Polona Caserman

Michelle Martinussen

Stefan Göbel

Effects of End-to-end Latency on User Experience and Performance in Immersive Virtual Reality Applications

Keywords: Immersive Virtual Reality, End-to-end Latency, Cybersickness, User Experience, Simultaneity Perception, Body Ownership, Sense of Agency, Sense of Presence, Full-body motion reconstruction

Immersive virtual reality (IVR) offers an opportunity to immerse oneself into a virtual world and experience an exciting adventure. However, latency between a user's movement and visual feedback has a big impact on user experience and performance. In this paper, we explore the effect of increased end-to-end latency in IVR applications by conducting a user study. Firstly, in the searching task, we analyze cybersickness level based on simulator sickness questionnaire. Secondly, in the reaching task, we measure the user performance by tracking the time they need to reach a target and the error they make during the execution. Lastly, in the embodiment task, we measure the sense of body ownership, agency, presence, and latency perception when only one body side is impaired by latency. We apply the Friedman test with Conover's test of multiple comparisons as a post-hoc test on all dependent variables to find significant results. Results show that the end-to-end latency above 63 ms induces significant cybersickness symptoms. In addition, user performance decreases with increasing delay and with end-to-end latency above 69 ms, the users need significantly longer to complete the task. Results also show that end-to-end latency affects body ownership significant later, namely, not until 101 ms.

Introduction

Many users experience adverse effects such as cybersickness when wearing a Head-Mounted-Display (HMD). The reasons for these symptoms are the subject of research. One possible theory is the mismatch between the senses which provide information about the body's orientation and motion, also known as the sensory conflict theory [START_REF] Reason | Motion Sickness[END_REF]. Thus, increased cybersickness is often reported when the delay between user input and visual feedback is too high [START_REF] Kawamura | Effect of HMD Latency on Human Stability during Quiescent Standing on one Foot[END_REF][START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF] or when the movements are forced upon the users, e.g., in flight simulations [START_REF] Stanney | Cybersickness is Not Simulator Sickness[END_REF]. Shafer et al. [START_REF] Shafer | Factors Affecting Enjoyment of Virtual Reality Games: A Comparison Involving Consumer-Grade Virtual Reality Technology[END_REF] argue that with increasing sensory conflict, users will experience a higher level of cybersickness.

High end-to-end latency can induce cybersickness [START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF]. End-to-end latency in immersive virtual reality (IVR) refers to the time delay between a user's action and when this action is visible on the HMD. High delays also impair user performance and experience. Attig et al. [START_REF] Attig | System Latency Guidelines Then and Now -Is Zero Latency Really Considered Necessary[END_REF] state that the lower limit of 100 ms is too high and suggests to update the guidelines.

In this paper, we want to recommend appropriate end-to-end latency so that the researchers and developers can prevent or minimize cybersickness, maximize user performance, and ensure immersive user experience. Our main contributions in this paper are:

-We aim to identify the lower limit for the end-to-end latency in IVR at which the users report increased symptoms for cybersickness.

-We analyze the impact of end-to-end latency on the sense of body ownership, agency, and presence. It should be investigated which maximal delay the users tolerate without losing the joy of the IVR application.

The paper is structured as follows. In Section 2, we present selected related work. In Section 3, we describe the IVR tasks to measure the effects of increased end-to-end latency on user experience as well as performance and furthermore propose our hypotheses. We describe the evaluation method in Section 4. We show the results in Section 5 and discuss them in Section 6. Lastly, we conclude in Section 7.

Related Work

The work of Albert et al. [START_REF] Albert | Latency Requirements for Foveated Rendering in Virtual Reality[END_REF] reveals that participants, even if instructed to search for artifacts, could not find them if the latency was not higher than 50 to 70 ms. Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF] stated that motor performance and simultaneity perception are worsened significantly at end-to-end latency above 75 ms, whereas the sense of agency and body ownership only decline at latency higher than 125 ms. However, in their study, the participants performed motor tasks inside a Cave virtual environment and did not use IVE with a HMD. Similarly, Kasahara et al. [START_REF] Kasahara | Malleable Embodiment: Changing Sense of Embodiment by Spatial-Temporal Deformation of Virtual Human Body[END_REF] observe that with an end-to-end latency of 104 ms, the participants lose the sense of body ownership.

Kawamura and Kijima [START_REF] Kawamura | Effect of HMD Latency on Human Stability during Quiescent Standing on one Foot[END_REF] studied the effect of latency on the ability to stand on one foot in IVR. They used a Wii Balance Board to measure the stability of the participants and found that balance decreased with increased latency of already 1 ms. Attig et al. [START_REF] Attig | System Latency Guidelines Then and Now -Is Zero Latency Really Considered Necessary[END_REF] expressed a similar view. The researchers report that the users are indeed able to perceive latencies down to the single millisecond and furthermore state that performance already gets impaired by latencies between 16 to 60 ms.

