA on various populations including healthy individuals (Bonaventura et al., 2020; Turriziani et al., 2021; Magnani et al., 2022), psychiatric populations (Magnani et al., 2022), and neurological populations, particularly those suffering from USN (Shiraishi et al., 2008; Serino et al., 2009) characterized by an improvement in daily activities such as reading, writing, and wheelchair navigation (Dijkerman et al., 2003; Angeli et al., 2004a,b; Rode et al., 2006; Jacquin-Courtois et al., 2008; Watanabe and Amimoto, 2010). Furthermore, PA based-rehabilitation has been shown to reduce deficits in various domains, including neglect (Rossetti et al., 1998; Làdavas et al., 2001; Dijkerman et al., 2003; Maravita et al., 2003; Jacquin-Courtois et al., 2010), visuospatial (Farnè, 2002) and postural deficits (Tilikete et al., 2001; Hugues et al., 2015).

Interestingly, the sensorimotor AEs of PA extend to cognitive domains, including mental imagery tasks that do not involve manual responses or overt visual scanning (Rode et al., 1999; Rossetti et al., 2004; Aiello et al., 2013). These AEs are influenced by the direction of visual displacement. For instance, while right-deviating PA improves neglect symptoms, and left-deviating PA is ineffective in neglect patients; however, healthy individuals experience significant cognitive changes after left-deviating, but not right-deviating PA.

Due to the enormous potential of PA therapy, recent studies have attempted to optimize PA by introducing a novel virtual reality-based PA protocol. Specifically, when implemented in VR, PA follows the same procedure as the traditional approach, but the pointing task may be executed with or without touch controllers while wearing virtual reality headsets.

With the increasing accessibility and widespread use of virtual reality (VR) systems, new opportunities for rehabilitation and psychotherapy support have emerged (Glize et al., 2017; Maggio et al., 2019; Leonardi et al., 2021; Lucifora et al., 2022; Maresca et al., 2022; Vicario and Martino, 2022; De Luca et al., 2023; Formica et al., 2023; Vicario et al., 2023a,b). VR is defined as interactive simulations created by computer hardware and software that immerse users in environments closely emulating real-world objects and events (Weiss et al., 2006) gaining popularity in rehabilitation settings for treating motor and cognitive disorders (Tieri et al., 2018). Three types of VR device can be identified based on immersion levels (Bamodu and Ye, 2013): non-immersive (NIVR), semi-immersive (SIVR), and immersive (IVR). NIVR, which utilizes 2D interfaces (such as mouse and keyboard or gamepad/joysticks) along with computer or console gaming systems, partially immerses users in the virtual environment (VE) while allowing them to remain aware of the outside world (Shahrbanian et al., 2012). SIVR involves the use of a large screen for VE projection and advanced interface devices such as cyber gloves, haptic feedback devices, or infrared cameras, enabling interaction with the VE while perceiving the real world (Bamodu and Ye, 2013). IVR, featuring a head-mounted display and 3D input devices, fully immerses users in the VE (Shahrbanian et al., 2012). VR offers a unique opportunity to modulate multiple parameters, enhancing the power of virtual prisms and isolating patients from external stimuli.

This new rehabilitation approach has the potential to address some limitations of PA. It can induce different degrees of deviation using the same setup and alleviate the traditional difficulty associated with moving the panel, especially for hospitalized patients. Furthermore, virtual PA (VPA) allows for a more precise digital quantification of both errors during PA and the occurrence of AEs.

Due to the high flexibility and personalization offered by the VPA, understanding its effectiveness will have important research and clinical implications, as well as improving rehabilitation outcomes. Therefore, here we conducted a systematic review of the current literature focusing on the use of VPA in healthy individuals and patients. The results of this review may pave the way to novel rehabilitation approaches for patients with different neurological conditions, including neglect.

A systematic review was conducted to investigate the efficacy of VPA. Although this review was not registered in a specific database, it was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Moher, 2009; Page et al., 2021).

