The study received approval from the ethics committee of the Faculty of Medicine at the University of Bonn (protocol number 105/23) and was preregistered on July 20, 2023, in the German Clinical Trials Register (DRKS00031259). All participants were informed in advance about the study’s objective and procedures, the use of technical equipment, and possible risks of participation such as motion sickness, and they provided written consent before taking part. Personal data such as names and addresses were recorded only on paper documents, including the consent form and the reimbursement form. A study ID was used for all data collection. Participants received a compensation of €40 (US $47.5) for full participation. Individuals who had to be excluded after the first session or who dropped out received prorated compensation.

A total of 63 individuals participated in the study, including 24 medicated adults with ADHD, 23 currently unmedicated adults with ADHD, and 16 unmedicated adults with MDD. The participants were recruited in various ways: direct approach of suitable individuals at the ADHD outpatient clinic of the University Hospital Bonn, advertisements on the internal website, posting flyers in outpatient clinics, writing to self-help groups, and a press release followed by a radio report. Participants could take part in the study if they fulfilled either ADHD or MDD criteria but no other severe psychiatric disorder, such as psychosis or severe addiction disorder. Participants were not screened for comorbid autism spectrum disorder, as autism spectrum disorder did not constitute an exclusion criterion and its prevalence in the population with ADHD is markedly lower in adults compared to children and adolescents [38]. Furthermore, participants had to be aged between 18 and 50 years, have normal or corrected-to-normal vision, and no history of epilepsy or other severe neurological disorders, pregnancy, or skin irritation on the head.

The study consisted of 2 separate appointments. During the first appointment, participants underwent a clinical assessment after providing informed consent. The assessment had 2 main objectives. First, to confirm the diagnoses of ADHD or MDD, depending on which group the participant should be assigned to, and to rule out the respective other condition. Second, to identify potential comorbidities that signified as an exclusion criterion. The assessment lasted approximately 1-2 hours and was conducted either in person at the psychiatry department of the University Hospital of Bonn or via a video call using the online platform Red Connect [39]. Two clinical interviews were administered: the Diagnostic Brief Interview for Mental Disorders (Mini-DIPS) [40] to screen for 17 mental disorders and the Integrated Diagnosis of ADHD in Adulthood [41] to confirm or rule out the diagnosis of ADHD. Both interviews are based on the criteria of the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders [Fifth Edition]) and ICD-10 (International Statistical Classification of Diseases, 10th Revision) and were conducted in German. After the clinical interviews, participants completed various self-rating scales via the online survey tool SoSci Survey [42], including the Patient Health Questionnaire-9 (PHQ-9) [43], and ADHD Self-Rating Scale (ADHS-SB) [44]. Additionally, a self-constructed questionnaire was administered to collect various sociodemographic data and the medication status. If no exclusion criteria applied during the initial assessment, participants were invited to the second appointment. During this session, the actual VR experiment took place in the VR laboratory of the University Hospital of Bonn. Once prepared, 3 motion trackers were affixed to the participants. Next, they were seated, and preparations for fNIRS recording commenced. Prior to the start of data acquisition, participants were fitted with the head-mounted display (HMD), and the VR environment was launched. Within the VR, a brief eye-tracking calibration was conducted, followed by a 30-second acclimation period to allow participants to familiarize themselves with the virtual setting. After that, participants received task instructions and completed a short practice block, before the actual experiment started. This consisted of 2 blocks, each lasting 24 minutes. Symptom assessments were conducted following both blocks, and the Virtual Reality Sickness Questionnaire (VRSQ) [45] was administered at the conclusion of the session. Throughout the experiment, the software Lab Streaming Layer (Christian Kothe) [46] was used to record and synchronize all data streams, including task performance, fNIRS, actigraphy, and eye-tracking data. Following completion of the experiment, all measurement devices were removed, and participants were debriefed and dismissed. Since the same study supervisor conducted both the diagnostic assessment and the VR session, the study did not include experimenter blinding.