Increased latency of 450 ms also negatively effects the ability to memorize objects in IVR [START_REF] Papadakis | The Effect of Tracking Delay on Awareness States in Immersive Virtual Environments: An Initial Exploration[END_REF]. Furthermore, Samaraweera et al. [START_REF] Samaraweera | Applying Latency to Half of a Self-Avatar's Body to Change Real Walking pPatterns[END_REF] investigated the impact of additional latency applied either to the left or right side of the avatar. The researchers found significant differences between the control (with base latency of 45 ms) and experimental group (end-to-end latency of 200 ms). However, only three out of ten participants noticed the added delay. Users in the experimental group walked slower towards the mirror and tried to compensate for the latency by increasing the step and swing time on the impaired side. Likewise, Meehan et al. [START_REF] Meehan | Effect of Latency on Presence in Stressful Virtual Environments[END_REF] argue that lower latency of 50 ms compared to 90 ms corresponds to a higher sense of presence.

Stauffert et al. [START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF] investigated the impact of latency jitter on cybersickness in IVR. In their study, almost half of the participants did not even notice latency jitter and the ones who did, found it to be annoying. The researchers furthermore found a significant correlation between heart rate and cybersickness. Similarly, Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF] also conclude that the users did not show cybersickness symptoms, even at end-to-end latency above 350 ms. Other works suggest that delays above 40 ms already evoke cybersickness and postural instability [START_REF] Dizio | Motion sickness side effects and aftereffects of immersive virtual environments created with helmet-mounted visual displays[END_REF].

Experimental Design

Tasks

We want to measure and compare the impact of different latency conditions on user experience and performance. In general, we apply latency to the virtual body and the HMD, controllers, as well as trackers. In other words, player movements are delayed with a certain amount of time. We analyze the impact of tracing delay in IVR to determine the upper latency limit that is still tolerated by users. Therefore, we developed three tasks:

Searching task is designed to measure cybersickness through the simulator sickness questionnaire (SSQ) [START_REF] Kennedy | Simulator Sickness Questionnaire: An Enhanced Method for Quantifying Simulator Sickness[END_REF]. As proposed by Stauffert et al. [START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF], the user has to find circular targets (platforms), spawned on the ground in one of three corridors. As soon as the user steps on one platform, a new one randomly spawns in another corridor. Thus, the user has to walk to find the next platform. Only one platform is present in the scene at the same time. The user is exposed to different artificial added delays, between 0 and 100 ms. In each round, the user has to find three platforms, one after another. After that, the user has to state how affected he/she is by each symptom on a four-point Likert scale: "None," "Slight," "Moderate," and "Severe." The scale is converted into integer numbers from 0 to 3. Finally, we calculate the final SSQ scale. Reaching task is designed to measure user performance, in particular, how much time a user needs to touch a cube appearing in a virtual scene and how many errors he/she makes during the execution. Similar to the searching task, we add delays between 0 and 100 ms. As proposed by Ware et al. [START_REF] Ware | Reaching for Objects in VR Displays: Lag and Frame Rate[END_REF], targets are spawned around the user in the field of view, at a distance so that he/she can reach them. The user has to sit on a stool and should use his/her dominant hand to reach the virtual target. As soon as their hand reaches the target, the virtual object disappears and a new one spawns randomly in the virtual environment. Thus, the users should reach the targets as fast as possible; however, they must avoid making mistakes. The user can access the object only once. If he/she fails, it is considered as an error. Embodiment task is designed to measure user experience while the users can observe themselves in a virtual mirror. As proposed by Samaraweera et al. [START_REF] Samaraweera | Applying Latency to Half of a Self-Avatar's Body to Change Real Walking pPatterns[END_REF], we apply latency to only one side of the body (either the left leg and left arm or the right leg and right arm). By using a seven-point Likert scale, we measure the sense of body ownership, sense of agency, sense of presence, and latency perception. The users are exposed to different artificial added delays, between 0 and 200 ms. The questionnaire consists of five questions, as listed in Table 1.

Table 1: Embodiment Questionnaire. Each question was assessed on a sevenpoint Likert scale: "Strongly Disagree" (-3) and "Strongly Agree" (3).

Condition Question Body Ownership It felt as if the virtual body in the mirror was my body.

Agency

My movements caused the movements of the virtual body in the mirror. Presence I felt as if I was really there. Impaired Side I noticed a difference in how the avatar responded to my movements between my left and right side. (If the participant answered the question with a value greater than 0, we furthermore asked which side was delayed. The participant could respond with "left',' "right side" or "not sure.") Simultaneity

The motion of the avatar was simultaneous to my movement.

Hypotheses

Studies on latency in IVR show that users performance may drop and they may notice latency smaller than 75 ms (see Section 2). However, studies evaluating the sense of agency and body ownership show that the users in IVR tolerate higher end-to-end latency between 90 and 200 ms.

For our experiment, we pose our hypotheses as follows:

H1: Higher end-to-end latency evokes cybersickness. H2: Higher end-to-end latency decreases user performance. H3: Higher end-to-end latency decreases user experience, in particular, sense of embodiment.

