In this experiment, we sought to determine how the peripersonal space boundaries would modulate when an agent, potentially perceived as another individual, was the visual stimulus. In particular, five different agents of varying visual features approached each participant using the same method as in the first experiment.
4.3.1 Participants
Forty participants (25 female, 15 male) between the ages of 18 - 40 took part in this study. Participants were recruited through our institution’s sign up system for psychology studies and flyers placed around campus. All participants had no prior knowledge of the study and had normal or corrected-to-normal vision. The protocol was approved by our institution’s IRB. Participants gave informed, written consent and were paid. A survey was given to participants about their virtual reality use. 80.9% of participants reported that they had used virtual reality before, but the majority (61.7%) had only used it once when they tried a demo.
Figure 12: Agents used in Experiment 2. From left to right: average female, intimidating female, average male, intimidating male and monster. ©IEEE 2020
4.3.2 Apparatus
We also used the HTC Vive Pro and built the immersive virtual environment using Unity Game Engine (version 2018.2.17f1) for Study 2. All Scripts were written in C#. Agents were either modeled using Adobe Fuse or taken from the avatar collection provided on Adobe Mixamo, and imported into Unity. There were five agents (see Figure 12) that each user saw during the study: an average female and male, intimidating female and male and a monster. The dimensions of these agents were as follows: the height and shoulder width of the average female were 1.65 m and 0.34 m respectively, of the average male were 1.8 m and 0.4 m respectively, of the intimidating female were 1.65 m and 0.43 m respectively, of the intimidating male were 2.15 m and 0.65 m respectively, and of the monster were 1.7 m and 0.68 m respectively. These agents were all given the same walking animation that was also downloaded from Adobe Mixamo. Users were given their own self-avatars, which were the same used as in Study 1 (see Figure 10). The Vive tracker configuration was the same as well, and avatars were tracked by using Ikinema Orion. The immersive virtual environment was the same as used in Study 1 with one slight modification: we modelled a doorway and enclosed room from which the agents emerged and placed it 2.3 m away from where the user would stand (see Figure 9).
4.3.3 Experimental Design
Before beginning the study, subjects were asked to fill a virtual reality experience questionnaire. Once completing the questionnaire, subjects were asked to don the HMD as well as the tracking shoes and belt, and were given the hand controllers. They were asked to adjust the headset to their comfort, including the IPD. Ikinema Orion
was started to track the movement of the subjects, and then they were placed in the virtual lab environment.
Subjects were allowed to familiarize themselves with the environment before beginning the study by looking around and viewing their self-avatar’s body. Once they were ready to begin, subjects stood behind a virtual piece of tape on the ground of the immersive environment that was approximately 2.3 m away from the doorway out of which the agents would emerge.
In a randomized order, one of the five different agents would emerge from the doorway and walk toward the participant at a speed of 75 cm/s. All agents followed the same walking animation. Subjects were instructed to watch as the agents approached, and to respond to the tactile stimulus (which was a vibration delivered to one of the controllers) by pulling one or both of the triggers on the controllers as soon as they felt the stimulus.
The tactile stimulus occurred randomly at each distance of 0.7 m, 1 m, 1.2 m, 1.5 m and 2 m. These distances were chosen due to the size of the agents, but still with the bounds of natural peripersonal space in mind. If some agents approached users at distances closer than 0.7 m, the user could see the inside of the agent. There were a total of 125 trials; each subject responded to each of the five agents on five different occasions per each of the five perceived distances. Reaction times were logged into a text file and data was extracted by a python script in an SPSS-ready format. Once all trials were completed, the study was concluded.
4.3.4 Agent Likeability Ratings
The same likeability survey that was administered in the first study [Bartneck et al. 2009] included the same questions regarding the agents used in Study 2. Again we wanted to determine the attitudes users would display when interacting with the agents. The average female and male, intimidating female and male and monster were all rated against the same Likert scales: dislike-like, unfriendly-friendly, unkind-kind, unpleasant-pleasant and awful-nice. Again, Mann-Whitney U Tests were performed on the data for each question and were Bonferroni corrected. Almost all comparisons were significant, with p-values smaller than 0.05 and no larger than 0.008.