The VR scenario was presented via the HTC VIVE Pro Eye HMD (HTC Corporation). The VR scenario (Figure 1) was created using Unity 3D (version 2019.4.11f1; Unity Technologies) and coded in C# [24]. To enable controller-free interaction with the virtual environment, a Leap Motion Controller (Ultraleap) was used to track participants’ hand movements and online transfer them to a virtual 3D hand model (“Leap Motion Realistic Hands,” Unity Asset Store). The VR environment, entered via the HMD, depicted an open-plan office equipped with typical office supplies, furniture, and virtual colleagues (Figure 1; for further specifications of hardware and software, see Multimedia Appendix 1 [47,48]). In front of the participant, the VEST was displayed on a virtual computer screen.

During the experiment, participants had to perform the VEST, a task that we developed and piloted in a previous study [24]). Compared to conventional CPTs, which often involve abstract stimuli, the VEST uses stimuli that mirror real-world office activities. In this task, participants assumed the role of a secretary in a marketing agency, where they were required to categorize incoming emails into 1/4 predefined categories: “spam,” “new customer,” “existing customer,” and “forward.” To ensure comparability with classical CPT paradigms, the task was structured such that 65% (389/599) of emails belonged to the spam category, making it the default response. Emails classified as “new customer” and “existing customer” each comprised 15% (90/599) of the total emails, while 5% (30/599) of emails fell into the “forward” category. Emails classified as “spam” were sourced from the Enron Spam Dataset (“Enron Spam Dataset,” 2015) but translated into German before usage. Emails for the remaining categories were composed by members of our research group and written from the perspective of a fictitious company or private individual and included both a subject line and a request relevant to a marketing agency. The emails contained clear identifying features corresponding to their respective categories, ensuring that each email had only one correct classification. In total, 599 mails were used in the VEST and presented in a randomized order.

The emails were displayed on a virtual computer screen in front of the participants (Figure 1). The screen simulated a touch interface, allowing participants to interact with it by using their virtual hands to select the appropriate category for each email (Figure 1). Emails did not expire after a certain time limit, so participants performed the task at their own pace—once an email was categorized, the next email would automatically open. Each block consisted of 4 DP and 4 NDP, each lasting 3 minutes and alternating. Consequently, over the course of the entire experiment, participants experienced 8 DP and 8 NDP. During each DP, 6 visual, auditory, or audiovisual distractors were presented in succession, each lasting 30 seconds. These distractors were randomly selected from a pool of 48 potential distractors. The order of the first phase in each block (ie, whether it was a DP or NDP) was counterbalanced across participants.

To assess the participants’ performance on the VEST, 3 distinct parameters were separately extracted for each DP and NDP: the average processing time and PTV (calculated by using the coefficient of variation) per email as measures of vigilance, and the error rate of misclassified emails as an indicator of inattention and impulsivity. For the statistical analysis, the mean values of each parameter were calculated across all DP and NDP per block.

To evaluate participants’ subjective symptom experience, a VR-embedded user interface controlled via gesture input was presented immediately before block 1, between the 2 blocks, and at the end of block 2. For each block, participants were asked to rate their momentary levels of inattention, impulsivity, and hyperactivity [18,20,24], as well as their emotional dysregulation. The items were rated on a 7-point Likert scale, ranging from –3 to +3. For statistical analysis, only the subjective symptom reports collected after block 1 and block 2 were used. Following the final subjective symptom report, the VRSQ was presented on the same interface and Likert scale. The VRSQ includes 8 items assessing potential physical discomfort from VR exposure, from which a total score is calculated [45].

During task execution in the VR environment, the movements of the participant’s head, torso, arm, and leg were recorded. Head movement data were acquired via the HMD, with its position and rotation continuously tracked in the 3D space defined by the base stations. The movements and rotation of the torso, arm, and leg were captured using HTC Vive trackers (HTC Corporation), each of which was also tracked in the 3D space relative to its own dedicated base station. The placement of the trackers depended on the participants’ handedness, that is, for right-handed participants, 1 tracker was attached to the left upper arm, 1 around the torso, and 1 to the right ankle. For left-handed participants, the position of the ankle and upper arm were reversed. In line with our previous studies [16,20,24], all motion data were processed using a uniform analytical approach. First, the data were down sampled to ~10 Hz. Subsequently, the Euclidean distance between successive data points was calculated to quantify the change in position and rotation over time. The mean changes per task phase were then computed to summarize movement dynamics.