The results of this systematic review suggest the potentiality of VPA in inducing visuomotor adaptation in healthy subjects, comparable to real PA. While research in this field is still in its early stages and only preliminary conclusions can be drawn, there is a growing interest in exploring the potential implementation of VR based PA therapy (Cho et al., 2020; Gammeri et al., 2020). It is noteworthy that, although the primary aim of the current study was not to directly compare traditional PA effects with VPA, many of the included studies examining VPA compared its effects to those of traditional PA. For instance, Ramos et al. (2019) compared prism AE in the real world after the removal of prism goggles, and the VR AE within the VE while wearing VR goggles, reporting larger prismatic AEs for VPA. Similarly, a recent study examining AE in VPA (Faity et al., 2023) found comparable AE similar to those observed with traditional PA (Faity et al., 2023). Interestingly, like traditional PA, these effects have been shown to vary across different cognitive tasks (Gammeri et al., 2020). However, this study highlights that the relationship between prism deviation and transfer effects is complex (Gammeri et al., 2020). Transfer effects were only evident with larger prismatic deviations (around 30 degrees) while they were absent with smaller deviations (20 degrees or less). This observation raises questions about the optimal degree of prismatic deviation for effective VPA and suggests that a one-size-fits-all approach may not be suitable.

An important limitation we found in the current literature is the lack of research involving patients. Specifically, we identified only two studies investigating the effect of VPA in patients with neglect (Bourgeois et al., 2022; Chen et al., 2022) which reported robust VPA effects on open-loop pointing. Unfortunately, in the study by Bourgeois et al. (2022), this effect did not translate into a significant improvement in clinical assessments, including cognitive tasks. The authors suggested that several factors might explain the lack of effect, including the small sample size and type of pointing task utilized in the study. Similar inconsistent results have also been reported for patients with neglect undergoing traditional PA (Morris et al., 2004). These discrepancies in findings have previously raised concerns about the effectiveness of PA as a rehabilitation method for this and other clinical populations. On the contrary, Chen et al. (2022) showed improvements as measured by BIT-c accuracy, but not in terms of laterality in patients with USN after VPA training. These findings contrast with those of Bourgeois et al. (2022) who reported no change in spatial bias or improvement in USN in any condition. However, there are substantial differences between the two studies that may have affected the results. In Bourgeois et al. (2022) participants utilized a handheld controller for responses, while in Chen’s study, participants used their hands directly. The VR program described by Bourgeois et al. (2022) may have lacked a game-like quality compared to Chen et al.’s (2022) program. Another significant difference lies in the assessment methods as Bourgeois et al. (2022) assessed participants within the VR headset environment, whereas Chen et al. (2022) evaluated participants using traditional 2-D tests outside the virtual environment. However, it is important to consider differences in methodology when comparing results across studies. Future research should aim to standardize experimental procedures for VPA application to identify the most effective PA approach, whether in virtual or physical reality, to maximize benefits for patients. Based on the evidence collected, we propose that an effective VPA approach should include precise visual deviations tailored to individual needs, customizable protocols that adapt in real-time, integration of multisensory feedback, and extended repetitive practices to enhance therapeutic outcomes. We emphasize the importance of developing standardized VPA protocols that incorporate these elements to maximize rehabilitation efficacy in virtual reality settings. Indeed, a recent study utilized fNIRS (Bero et al., 2018) to examine brain activity during VPA, revealing a significant activity in the right frontal eye field and in the dorsolateral prefrontal cortex (DLPFC) during and following VPAT. This result aligns with previous literature on traditional PA, which has consistently reported activation in a broad network including the frontal lobes, the parietal cortex and cerebellum as assessed with functional magnetic resonance imaging (fMRI) (Luauté et al., 2009; Chapman et al., 2010; Küper et al., 2014; Panico et al., 2020). Future MRI studies should further explore whether the same network involved in traditional PA is also implicated in VPA.

The selection of VR hardware and tracking systems is also critical in VPA research. High-cost, space-consuming PC-connected VR system (Kim et al., 2017; Cho et al., 2020) may limit the accessibility of VPA interventions. To address these challenges, Ishida and Higa (2023) recently proposed a novel VPA system designed to create a rehabilitation space (for instance for USN) suitable for use in both hospital and home setting.

