Participants
Healthy participants (N = 27; 9 males; mean age = 23.7 years, SD = 3.48 years, range = 18 - 32 years) within a healthy range of BMI (18.9 – 24.9 kg/m²) were recruited through mailing lists and flyers from the Tübingen student population. All participants were >18 years old, had normal or corrected-to-normal vision and were right-handed according to Oldfield’s handedness inventory [40]. Under- or overweight (i.e., a BMI exceeding the 18.5-24.9 kg/m² range) was considered exclusion criterion; furthermore, left-handedness, epilepsy, or any other history of severe mental or physical disease led to exclusion. Participants reported moderate levels of hunger in the beginning of the experiment [mean = 6.00 (SD = 0.96); range = 5-8 on a scale from 1-10]. There was no indication of eating disorders and the mean global score in the eating attitudes test (EAT-26D) was below the cut-off score of 20 (M = 4.59, SD = 5.09).
The sample size was justified by available resources and acceptable power to reproduce previously reported behavioral differences between food and non-food 3D objects [18] in a within-subjects design. This sample size was also sufficient to replicate a behavioral bias for food in VR-mediated motor responses (f = 0.48) in a factorial design with a significance level of α = .05 and a power of 1-β = 0.95. Participants provided informed consent and received either course credit or a small monetary compensation for their participation. Ethical approval for the study was obtained from the University Hospital Tübingen Ethical Commission (No. of approval: 207/2015BO2). All data for this study were collected in 2015 before the Coronavirus pandemic.
Apparatus
To immerse participants in the VR, they were equipped with an Oculus Rift© DK2 stereoscopic head-mounted display (HMD; Oculus VR LLC, Menlo Park, California). Motion tracking of hand movements was realized with a LeapMotion© near-infrared sensor (LeapMotion Inc., San Francisco, California, USA, SDK version 3.1.3). The LeapMotion© sensor provides positional information regarding the palm, wrist, and phalanges. This data can be used to render a hand model in VR, as in previous studies [18, 38, 41]. The whole experiment was implemented within the Unity® engine 4.5 using the C# interface provided by the application programming interface. Instructions and feedback were presented on different text-fields, aligned at eye-height.
Procedure
Prior to the actual experimental testing, participants received a verbal instruction regarding proper handling of the VR equipment. Then they were equipped with the HMD and the experiment started with practice trials. A researcher was present throughout the entire time of the experiment. All participants completed the same virtual experience and all independent variables were within-subject manipulations.
The supplementary video 1 shows examples of the trial sequence and Figure 1 displays screenshots from the participants’ view. The VR procedure was identical to our pilot study [18] except for the additional stimulus categories. Each trial consisted of two parts: (1) Preconditions had to be met for an interaction to begin: Participants had to move their right hand into a predefined and fixed starting position, indicated by red, semi-transparent spheres, at a comfortable height close to the participant. If the palms were within the positions and the hand was open, the semi-transparent spheres turned green. Furthermore, participants had to center their field of view on a fixation cross located at the outer bound of the task space. (2) Once the center of the visual field had been directed towards the fixation cross for at least 500 ms, the spheres and the fixation cross disappeared (stimulus-onset asynchrony: 200 ms), and a colored cue indicated the upcoming position of a target and indicated the requested action (e.g., blue cue for pushing and purple cue for grasping movements, 400 ms). This cue was then replaced with the target object, which appeared with slight motion directed towards the participant. Objects always appeared approximately 20 cm in front of the participants close to the position of the (removed) fixation cross, but exact location, rotation, and speed were jittered to make the task more challenging (please see supplementary video 1, for demonstration of these subtle variations). In the case of grasping, participants were requested to close their hand surrounding the virtual object, move it towards themselves, and place it in a box in front of them. In the case of pushing, participants were requested to hit the target object with their hand, which would then fly away after the collision. The three most recently collected objects remained in the box adjunct to the participants’ feet, whereas pushed objects were cleared always before the next trial.
Trials were cancelled if the movement initiation took longer than two seconds. In case of such time-outs, early movements (earlier than 250 ms), or wrong responses, participants received according feedback in the form of a semi-transparent text-field. The whole experiment was self-paced, since trials only started when participants took the initial position and fixated the fixation cross. Hence, participants could (and they were encouraged to) take breaks between trials at any time. After half of all trials (i.e., approximately 30 minutes of VR), participants were asked to take off the HMD for a slightly longer break of some minutes.
In extension to ball and high calorie virtual food objects that were already studied before in a different sample of participants [18], two further categories of low calorie food and complex office-tools were selected All stimuli were created with 3D objects from the Unity® asset store and were modified to be equally sized Since no low-calorie and office-tools 3D stimuli were available from previous studies, the authors tried to maintain a consistent graphical style (low-poly, cartoon-like stimuli) with low rendering load to compliment fast refresh cycles and continuous motion capture. Screenshots of the object categories are shown in Figure 2. Grasp and push interactions were requested in randomized order across the whole experiment for each of the 12 objects. Each object was presented 30 times, resulting in 360 trials in total. The experiment was subdivided into a total of 10 blocks, each comprising 36 trials, for a total duration of approximately 45 minutes. After the first part of 5 blocks, participants were encouraged to take a break before the second part of 5 blocks started. The entire experiment lasted approximately 2 hours.
