[ARTICLE] Transfer of motor skill between virtual reality viewed using a head-mounted display and conventional screen environments – Full Text

Abstract

Background

Virtual reality viewed using a head-mounted display (HMD-VR) has the potential to be a useful tool for motor learning and rehabilitation. However, when developing tools for these purposes, it is important to design applications that will effectively transfer to the real world. Therefore, it is essential to understand whether motor skills transfer between HMD-VR and conventional screen-based environments and what factors predict transfer.

Methods

We randomized 70 healthy participants into two groups. Both groups trained on a well-established measure of motor skill acquisition, the Sequential Visual Isometric Pinch Task (SVIPT), either in HMD-VR or in a conventional environment (i.e., computer screen). We then tested whether the motor skills transferred from HMD-VR to the computer screen, and vice versa. After the completion of the experiment, participants responded to questions relating to their presence in their respective training environment, age, gender, video game use, and previous HMD-VR experience. Using multivariate and univariate linear regression, we then examined whether any personal factors from the questionnaires predicted individual differences in motor skill transfer between environments.

Results

Our results suggest that motor skill acquisition of this task occurs at the same rate in both HMD-VR and conventional screen environments. However, the motor skills acquired in HMD-VR did not transfer to the screen environment. While this decrease in motor skill performance when moving to the screen environment was not significantly predicted by self-reported factors, there were trends for correlations with presence and previous HMD-VR experience. Conversely, motor skills acquired in a conventional screen environment not only transferred but improved in HMD-VR, and this increase in motor skill performance could be predicted by self-reported factors of presence, gender, age and video game use.

Conclusions

These findings suggest that personal factors may predict who is likely to have better transfer of motor skill to and from HMD-VR. Future work should examine whether these and other predictors (i.e., additional personal factors such as immersive tendencies and task-specific factors such as fidelity or feedback) also apply to motor skill transfer from HMD-VR to more dynamic physical environments.

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) [16,17,18]. 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) [22,23,24,25,26]. 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 [24,25,26].

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 [22,23,24,25,26, 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.

Methods and materials

Participants

Seventy-four healthy adults were recruited. Participants were randomized into two groups (Train-HMD-VR, Train-Screen). Three participants in the Train-Screen group were excluded from the analysis as a result of performing all trials in the Baseline training block incorrectly (see Analyses) and one participant in the Train-HMD-VR group was excluded from the analysis as a result of being an outlier, which was defined as being beyond three standard deviations from the group mean motor skill in at least one of the blocks. This resulted in a total of seventy participants (53 females/16 males/1 other, aged: M = 25.81, SD = 4.71) with thirty-five participants in each group included in the analysis. A statistical power analysis was performed for sample size estimation based on data from a pilot study of this work (N = 12) [32]. The effect size in this study was d = 0.38. With an alpha = 0.05 and power = 0.60, the projected sample size need with this effect size was approximately N = 35. Eligibility criteria included healthy, self-reported right-handed individuals and no previous experience with the motor skill task (see Experimental design). Written informed consent was obtained from all subjects. The experimental protocol was approved by the University of Southern California Institutional Review Board and performed in accordance with the 1964 Declaration of Helsinki.

Experimental design

Figure 1a provides an overview of the experimental design. The experiment consisted of training and testing blocks in which participants completed a modified version of the Sequential Visual Isometric Pinch Task (SVIPT) [33]. In this task, participants were instructed to apply varying degrees of isometric force between their thumb and index finger to a small pinch force sensor (Futek Pinch Sensor FSH01465; Futek IPM FSH03633; Fig. 1b) to move a cursor between numbered colored gates as quickly and accurately as possible (Fig. 1c). A small circle at the bottom of the screen changed from red to green to indicate the start of each trial. For each trial, no time limit was given and trial completion time was recorded. At the end of each trial, the small circle at the bottom of the screen changed from green to red and participants received auditory feedback (a pleasant “ding” if the cursor correctly entered all the gates or an unpleasant “buzz” if the cursor missed one or more of the gates). A two-second time interval was given between each trial.

Experimental paradigm. a Experimental design. b Pinch force between the thumb and index finger was applied to a small force transducer to move the cursor in the SVIPT. c Sequential Visual Isometric Pinch Task (SVIPT) display. Participants were asked to apply force to the force transducer, which translated into the movement of a small black cursor (shown at the home position in the white bar) moving horizontally to the right in the environment. The cursor moved left by reducing force. Instructions were to move the cursor between the gates, in order from 1 to 5, as quickly and accurately as possible, without over- or under-shooting any of the gates

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