To track the participant’s horizontal and vertical eye movements, the Tobii eye tracker (Tobii Technology) built into the HTC Vive Pro Eye was used (for further specifications of hardware and software, see Multimedia Appendix 1). Here, we tracked the participant’s duration of momentary gaze focus on the canvas on which the task was displayed on and the duration spent looking away from the canvas (off-task gaze) [24]. The momentary gaze focus time spent off-task was calculated for each NDP and DP.

As in previous studies [16,20,24], the participants’ fNIRS brain activity was recorded during task performance by the NIRSport 2 system (NIRx Medical Technologies) and acquisition software Aurora fNIRS (NIRx Medical Technologies). We focused on oxy-HB in the dlPFC tracked by 2 channels (Fc1-Fc3, Fc2-Fc4; see also [20]), taking previous studies on ADHD using fNIRS [36] and spatial constraints into consideration (for further specifications of hardware and software used for recording and preprocessing, see Multimedia Appendix 1). For following statistical analyses, DP and NDP were first split into six 30-second units, equivalent to the length of distractor intervals in DP. Next, block averages across all units per phase were determined, and mean concentration of oxy-Hb was calculated for each phase. Lastly, each NDP was subtracted from its corresponding DP, and mean DP-NDP contrasts across all DP-NDP pairs were determined for each participant.

One participant was not able to conclude the experiment due to motion sickness in the second block and was excluded from analysis. For the VEST performance and head actigraphy, all remaining 62 datasets were complete and included; for eye tracking analysis, the sample was reduced to 61 datasets. Due to intermittent data transfer issues with the body trackers, some datasets were incomplete. If at least 3 out of 4 phases were successfully recorded, the phase average for each block was calculated using the 3 available values. However, datasets with more than 1 missing phase were excluded from the analysis. Using this approach, 58 datasets were included in the analysis of arm movements and 45 datasets in the analysis of torso movements. For leg movements, only 35 datasets remained, which led to the exclusion of this parameter from the analysis. Regarding the fNIRS data, 33 datasets were excluded for the left dlPFC and 39 for the right dlPFC due to insufficient data quality, leading to a reduced sample of n=29 and n=23, respectively. Since fNIRS was the only method used for analyzing brain activity in this study, the remaining datasets were still analyzed despite the decreased sample size. Multimedia Appendix 2 [49] presents descriptive summaries of missing data by modality and binary logistic regression analyses assessing whether missingness was associated with group, gender, and age.

For the majority of parameters of interest (eg, error rate; processing time; PTV; head, torso, and arm rotation and position; and off-task gaze), separate 3×2×2 mixed-design ANOVAs were conducted, with group as a between-subjects factor and block and phase as within-subject factors. For the experience sampling data, a 3×2 mixed-design ANOVA was performed with group and block as factors. Analyses of fNIRS and VRSQ data were conducted using 1-way ANOVAs with group as the factor. Furthermore, we conducted an exploratory correlational analysis of the objective parameters and subjective, task-related symptom intensity. For this purpose, we used aggregated values across phases and blocks. Because of the exploratory nature of the correlational analysis, we decided not to correct for multiple testing. Statistical significance was set at α<.05 and α between .05 and .10 was considered as a statistical trend level for all calculations.

Due to right-skewed distributions, several variables (eg, error rate, off-task gaze, and actigraphy measures for arm and torso) were subjected to square root transformations prior to analysis. Inattention scores exhibited left skewness; thus, these data were first mirrored and then square root transformed. Homogeneity of variances was assessed using Levene test, with P>.05 indicating acceptable homogeneity for all variables except inattention, for which homogeneity was not achieved despite transformation.

To control type I error inflation due to multiple testing, Bonferroni corrections were applied to the main effect of the 3-level group factor in the ANOVA, to interaction terms, and to all post hoc group comparisons. Effect sizes were interpreted according to Cohen [50] with partial eta squared (η²_p) values of 0.01, 0.06, and 0.14 representing small, medium, and large effects, respectively for ANOVA results and Cohen d with values of 0.2, 0.5, and 0.8 representing small, medium, and large effects for post hoc comparisons.

The sample consisted of 46 participants with ADHD, equally divided between the medicated and unmedicated groups, and 16 individuals with MDD. The 3 groups did not significantly differ in terms of gender, age, handedness, or level of education. Regarding self-reported clinical measures, the groups only differed on those that were specific to ADHD. Further demographic details can be found in Table 1.

