Background
The use of virtual reality (VR) in rehabilitation has been growing exponentially over recent years [
1,
2]. Clinical applications of VR have been shown to be engaging and motivating [
3,
4] with promising results suggesting VR interventions are comparable [
5] or in some cases superior [
6,
7] to conventional rehabilitation. However, while a number of studies have reported benefits of using VR for cognitive and motor rehabilitation, there are also reports on the limitations of using these devices for clinical applications [
8,
9]. In particular, some studies have shown that VR interventions are not effective at improving motor performance in the real world due to a lack of motor skill transfer (i.e., the application of a motor skill in a novel task or environment [
10]) [
11,
12].
Concerns about motor skill transfer from virtual to real environments are even greater when specifically considering the use of VR viewed using a head-mounted display (HMD-VR). HMD-VR provides a more immersive experience compared to conventional environments (e.g., computer screens) and results in increased levels of presence (i.e., the illusion of actually being present in the virtual environment) and embodiment (i.e., the perceptual ownership of a virtual body in a virtual space) [
13,
14] that modulate behavior [
15] and impact performance on motor learning and rehabilitation applications (e.g., gait, balance, neurofeedback tasks) [
16‐
18]. Additionally, motor learning in HMD-VR (e.g., upper extremity visuomotor adaptation) has been shown to rely on different learning processes compared to a conventional screen environment [
19]. Given the differences in immersive experiences and learning processes between HMD-VR and conventional environments, it can be assumed that individuals may experience these environments as separate contexts. Studies have found the context of the training environment to affect the transfer of motor skills [
20], where motor performance may decrease when testing occurs in an environment different from training [
21]. However, only a small number of studies have specifically explored motor skill transfer of from an HMD-VR training environment to a more conventional environment (e.g., computer screen or real world) [
22‐
26]. Among these studies, there are again conflicting results, with some studies finding successful motor skill transfer from HMD-VR to the real world [
22,
23], and others not [
24‐
26].
There is also large interindividual variability within the results, and this variability suggests there may be particular tasks or particular individuals that will be more successful in transferring HMD-VR motor skills to the real world. Understanding the task-related or personal factors that mediate learning and transfer from HMD-VR environments should be examined in order to understand what makes HMD-VR interventions effective. One advantage of HMD-VR over conventional screen environments is the ability to realistically simulate the real world which allows for greater task specificity [
27]. Task-related factors such as fidelity (i.e., imitation of the real environment) and dimensionality (i.e., matching dimensions between virtual and real environments) between HMD-VR and the real world have been shown to influence lower extremity motor performance [
28] and have been suggested to have an influence on transfer in both lower and upper extremity motor transfer [
29,
30]. Individual differences in personal factors such as gender, age, video game experience, prior technical computer literacy, and computer efficacy seemed to influence transfer from HMD-VR to the real world in studies examining the transfer of spatial knowledge acquired in an HMD-VR environment [
26,
31]. However, the individual differences on both task-related and personal factors have not been extensively examined in HMD-VR motor skill transfer. We begin to address this gap by examining whether individual personal factors facilitate better transfer from upper extremity motor skill acquisition in HMD-VR to a conventional screen environment.
In the current study, we examined: (1) whether transfer of upper extremity motor skills occurs between HMD-VR and conventional screen environments, and (2) what personal factors predict transfer between environments. Given the variability of motor skill learning and transfer in previous studies [
22‐
26,
29], we hypothesized that individual motor performance would vary after transfer to a novel environment, and that this variability could be predicted by individual differences in variables such as presence in the training environment, prior experience with HMD-VR, or non-VR video games.
Discussion
In this study, we examined motor skill transfer from an HMD-VR environment to a conventional environment (i.e., computer screen), and vice-versa. First, we confirmed that motor skill acquisition occurs in both HMD-VR and conventional screen environments and demonstrated that acquisition occurs at a similar rate in both environments, suggesting that task difficulty was not different between the environments. We then demonstrated that while motor skill transfer occurs after training in either environment, there are individual differences in the amount of motor skill that transferred.
In examining whether motor skills acquired during training in HMD-VR transferred to a conventional screen environment, we found a significant decrease in motor skill performance as a result of the transfer. To see if this decrease in motor skill transfer could be explained, we examined whether individual differences in five presence themes (realism, possibility to act, quality of interface, possibility to examine, self-evaluation of performance), age, gender, video game use, and previous HMD-VR experience could be used as predictors. We found trending but nonsignificant evidence that a combination of two presence themes, positively correlated possibility to act and negatively correlated self-evaluation of performance, best predicted this observed decrease in motor skill. Additionally, we found trending evidence that previous experience using HMD-VR independently may predict the decrease in the motor skill transfer. Overall, these results suggest that while the motor skills acquired in HMD-VR may not transfer to a conventional environment, the factors mentioned could mitigate this decrease.