However, for all questions, responses to the intimidating female and male were never significantly different.
Additionally, responses to the average and intimidating males were not significantly different for questions 1 (dislike-like) and 4 (unpleasant-pleasant) and were not different between the average male and intimidating female for questions 1 (dislike-like), 3 (unkind-kind) and 4 (unpleasant-pleasant). A full list of significance values
Agent Type Likeable Friendly Kind Pleasant Nice
Average Male 5% 18% 24% 13% 18%
Average Female 47% 45% 47% 47% 42%
Intimidating Male 18% 8% 13% 11% 18%
Intimidating Female 11% 2.6% 8% 11% 11%
Monster 18% 5% 8% 8% 5%
Table 6: Depicted in this table are the positive ratings (meaning ratings of 4 or a 5 on the Likert scale) for each question in the likeability survey for the agents used in Study 2. ©IEEE 2020
for each question and agent comparison can be found in the supplementary material included with this paper.
Table 6 depicts how respondents felt each agent type to be likeable, friendly, kind, pleasant and nice. Overall, the female character is rated most positively amongst survey participants, while the monster is rated most negatively. We would expect participants of Study 2 to react differently to the agents according to the positive and negative perceptions recorded here.
4.3.5 Results
Our analysis follows the first experiment. We first chose to remove all outliers from our data. We did so by using Tukey’s Method. We calculated the upper fence for each set of reaction times at each distance for each agent and rejected any reaction time greater than that fence. Again, we did not calculate the lower fence. This removed approximately 7% of the data, and the majority of the outliers removed were instances where the subject did not respond. Once outliers were removed, we then averaged all reaction times for each of the trials at each distance for each agent. For ease of understanding we chose to collapse the agents into three groups:
“average" agents, “intimidating" agents, and the monster. The “average" agent group consisted of the average female and male, and the “intimidating" agent group consisted of the intimidating female and male.
Using IBM SPSS Statistics, we submitted this data to a repeated measures analysis of variance (RM-ANOVA) with factors of distance and agent. Assumptions were checked and corrected for by SPSS. The RM-ANOVA found main effects of distanceF(1,33) = 8.314,p < .01and agentF(1,33) = 14.794,p < .01, and no interactions. Bonferroni corrected paired samples t-tests revealed that all reaction times at each distance were significantly differentp<0.001, with the exception of reaction times at 1m and 1.2 m.
To delimit exact peripersonal space boundaries, we fitted the data to the same sigmoidal function as described in Study 1. We performed a fitting on the averaged data of each agent grouping. This sigmoidal fitting found the average peripersonal space boundary to be around 1.36 m (1.35 m for the “average" agents,
1.39 m for the “intimidating" agents, and 1.33 m for the monster). Again, the goodness of fit measure (R2) was greater than0.9for all fits.
0.200.250.300.350.40
Distance (Meters)
Reaction Time (Seconds)
0.7 1 1.2 1.5 2
Reaction Time Per Distance
Normal Intimidating Monster
Figure 13: This graph depicts the average reaction time recorded at each perceived distance for average, intimidating and monster agents. Error bars represent the standard error of the mean. ©IEEE 2020
Additionally to determine if the peripersonal space boundaries were modulated by the agent type, we again performed Bonferroni corrected paired samples t-tests. These tests revealed a significant difference in the peripersonal space boundaries defined by the monster and the “intimidating" agentsp < 0.001, but there were no other significant results. Thus we performed Bayes factor analyses to further shed light on these results.