Performance data of the VEST are presented in Figure 2. The analysis of mean processing time revealed significant main effects of block (F_1,59=68.22; P<.001; η²_p=0.54) and phase (F_1,59=23.99; P<.001; η²_p=0.29), but not of group (F_2,59=1.49; P=.24). Mean processing time was higher in block 1 than block 2 (mean difference [MDiff]=4.48, 95% CI 3.40-5.57) and higher in DP than NDP (MDiff=1.86, 95% CI 1.10-2.61). In addition, there was a significant interaction of block × phase (F_1,59=4.00; P=.05; η²_p=0.06), which was not further interpreted.

For PTV, the analysis yielded a significant main effect of block (F_1,59=8.39; P=.005; η²_p=0.12), indicating a higher PTV in block 2 than block 1 (MDiff=–0.05, 95% CI –0.09 to –0.02). There were no significant main effects of phase (F_1,59=0.46; P=.50) and group (F_2,59=1.18; P=.32). However, the analysis revealed significant interactions of block × group (F_2,59=3.35; P=.04; η²_p=0.10), phase × group (F_2,59=5.09; P=.009; η²_p=0.15) and a trend for block × phase (F_1,59=3.79; P=.06; η²_p=0.06), which was not further interpreted. In the pairwise comparisons, we found a significantly higher PTV in block 2 than 1 in the unmedicated ADHD group (P=.03; d=0.39; MDiff=–0.07, 95% CI –0.13 to –0.01) and in the MDD group (P=.005; d=0.95; MDiff=–0.11, 95% CI –0.18 to –0.03), but not in the medicated ADHD population (P=.73). For phase × group, pairwise comparisons revealed a significant difference between DP and NDP in the medicated ADHD group (P=.02; d=0.78) with higher PTV in DP than NDP (MDiff=0.07, 95% CI 0.02-0.12), while for the unmedicated ADHD group (P=.09) and the MDD group (P=.88) there was no significant difference.

The analysis of square root–transformed error rates yielded a significant main effect of phase (F_1,59=41.12; P<.001; η²_p=0.41), indicating a higher error rate in DP than NDP (MDiff=0.02, 95% CI 0.01-0.02). Apart from that, no significant main effects of block (F_1,59=1.79; P=.19) or group (F_2,59=0.04; P=.96) or significant interactions were found.

Eye tracking data are depicted in Figure 4. The ANOVA for square root–transformed off task gaze data revealed a significant main effect of phase (F_1,58=4.72; P=.03; η²_p=0.075), in that across groups, there was a higher off task gaze in DP than NDP (MDiff=0.31, 95% CI 0.02-0.59). Apart from that, no main effect of block (F_1,58=0.50; P=.48) or group (F_1,58=1.16; P=.32) and no significant interactions were observed.

One-way ANOVAs revealed no significant group differences in DP-NDP oxy-Hb contrast on either channel of interest with F_1,28=0.29; P=.73; η²_p=0.02 for the left dlPFC and F_1,22=0.06; P=.94; η²_p=0.07 for the right dlPFC.

Results of the participants’ reported ADHD symptom intensity during and after task performance are depicted in Figure 5. The ANOVA of self-reported square root–transformed inattention intensity revealed a significant main effect of block (F_1,58=9.52; P=.003; η²_p=0.14), with higher scores in block 2 than 1 (MDiff=–0.09, 95% CI –0.03 to –0.15). A significant main effect of group also emerged (F_1,58=9.28; P<.001; η²_p=0.24). Pairwise comparisons showed significantly higher inattention scores in both medicated (P=.007; d=0.91; MDiff=0.27, 95% CI 0.48-0.06) and unmedicated (P<.001; d=1.68; MDiff=0.36, 95% CI 0.57-0.15) participants with ADHD compared to those with MDD, while the ADHD groups did not differ significantly (P=.69). No significant block × group interaction was found.