We also examined whether motor skills acquired during training on a conventional screen environment transferred to HMD-VR. We found that motor skills learned in a conventional screen environment transfer to HMD-VR; however, not only do the motor skills transfer, but performances seem to improve in the novel HMD-VR environment. We found that the combination of the quality of interface, gender, age, and video game use best predicted this motor skill transfer. Additionally, we found evidence that age and video game use independently may predict the increase in motor skill transfer between computer screen and HMD-VR. This supports previous findings that age and video game use affect acquisition and transfer in non-immersive virtual environments [
44,
45]. We also found trending evidence that the quality of interface independently may predict the increase in motor skill transfer between a computer screen and HMD-VR, further supporting the involvement of presence in the transfer of motor skill. These predictors may be useful to consider in cases when a HMD-VR rehabilitation intervention is introduced after motor skills have already been acquired in the real world.
Our work adds to the limited knowledge of personal factors that could potentially drive motor acquisition in HMD-VR and the transfer of motor skill to other environments. While other studies have identified potential mechanisms for HMD-VR transfer by examining existing literature [
7], there is inconclusive evidence for why motor skill acquisition in HMD-VR and transfer to other environments may be more effective for some individuals compared to others. The two presence themes identified support previous findings that levels of presence relate to motor performance in an HMD-VR enviorment [
46] and extend these findings to the transfer of motor skill acquisition. Additionally, previous experience with the training device, which is HMD-VR in the present case, support findings that the transfer of spatial knowledge is influenced by previous experience with the environment [
31]. Increased exposure to HMD-VR may decrease the novelty, and subsequent attention evoked during the task, which may decrease motor performance. Future studies should examine whether individuals with more HMD-VR experience have greater motor skill transfer to the real world.
In addition to the personal factors that we have examined in this study, there are undoubtably more mechanisms that could either drive or predict HMD-VR motor skill transfer, and this should be further explored. Future studies should also consider other personal factors such as participants’ immersive tendencies, the likelihood that an individual will feel immersed in a new environment [
47] as well as avatar embodiment, if applicable [
48]. In addition to personal factors, task-related factors likely contribute to differences in motor skill acquisition and transfer from HMD-VR to conventional environments, and vice versa. Previous findings have suggested that fidelity and dimensionality influences the transfer of motor skills from HMD-VR environments [
29]. In the current study, a possible explanation for the decrease in performance on a computer screen could be that the visual representation of the HMD-VR environment did not reflect what individuals expected and therefore, motor skill performance was not maintained with transfer. Future studies should consider examining the level of fidelity and dimensionally in HMD-VR needed to optimize motor skill transfer to the real world, and vice versa. Motor skill transfer has also been shown to be influenced by other task-related factors such as task variability, engagement, and feedback [
49,
50]. In the current study, the increase in performance in HMD-VR could be a result of an increase in attention or engagement after transfer from the computer screen. Future studies should also examine how these task-related factors influence HMD-VR motor transfer to the real world, and vice versa.
It has also been suggested that HMD-VR may require additional cognitive resources and that additional information and stimuli must be processed in order to solve tasks in virtual reality [
51]. One study found that the motor skills acquired in HMD-VR through the reliance of spatial cognitive capabilities did not transfer to the same task in the real world [
25]. Our own previous work has shown that visuomotor adaptation in HMD-VR requires a greater reliance on cognitive strategies than performing the same task on a computer screen [
19]. Taken together, this suggests that the decrease in motor skill transfer observed when moving to a conventional screen environment could also be due to less engagement of the cognitive processes used when in HMD-VR. The utilization of these cognitive processes during performance in either environment could be influenced by any of the personal or task-related factors described. Future work should examine whether specific cognitive processes have a role in HMD-VR motor skill transfer to the real world.
One limitation of this study was the use of a computer screen as the transfer condition from HMD-VR. Although this was purposefully designed to provide the most well-controlled and subtle differences between HMD-VR and the conventional environment, and previous studies have reported significant differences between HMD-VR and computer screen environments [
19,
52‐
54], future work should examine whether presence, gender, age, video game use, or previous HMD-VR experience has an effect on HMD-VR motor skill transfer to more dynamic, real world physical applications (e.g., throwing a ball in HMD-VR versus throwing a ball in real life). Future research should also look to see if the identified factors apply to different clinical populations and examine whether mechanisms such as functional independence or cognitive status could predict success of HMD-VR rehabilitation interventions [
55,
56]. Another limitation was that our definition of motor skill transfer reflects the transfer of motor skill acquisition rather than motor skill learning. Experimental designs of motor skill learning typically examine transfer after a retention interval and compare transfer performance to baseline performance in the transfer context [
22]. Future studies should examine whether the personal factors identified here are also predictors for this type of experimental design. Lastly, the use of a subjective questionnaire to measure presence is also a limitation; future work should use alternative objective measures, such as physiological responses, in addition [
57]. Overall, despite these limitations, we believe that the work presented in this study provides an initial examination into the transfer of motor skills between HMD-VR and conventional screen environments as well as insight into the factors that may mediate this transfer.
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