Comparing the “average" and “intimidating" agents generated a JZS Bayes factor of 3.11 substantially in favor of the null, comparing the “average" agents and the monster generated a JZS Bayes factor of 7.04 substantially in favor of the null, and comparing the “intimidating" agents and the monster a JZS Bayes factor of 6.57 substantially in favor of the alternative. These results indicated that the peripersonal space boundary was modulated by the most negatively perceived agent, or the monster.
4.3.6 Discussion
It is noteworthy that the agent had a significant effect on how users responded to the tactile stimulus. The results of the RM-ANOVA paired with the Mann-Whitney U Test results for the likeability survey are a clear
representation of the significant difference we see in reaction times. Reaction times to the “average" agents (0.224s, 0.272s, 0.251s, 0.331s, and 0.374s at 0.7 m, 1 m, 1.2 m, 1.5 m, and 2 m respectively) were slower than those for both the “intimidating" agents (0.215s, 0.250s, 0.244s, 0.307s, and 0.359s) and the monster (0.215s, 0.230s, 0.251s, 0.315s, and 0.348s). The results from the likeability survey demonstrated that the average female was the most desirable of all of the agents, since she was always viewed the most positively by survey participants. There were mixed results for the average male in comparison to the average female, as the average male was sometimes not seen as different than the intimidating female, male or both, but this result is similar to conclusions drawn from other works that the comfort distance of users of immersive virtual environments from males is extended further than that for females and that males are viewed as a more intimidating presence [Bailenson et al. 2003; Iachini et al. 2016; Yee et al. 2007]. The intimidating agents were most always viewed in a more negative light than the average female and sometimes the average male and were never different from one another, and the monster was always considered in the most negative light. Upon reviewing Figure 13 we can see a visual representation of the significance between the reaction times to different types of agents as discussed just above.
It is also notable that there is a shift here in the peripersonal space boundary from the first experiment.
The peripersonal space boundary has extended to 1.36 m here instead of the 1.27 m boundary found in the first experiment, showing that users of an IVE give more space between themselves and humanoid agents instead of objects. This result is also consistent with previous literature [Bailenson et al. 2003; Iachini et al.
2014; Llobera et al. 2010; Wilcox et al. 2006] that has found the same. It is worth mentioning that it may be assumed that the peripersonal space boundary may be modulated by the size of the agents since they are significantly larger than the balls used in the first experiment. However, prior literature has shown virtual reality users to modulate peripersonal space differently even when an object’s size is comparable to that of an agent [Bailenson et al. 2001; Sanz et al. 2015]. The peripersonal space boundary was modulated by the monster, but remained the same for all other agents when compared. While more work needs to be done to confirm and replicate the findings here, this gives the implication that there is some negative quality that is required in immersive virtual environments for peripersonal space boundaries to retract as has been shown in prior work [Coello et al. 2012; Pellencin et al. 2018; Ruggiero et al. 2017]. Body size, or the volume of the agents,
may have contributed to the decreased peripersonal space boundary since it has been shown that larger perceptions of body size decrease personal space [Phillips 1979; Sanders 1976]. But our results do not show a clear correlation since the monster has a body size intermediate between the intimidating male and the rest of the avatars.
Figure 13 also shows us that once the peripersonal space boundary is crossed, reaction times trend slower for both the average and intimidating agents. The reaction times for both of these agent groupings appear to be non-monotonic. The implication here is that depending on the social characteristics that are conveyed, there may be an initial acceptance of an agent once the peripersonal space boundary has been crossed. This may be tied to the idea that when a person breaches another’s peripersonal space, behavior is adjusted to respond appropriately to this new situation fast (i.e., a fight or flight situation). Under such circumstances, one’s capacity is tied up to cope with the emergency situation of having another body directly in the space around his or her own body. Distress could lead to an increase in reaction time and thus a decrease in attention and concentration in cognitive tasks Kato et al. [2009]. This trend did not happen with the monster, as the reaction times to this agent monotonically increase. The latter two findings, to our knowledge, have not been reported previously.