Regarding subjective impulsivity, a significant main effect of group was found (F_2,58=6.16; P=.004; η²_p=0.18). Post hoc comparisons indicated that unmedicated participants with ADHD reported significantly higher impulsivity than participants with MDD (P=.003; d=1.22; MDiff=1.40, 95% CI 0.42-2.38). Additionally, there was a trend (P=.10; d=0.73; MDiff=0.88, 95% CI –0.11 to 1.86) for higher impulsivity scores in medicated participants with ADHD than in participants with MDD, while the ADHD groups did not differ significantly (P=.44). No significant main effect of block was found (F_1,58=0.38; P=.54), nor was there a significant interaction between group and block.

For self-reported hyperactivity, a significant main effect of block was observed (F_1,58=16.87; P<.001; η²_p=0.22), indicating higher hyperactivity levels in block 2 compared to block 1 (MDiff=–0.72, 95% CI –1.08 to –0.37). There was no significant main effect of group (F_2,58=0.54; P=.59). However, a trend-level interaction between block and group emerged (F_2,58=2.53; P=.09; η²_p=0.08). Pairwise comparisons revealed significant increases in hyperactivity from block 1 to block 2 in the medicated ADHD group (P<.001; d=0.68; MDiff=–1.09, 95% CI –1.65 to –0.52) and MDD group (P=.02; d=0.70; MDiff=–0.87, 95% CI –1.56 to –0.17), whereas hyperactivity levels remained stable across blocks in the unmedicated ADHD group (P=.44).

For emotional dysregulation, ANOVA revealed a significant main effect of block (F_1,58=9.20; P=.004; η²_p=0.14), with higher scores observed after block 2 compared to block 1 (MDiff=–0.53, 95% CI –0.88 to –0.18). A significant main effect of group also emerged (F_1,58=3.74; P=.03; η²_p=0.11). Post hoc comparisons indicated that the medicated ADHD group reported significantly higher emotional dysregulation than the MDD group (P=.03; d=0.94; MDiff=1.55, 95% CI 0.15-2.96). No significant differences were found between the two ADHD groups (P=.86) or between the unmedicated ADHD and MDD groups (P=.24).

Across groups, mean VRSQ scores were in a low positive range (mean 0.31, SD 1.23), indicating a low level of motion sickness during the experiment. The conducted 1-way ANOVA revealed no significant differences between groups (F_2,60=1.04; P=.36).

Core findings of the correlational analysis are presented in Table 2, whereas results of all computed correlations with respective sample sizes are available in Multimedia Appendix 4. Regarding associations between task-related symptom intensity and psychophysiological markers, inattention was correlated with off-task gaze (r=0.28; P=.03). Hyperactivity correlated with both mean error rate (r=0.34; P=.008) and mean processing time (r=0.33; P=.01), as well as with head movement (r=0.35; P=.006) and head rotation (r=0.30; P=.02). Analyses further showed a correlation between impulsivity and mean error rate (r=0.35; P=.006), and between emotional dysregulation and off-task gaze (r=0.29; P=.02). For the correlational analysis between the objective measures, we found, among other things, a significant correlation between off-task gaze and mean PTV (r=0.33; P=.01), head movement (r=0.43; P<.001), and arm movement (r=0.40; P=.002). Additionally, to its significant correlation to off-task gaze, mean PTV was also found to be correlated to head movement (r=0.35; P=.006) and rotation (r=0.26; P=.04). Furthermore, we also found a significant association between head rotation, mean processing time (r=0.30; P=.02), and head movement (r=0.51; P<.001).

In this study, we applied our virtual office room scenario with its VEST to a sample of medicated participants with ADHD, unmedicated participants with ADHD, and unmedicated participants with MDD. Using a multimodal approach, we explored the paradigm’s potential to detect differences in various outcomes across the 3 groups, extending previous research on VR-based assessment methods [20,24]. While we expected more movement in the ADHD groups than in the MDD group, only partial evidence supported this hypothesis. Unmedicated individuals with ADHD showed a distraction-induced increase in head rotation, whereas those with MDD showed a decrease in rotation during distraction. A trend-level effect also indicated more head movement during distraction in both ADHD groups compared to the MDD group. Additionally, regarding our research questions, we found an increase in PTV in unmedicated individuals with ADHD and in those with MDD, but not in medicated individuals with ADHD. Furthermore, we found an increase in PTV during distraction in medicated individuals with ADHD. At least one of the ADHD groups also reported higher task-related levels of inattention, hyperactivity, and emotional dysregulation than the MDD group. No differences between groups were found in eye movement, brain activity in the dlPFC as measured via fNIRS, and movement of the arms and torso. These findings indicate differences between the investigated groups in some of the parameters measured, and distinct responses to distractors and time-on-task, which will be examined in more detail in the following discussion.

