Study design and participants
This was a cross-sectional study. Thirty healthy adults 20 years of age or older were enrolled through convenience sampling from the study institute (Table 1). The exclusion criteria were a history of neurological conditions, vestibular dysfunction, vertigo, psychiatric conditions, significant visual problems (such as visual loss, cataract, glaucoma, or other major eye conditions), upper limb musculoskeletal disorders causing difficulty using a keyboard or controller, and an inability to tolerate HMD-VR. They were asked not to consume alcohol, nicotine, or hypnotics on the day of testing. All participants agreed to undergo retesting. This study was approved by the ethical committee of the National Taiwan University Hospital (approval number: 202104030RINC, date of approval: 05/13/2021), and written informed consent was obtained prior to participation. All procedures were carried out in accordance with the ethical rules and the principles of the Declaration of Helsinki.
VR- and PC-based SRT evaluation
The concurrent validity was tested against the PC-based software Alertness (ALET) module in RehaCom software (version 6.9.0.0, HASOMED GmbH, Magdeburg, Germany), which is used for training and evaluation in clinical settings [43]. The software was run on a desktop computer with Windows 10, Intel i5-8500 CPU, 8 GB RAM, and an Intel UHD Graphics 630 graphics card. The display was a 21-inch screen. The stimulus was a 50 x 50 mm red square and appeared in a fixed position on the center of the display on a light-green colored background. The interstimulus time intervals were random within 3 seconds, and no audible warning before the stimuli was provided. The participants responded to the stimuli by pressing a button on a special custom keyboard.
The VR-based SRT was tested using the Vive ProEYE system (HTC, Inc. Taiwan), which includes an HMD with eye tracking and two infrared laser emitter units (lighthouse 2.0). The HMD was connected to the measurement computer through a cable and Vive link box, which provided an HMDI connection, a USB connection, and a power supply. Two lighthouses were positioned diagonally and mounted at a height of 2.1 m, angled downward at an angle of 25–30°, and connected by a synchronization cable. A custom C# script and the SteamVR (Valve Corp, Washington, USA) plugin for Unity3D were used to provide integration with the HTC Vive to design the software for testing the SRT. The VR software operated on a laptop computer, Asus ProArt StudioBook Pro H500GV (Asus, Taiwan), equipped with Windows 10, Intel i7-8700, 16 GB DDR4 RAM, and an Nvidia GeForce RTX 2060 graphics card. We set SRT testing environments similar to those for the PC-based testing by building a virtual board as the screen of a Vive tracker, which was adjusted to a position of 65 cm in front of the user. The stimulus was also a 50 x 50 mm red square block presented in a fixed position on the virtual board, with random time intervals between 1000–3000 ms. The participants responded to the stimuli by pressing the “space” key on the keyboard of the laptop computer, and the values for all RTs to relevant stimuli were recorded in ms (Fig. 1).
Procedure
The procedure of our study design was shown in Fig. 2. The demographic data, including age, sex, and handedness, were obtained via a questionnaire. The participants were randomized to two groups in the order of the PC- and VR-based tests. This is, some participants underwent the PC-based tests prior to the VR-based one, and others encountered in an opposite order. There was a 10-minute break between the 2 testing modalities. For both PC- and VR-based tests, the participants were seated in a comfortable chair in a quiet room. To control the effect of the weight of the HMD, the participants wore the HMD on top of their heads without blocking their vision during the PC-based SRT test. The participants were asked to complete two tests, one responding with the right index finger and another with the left one, in each modality. A practice of 20 trials was provided prior to each formal test, with the instruction of responding to the stimuli as fast as possible without error and anticipation. Then, the participants completed two 100-trial tests with either the right or left hand, respectively, in a randomized order, similar to the protocols used in previous studies [14, 44]. There was a 2-minute break between each testing condition. They were allowed to stop the test if there was any discomfort. The total time to complete the VR-based test was computed from the recorded raw data. They were verbally asked about any symptoms of cybersickness, such as dizziness, nausea, eye strain, or headache, after the VR-based test. The participants repeated the VR-based SRT test within 1 week. The schedule of the retest was arranged in the morning or afternoon, as in the first test.
Statistical analysis
The PC-based SRT was automatically generated in ms by the RehaCom software as a median value. For the VR-based SRT, we excluded the data with RTs below 100 ms or above 1000 ms. We further divided the data of the 100-trials into 4 blocks, with each containing 25-trials, and computed the median values for the 100-trial and each block of 25-trial. The descriptive statistics of the median SRTs were checked for normality of the data distribution using the Shapiro–Wilk test. Parametric or nonparametric methods for statistical analysis were chosen according to the distribution.
The differences in the median SRTs of each 25-trial block of the VR-based SRTs were examined with Friedman’s test or repeated measures analysis of variance, and the p values for the post hoc examination among multiple comparisons of tasks were examined by the Wilcoxon signed rank test or paired t-test with Bonferroni correction. The test-retest reliability of VR-based SRT was assessed with the intra-class correlation coefficient (ICC) based on a single rating, absolute-agreement, 2-way mixed-effects model. An ICC higher than 0.75 was considered excellent, an ICC between 0.6 and 0.74 was considered good, an ICC between 0.4 and 0.59 was considered fair, and an ICC below 0.4 was considered poor [45]. The ICC was calculated for 25-trial, 50-trial, 75-trial, and 100-trial SRTs to assess the reliability of various trial numbers. The convergent validity was tested against the PC-based SRT obtained by RehaCom with Pearson’s correlation coefficient and Bland–Altman plot [46]. The size of the correlation coefficient was interpreted as being very high (0.90), high (0.7 to 0.9), moderate (0.5 to 0.7) or low (0.3 to 0.5) [47]. Statistical analyses were performed using SPSS 15.0 for Windows (SPSS Inc., Chicago, USA) with a statistical significance of p < 0.05.