This system may expand the use of prism adaptation to involve full-body participation, enhancing the therapeutic process by integrating postural feedback and encouraging spontaneous head orientation toward the neglected side (Ishida and Higa, 2023). The development of the VR system opens new perspectives for applying PA to a collection of multiple motor units, such as the upper trunk system, and highlights the potential of this approach for postural correction in USN patients, along with possible applications in detecting cerebellar disorders (Ishida and Higa, 2024). Future studies are needed to explore the applicability and effectiveness of this system. Regarding the methodologies and devices used in the studies reviewed, such as Oculus Rift and Oculus Quest, they present challenges in synthesizing a comprehensive understanding of VPA’s efficacy. Different VR devices vary in display resolution, field of view, and tracking capabilities, impacting user experience and the effectiveness of VPA studies. Higher resolution and better tracking can improve visual feedback, crucial for prism adaptation. The comfort and usability of VR headsets also affect participation and outcomes, as heavier or less ergonomic headsets can cause fatigue, limiting engagement time. Furthermore, variability in device calibration and customization can influence how users perceive and adapt to virtual environments, potentially skewing study results. For example, the Quest 2 offers a cable-free experience with a built-in battery and a higher-resolution, higher-refresh rate and brighter display compared to the Rift, which requires connection to a PC via DisplayPort and USB (Jost et al., 2021; Carnevale et al., 2022). Therefore, the selection of devices should be carefully considered when planning VPA studies.

Additionally, the current literature highlights some disadvantages associated with the use of VPA. For example, actual VR systems may cause discomfort or head pain thus affecting user experience (Cho et al., 2020). Moreover, the occurrence of motion sickness, a common side effect associated with immersive VR or head-mounted devices, is an aspect requiring careful consideration in the use of VPA (Munafo et al., 2017). Furthermore, participants may encounter challenges in performance due to the weight and design of the headset, potentially affecting user comfort and compliance during longer sessions (Ramos et al., 2019; Cho et al., 2020). However, these discomforts can be alleviated by strategic configuration of the virtual environment (Cho et al., 2020).

On the other hand, the current literature highlights several key advantages of VPA, particularly its ability to offer precise control over the degree of optical deviation, a challenge often encountered with traditional wedge prisms (Bourgeois et al., 2022). This precision is crucial for accurately tracking and quantifying pointing errors throughout therapy, a feature that can be pivotal for establishing effective home-based rehabilitation programs (Cho et al., 2022). In experimental setup, VR provides a more effective blinding of participants if compared with traditional PA (Gammeri et al., 2020). The removal of extraneous visual stimuli enabled by VPA ensures a controlled environment for evaluating adaptation effects. In other words, the immersive nature of VR may decrease the interference of potential external distractors, which could influence the overall adaptation process and its outcomes. Further, Bourgeois et al. (2021) found that VPA progressively induce the mismatch between hand and target across trials, which reduces conscious perception of any visual bias. This progressive adaptation limits the use of conscious strategies by the subjects, potentially leading to more robust and reliable experimental outcomes (Michel et al., 2007). In summary, current research on VPA in healthy individuals is promising, demonstrating its effectiveness in inducing adaptation processes similar to PA. Moreover, there appears to be consistency in the results across studies. However, there is an urgent need to standardize methodologies and deepen our understanding of the mechanisms that produce reliable transfer effects, as well as to confirm the practical and clinical applicability of this technology. Additionally, future research should focus on assessing the ecological validity of this adaptation methodology in everyday contexts. Addressing these aspects will greatly contribute to refining VPA research and maximizing its potential clinical value.

Overall, this review emphasizes the potential of VPA and suggests that further investigations of its effects may support its application as possible rehabilitation approaches. VPA offers several advantages over traditional prism goggles, overcoming well known limitations. Specifically, VPA enables a gradual increase in deviations to produce greater aftereffects, reduces distraction from external stimuli, and it is suitable for bedridden patients.

Finally, there is a notable gap in studies investigating VPA in those neurological populations that may benefit from PA therapy, such as neglect patients. Understanding the duration and generalization of aftereffects to daily activities in these populations is essential. On the other hand, investigating the effects of VPA in healthy individuals represents a crucial first step paving the way for a more reliable and precise application of this innovative technology in patients, aiming to maximize benefits and reduce risks.

LC: Conceptualization, Writing – original draft. AG: Writing – review & editing. VL: Writing – review & editing. VC: Writing – review & editing. AF: Supervision, Visualization, Writing – review & editing. CV: Supervision, Visualization, Writing – review & editing. AQ: Supervision, Visualization, Writing – review & editing. SM: Supervision, Visualization, Writing – review & editing.