Investigating task performance, we found a significant increase in PTV over time in participants with unmedicated ADHD and participants with unmedicated MDD, but not in medicated participants with ADHD. This finding indicates a time-on-task decline of sustained attentional performance or vigilance in unmedicated ADHD and MDD. This result aligns partially with previous research using VR-based CPTs in children, reporting lower reaction time variability in medicated compared to unmedicated individuals with ADHD [25-27]. Moreover, a meta-analysis by Kofler et al [51] found significant effects of stimulant treatment on reaction time variability, whereas no consistent reaction time variability differences were found between adults with ADHD and other clinical populations, including MDD. Our results indicate that medication helps maintain attentional focus over time in ADHD and that unmedicated individuals with ADHD or MDD show a comparable decline in attentional performance.

Additionally, medicated participants with ADHD showed a significantly higher PTV in DP than NDP. Although the effect was observed only in the medicated ADHD group, this finding suggests that the task is sensitive to distractor-specific influences on performance variability. The absence of the effect in the unmedicated ADHD group remains unclear. One possible explanation concerns the measure of variability; while the coefficient of variation is a common metric for reaction time variability, some argue that controlling for mean reaction time may be inappropriate, particularly in ADHD populations [52]. In this study, while not significantly different, unmedicated individuals with ADHD showed descriptively higher mean processing time than the medicated ADHD and the MDD group. Additionally, it is unclear to what extent processing time for emails is comparable to reaction times measured in tasks such as CPTs, which may also suggest the need for an alternative measure of variability.

Of note, no group differences emerged in mean processing time and error rates. This means that we were unable to use the VEST to map the comparatively slower information processing of individuals with MDD that has been observed in other neuropsychological tasks [53]. The error rate, which can be considered a measure of impulsivity, also showed no group differences. This may be due to the task itself, as the stimulus remains visible until a response is made rather than disappearing after a brief interval. It is also possible that, alongside the well-established impulsivity in ADHD, individuals with MDD exhibit impulsivity problems that reduce group differences, which has previously been shown [54]. However, it should be noted that the previous study likewise found no differences in error rates between ADHD and HC [24].

In addition, we observed a longer processing time in DP compared to NDP across groups. While this finding suggests that the experimental induction of distractibility was generally effective in observing reduced attentional performance across all groups, the distractors did not appear to elicit symptom-specific patterns of distractibility that would differentiate between diagnostic groups or between medicated and unmedicated participants.

We hypothesized that both ADHD groups would exhibit increased movement compared to the MDD group. This hypothesis is based on prior research that suggested that the overall activity level of individuals with MDD is decreased compared to those with ADHD. In our task, we measured specific body movements while sitting during task performance, which participants with ADHD and MDD did not differ in. Hence, differences might be found in overall motoric activity but not necessarily in physical activity during cognitive tasks. Nonetheless, the hypothesis was partially supported by a trend-level increase of movement during DP in both ADHD groups, but not in the MDD group. Similarly, we previously found a more pronounced increase in head movements from NDP to DP in ADHD compared to HC [24]. This pattern may reflect core symptomatology of ADHD, particularly hyperactivity and distractibility. If replicated, this differential movement response to environmental distractors may serve as a clinically relevant marker to distinguish between ADHD and MDD populations. In line with this, unmedicated participants with ADHD also displayed a significantly greater head rotation in DP than NDP. This finding further supports the potential use of actigraphy-derived motion parameters as objective, ecologically valid markers for differential diagnosis. By following up the phase and group interaction, we also found a higher head rotation in NDP compared to DP in participants with MDD. While seemingly counterintuitive at first, this finding might be due to the paradoxical effects of distractors. Prior research has presented evidence that distractors can sometimes facilitate performance [55,56], depending on the task characteristics and properties of the distractors, and the specific population explored, including MDD and ADHD [57,58]. Given the unclear directionality and inconsistency with baseline expectations, further research is needed to clarify the mechanisms behind this effect.