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Questionnaires & Ratings
Hunger, BMI, Presence, Simulator Sickness
Before entering VR, participants had to indicate their hunger on a 1-10 hunger scale with descriptive anchors (10 = painstakingly full, uncomfortable; 1 = need to eat now). After the experiment, all participants were asked to rate the VR exposure regarding experienced presence [42] and possible symptoms of simulator sickness [43]. For presence, we used the established Igroup Presence Questionnaire [42] with subscales for spatial presence, involvement, realism, and a general presence item. Items were rated on a scale from -3 (e.g., “not at all”, “about as real as an imagined world”) to +3 (e.g., “very much”, “indistinguishable from the real world”). Internal consistencies were weak in this study (Cronbach’s α = .60 (spatial presence), α = .60 (involvement), α = .50 (realism), For the simulator sickness questionnaire [43], symptoms were rated from 1 (“not at all”) to 4 (“strong”) and internal consistency of the total score was acceptable (α = .79).
All participants self-reported their weight and height for computation of BMI (weight in kg / (height im cm)²). The hunger and BMI scores were explored regarding their possible contribution to an approach food bias [7, 44, 45] in correlation analyses.
Rating of stimuli: Valence and arousal
To evaluate the composition of object categories and differentiate subjective from behavioral responses to high-calorie food, participants also had to rate all objects for valence (negative – positive) and arousal (not at all – very arousing) on visual analog scales (VAS). The rating was performed after the experiment, participants’ were shown a screenshot of each stimulus on a separate page in randomized order and were given two VAS items with the anchors “negative – positive” and “not at all – very arousing”. For analysis, VAS ratings were saved on scales from 0.0 - 100.0. Internal consistency for high-calorie food valence was questionable (α = .53), consistencies for low-calorie food, ball, and office-tool valence ratings were acceptable (α = .63 - α = .74). Internal consistencies for arousal ratings were acceptable to excellent (α = .63 - α = .90).
Eating attitudes test
We screened for eating disorder symptoms using the EAT-26 questionnaire [46] in its German translation [47]. The questionnaire assesses eating-related thoughts and attitudes and discriminates (1) Dieting, (2) Bulimia and Food Preoccupation, and (3) Oral Control on three subscales. Responses are scored on six levels (never-always), but are recoded to only consider the levels “often”, “usually”, and “always“ for all items except food novelty [48]. A global sum score of 20 or more indicates an increased risk for having an eating disorder. Internal consistencies of most subscales were acceptable (Cronbach’s α = .80 (dieting), α = .76 (bulimia), α = -.48 (control), α = .76 (global)).
Dependent Measures and Data Treatment
Mean subjective ratings from VAS were calculated for each stimulus category. Response times (RTs) were recorded within the VR in milliseconds at three different stages of an experimental trial. Movement onset was defined as the time-point when the grasping hand left the starting position, object contact was triggered by the collision of the virtual hand with the target object, either due to grasping or pushing the object away. Finally, in grasp trials only, collection time was recorded once the object had been placed in the box next to the participant. All of these events were automatically triggered by the physics engine in Unity3D. Please see Figure 1 for a visualization of these experimental stages within a sample trial.
This study was a repeated measures factorial design with the factors interaction (grasping vs. pushing) and stimulus (high-calorie food, low-calorie food, balls, office tools). Data from correct trials were individually aggregated to the four categories for each type of interaction (grasping vs. pushing).[1] Incorrect trials were excluded because of erroneous responding (14.2 %) or detection of early hand movements prior to onset of the stimulus (3.3 % of all trials). Further, an outlier correction of 2.5 SD was applied for each RT (1.83 % movement onset, 2.53 % object contact, 2.57 % object collection) before aggregation to the different condition means for each individual. Mean RTs and arousal VAS ratings were not normally distributed according to Kolmogorov Smirnov tests for normality (W = 0.95-0.97, ps < .002). Accordingly, we employed non-parametric statistical tests. We used the aligned rank transformed nonparametric factorial analysis (ART) [49] to investigate the two-way interaction of interaction and stimulus in movement onset and object contact RTs and the main effect of stimulus in collection times, for which only the interaction-level “grasp” was available. The ART procedure aligns data for each main effect and interaction, assigns ranks, and subsequently submits aligned ranked data to an F-test. Simple effects were implemented using paired nonparametric Wilcoxon tests with Bonferroni correction for multiple comparisons. The same analytic strategy was employed for VAS ratings for the main effect of stimulus. Finally, we explored Spearman’s rank-order correlations between behavior and subjective variables such as BMI, hunger, stimulus ratings and questionnaire scores. Analyses were performed using R 4.1.2 [50], the ARTool package [49] and rstatix [51] for nonparametric testing the outlier()-function from schoRsch [52] and the tidyverse package [53].
Footnote:
[1] We also checked in a sensitivity analysis whether duration of the experiment had an influence. Specifically, we included another repeated-measures factor part (first part, second part), because the entire procedure was intermitted by a break after half of the trials. As expected, RTs in all conditions decreased for the second part of the experiment. For movement onset and object contact, all reported effects were comparable at large to the reported main analyses and there was no interaction between part × stimulus or part × stimulus × interaction. For object collection, a behavioral bias for high-calorie relative to low-calorie food was larger during the first part (241 ms) compared to the second part of the experiment (9 ms; F(3,75) = 3.52, p = .019, ηp² = 0.12).