The increase in movement of the head, arms, and torso across groups is in line with our previously reported increase in head movement over time in the same paradigm in HC and individuals with ADHD [24]. Increased body movement during sustained attention tasks may reflect a compensatory mechanism to maintain attention [59,60] and can be viewed as an adaptation to the demands of prolonged sustained attention, such as decreasing arousal or accumulating fatigue.

As pertains eye tracking, we found a reduced focus on the task in phases with distractions compared to phases without distractions, but no group differences. Regarding ADHD medication effects, this null finding aligns with our previous study [20], which also revealed no significant differences in eye movement patterns between these 2 subgroups. We also did not find any differences between ADHD and MDD, indicating that all groups spent a similar amount of time looking at the task. Nonetheless, it remains possible that subtle effects exist but were not detected due to limited statistical power resulting from relatively small sample sizes.

In the fNIRS analysis, we did not observe any group differences in the DP-NDP contrast in the dlPFC. This may partly be due to the loss of data caused by technical issues, reducing the analyzable sample on both channels of interest to 29 and 23, respectively. In contrast to traditional inhibitory control tasks, where reduced activation in the dlPFC is consistently reported in individuals with ADHD compared to HC [36], we found no difference between medicated and unmedicated individuals with ADHD. It is also possible that the type of inhibitory control required in classical tasks is different from inhibiting distractors, or alternatively, that the distractors used in our study were not strong enough to elicit such effects [24].

Pertaining to self-reports of task-related symptom experience, both ADHD groups reported higher levels of task-related inattention and impulsivity compared to the MDD group. Assuming that our PTV measure partly captures inattention, this contrasts with our finding that unmedicated individuals with ADHD or MDD showed greater PTV increases than medicated individuals with ADHD. Hence, there appears to be a partial discrepancy between behavioral and self-reported measures, which could mean that they capture different aspects of attentional dysfunction or be attributed to a greater sensitivity of PTV as an index of inattention. For impulsivity, although elevated levels have also been reported in MDD [54], the higher scores observed in the unmedicated ADHD group and, at a trend level, in the medicated ADHD group are consistent with the classification of impulsivity as a core symptom of ADHD. In terms of emotional dysregulation, only the medicated ADHD group reported higher scores than the MDD group, whereas both ADHD groups showed comparable levels to each other. Difficulties in emotion regulation are frequently reported in association with ADHD [61], and in our sample, medication use does not appear to have exerted a beneficial effect, with descriptively higher scores in the medicated group. The MDD group showed the lowest scores, which differed significantly from those of the medicated ADHD group. Because participants in our MDD group were affected by mild to moderate depressive symptoms that did not currently require medication, it is conceivable that they were able to use emotion-regulation strategies such as acceptance. Nevertheless, previous research has shown that individuals with MDD, even when remitted, still tend to suppress emotions, which could also manifest in lower scores [62]. Importantly, emotional dysregulation is a construct that includes various dimensions such as emotional reactivity and intense experience of emotion, but also decreased emotional awareness [63]. Therefore, it is possible that the item used to assess task-related emotional dysregulation might have only encapsulated one facet of emotional dysregulation. Also, it should be noted that, irrespective of absolute levels, all 3 groups reported values in the positive range of emotional dysregulation after the second block.

No group differences emerged in self-reported hyperactivity, which contrasts with the distractor-induced differences detected via head actigraphy, but aligns with the null results from actigraphy-derived torso and nondominant arm movements as discussed above.

Lastly, we conducted an exploratory correlational analysis of the task-related symptom intensity and the objective parameters we measured. Two key points emerged: first, self-reported, task-related intensity of inattention, hyperactivity, impulsivity, and emotional dysregulation were each correlated with different objective measures. For instance, inattention and emotional dysregulation were associated with higher levels of off-task gaze; impulsivity was associated with a higher error rate; and hyperactivity was associated with increased head movement, higher error rate, and longer processing time. Although exploratory, this pattern might suggest that a multimodal assessment combining task performance, eye movements, and body movements (particularly head movement) may capture distinct symptom dimensions of ADHD. Second, off-task gaze and head movement and rotation showed a correlation with mean PTV, and, in the case of head rotation, with mean processing time. While these findings present a first glance into the potential relationships of different behavioral modalities during the VEST, the causal relationship of these parameters needs further investigation.

While the VEST was specifically designed to increase ecological validity compared to traditional neuropsychological tasks by applying a function-led design approach, it would be of interest to investigate the specific cognitive domains it draws upon [24]. Therefore, future studies should examine its associations with established (neuropsychological) measures, for example, attention and reading comprehension. Additionally, although the VEST is designed to simulate a realistic setting and task, a systematic investigation of its ecological validity, meaning the correspondence between behaviors exhibited in the virtual environment and in real-world settings [64], is still needed. While it would be very challenging to assess eye movement behavior, brain activity, and performance measures during the participants’ work to evaluate the equivalency of behavior in the virtual office room to real-world behavior, several alternative approaches could be applied. For example, cognitive functioning at work could be assessed using questionnaires [65,66]. Moreover, office-based participants could be asked to wear a mobile eye tracker and an actigraph during work sessions, while ambient noise levels would be simultaneously measured to assess auditory distractions. This would allow investigation of the relationship between distractor-related shifts of attention and movements in the virtual office room and real life.

Further, although the total sample comprised 62 participants, the sizes of the diagnostic subgroups, particularly the MDD group, were limited. As indicated by a sensitivity analysis, our sample consisting of 63 participants provided 80% power to detect large main effects (η²_p=0.14, as calculated via G*Power, Heinrich Heine University Düsseldorf [67], based on the harmonic mean group size). Consequently, small- to medium-sized differences between the investigated population may not have been detected. Future studies with larger and more balanced samples are necessary to replicate and extend these results. For example, to reliably detect medium main effects, at least 50 participants per group would be needed. Furthermore, while the MDD diagnosis of participants in the MDD group was confirmed using the structured clinical interview Mini-DIPS, some MDD participants only reported mild depressive symptoms in the PHQ-9 [43]. Therefore, it is unclear whether the results can be generalized to individuals with MDD with elevated symptom intensity.

While we provide an analysis across multiple levels of behaviors by group, examining the influence of time-on-task and distractors, and conduct an exploratory correlational analysis, more in-depth integrative approaches combining these measures could further enhance our understanding of behavior in ADHD. For example, a machine learning approach similar to our analysis in Wiebe et al [18] could integrate the various parameters to classify between medicated versus unmedicated individuals with ADHD, and individuals with ADHD versus HC if combined with the dataset reported in Pakos et al [24]. Additionally, a more thorough analysis of distractor-related movement and gaze behavior, and their interplay during task errors, could illuminate potential underlying causes of said errors.

Moreover, while assessing the VEST’s discriminant validity in differentiating between ADHD and MDD represents an important step toward evaluating its potential use in a clinical context, future research might also explore the overlap and divergence of symptom profiles in individuals with comorbid ADHD and MDD. This could further inform the refinement and diagnostic use of VR-based assessments like the VEST. Consequently, although the actual clinical applicability of the VEST still requires further investigation, it is conceivable that it could be used, for example, as a complementary tool in differential diagnostics or for monitoring symptom severity or symptom changes associated with medication. Its administration is straightforward, and even if the space or financial resources for a fully equipped VR laboratory are not available, it would be possible to use the application on a smaller scale, for example, on a smartphone with an inexpensive VR headset alternative.

This study examined, for the first time, the potential of our recently developed multimodal VEST to distinguish individuals with ADHD and MDD, and to detect medication-related effects in ADHD. An increase in PTV over time was observed in unmedicated ADHD and MDD participants, but not in those with medicated ADHD, suggesting a stabilizing effect of medication on attentional capacities. Moreover, medicated individuals with ADHD showed an increase in head rotation during distractions, and, on a trend level, both medicated and unmedicated individuals with ADHD showed increased distractor-associated head movements, an effect not observed in the MDD group. This finding underscores the potential of actigraphy in combination with distracting events during task performance as a tool for differentiating ADHD from other clinical populations. Additionally, exploratory correlational analyses showed correlations between various subjective and objective, as well as psychophysiological markers. While we also observed higher scores in task-related inattention, impulsivity, and emotional dysregulation in at least 1 ADHD group compared to MDD, no differences were found in gaze behavior and brain activity measured by fNIRS. To solidify the results and be able to reliably detect potential small size differences between the groups, and verify the task’s potential as an assessment tool with potential applications in research and a clinical context, higher-powered studies are needed.