The evaluation of H1 is based on the SSQ, as proposed by Kennedy et al. [START_REF] Kennedy | Simulator Sickness Questionnaire: An Enhanced Method for Quantifying Simulator Sickness[END_REF]. Another way to detect cybersickness symptoms could be by using an ECG to measure the electrical activity of the heartbeat as proposed in our previous work [START_REF] Garcia-Agundez | Identifying Cybersickness through Heart Rate Variability Alterations[END_REF]. To test H2, we measure the time it takes for a user to reach a cube and the errors that the user makes. Furthermore, to test H3, we analyze an embodiment questionnaire on a seven-point Likert scale.

Method

We recruited 21 participants, aged between 19 to 53 years (M = 28.6 years, SD = 8.79, six females). At the beginning of the experiment, all participants filled out a demographics questionnaire and were informed about the risks.

The experiment was conducted on a laptop with an Intel i7-7700HQ processor and an Nvidia GeForce GTX 1080. This setup fulfills the minimal hardware requirements of the HTC Vive Pro. 1 The tracking area was approximately 3.1 × 2.9 m large. The application was built with Unity 3D on a Windows 10 system. We used an HTC Vive Pro HMD with two controllers and three trackers to enable full-body motion reconstruction. The users had to bind two trackers to left and right ankle as well as one on the hip. Additionally, they had to hold both controllers in the hands to track arm movements. One additional tracker was bound to a stool so that it can be shown in the IVR application.

Each participant could choose between two virtual bodies (a female and male avatar). After the avatar calibration, the participants then viewed a simple scene with a mirror in from of them and a stool, as shown in Figure 1. Thus, they could view their avatar from the first-person view, i.e., by looking down towards their body or by observing themselves in a virtual mirror. The study had a fully within-subjects design. Thus, all participants tested all tasks and all conditions. To ensure that the participants do not adapt to the latency during execution, we randomized the task order as well as latency conditions. The experiment started with a SSQ to provide a baseline measurement.

After that, a total of 165 (11 × 3 different platform positions in the searching task, 11 × 6 different cube positions in the reaching task; 11 × 6 tries in embodiment task) randomly conditions were presented. In searching and reaching task, we added a delay between 0 and 100 ms (0 ms, 4 ms, 8 ms, 13 ms, 19 ms, 25 ms, 33 ms, 42 ms, 54 ms, 71 ms, 100 ms). In the embodiment task, we added higher delays between 0 and 200 ms (0 ms, 8 ms, 17 ms, 27 ms, 38 ms, 51 ms, 66 ms, 84 ms, 108 ms, 142 ms, 200 ms).

At the end of each latency condition in the searching task, the participants had to complete the SSQ. The surveyor read the questions and noted the participant's responses. In the reaching task, the participant did not need to complete any questionnaire. Instead, we measured how long each participant needed to grab the cube and how many errors he/she made. In the embodiment task, the participants again had to complete a questionnaire (see Table 1). Note that the order of the tasks and different latency conditions were randomly selected for each participant. Exposure time was approximately 55 minutes.

Results

Base End-to-end Latency

We firstly measured the system latency of our setup by using a frame counting approach, as proposed by Friston and Steed [START_REF] Friston | Measuring Latency in Virtual Environments[END_REF]. The end-to-end latency is the delay between the player's movement until the corresponding avatar movement is displayed on the HMD. Due to the calculation of inverse kinematics for fullbody motion reconstruction, our base latency is slightly higher than those found in related work, which is often between 22 and 36 ms [START_REF] Albert | Latency Requirements for Foveated Rendering in Virtual Reality[END_REF][START_REF] Niehorster | The Accuracy and Precision of Position and Orientation Tracking in the HTC Vive Virtual Reality System for Scientific Research[END_REF][START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF]. Other similar studies using full-body motion capturing reported higher end-to-end latencies, between 45 and 80 ms [START_REF] Kasahara | Malleable Embodiment: Changing Sense of Embodiment by Spatial-Temporal Deformation of Virtual Human Body[END_REF][START_REF] Kondo | Illusory Body Ownership of an Invisible Body Interpolated Between Virtual Hands and Feet via Visual-Motor Synchronicity[END_REF][START_REF] Samaraweera | Applying Latency to Half of a Self-Avatar's Body to Change Real Walking pPatterns[END_REF][START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF]. We measured end-to-end system latency of approximately 50 ms.

Cybersickness

To evaluate the level of cybersickness, we calculate the final SSQ scores at the end of each sub-task. Figure 2 shows the final SSQ scores for different end-to-end latency conditions. Error bars represent 95% confidence intervals of the mean.

Results of the Friedman test show that higher end-to-end latency has a significant effect on final SSQ scores (χ 2 (2) = 100.6, p ≤ .001, Kendall's W = .57). The internal reliability for cybersickness was very high (Cronbach's α = .922). Post-hoc analysis with Conover's test and α = .05 reveals significant differences between 50 ms and ≥ 75 ms, 54 ms and ≥ 75 ms, 58 ms and ≥ 75 ms, 63 ms and ≥ 83 ms, 69 ms and ≥ 75 ms, 75 ms and ≥ 121 ms, 83 ms and 150 ms, 92 ms and 150 ms, as well as 104 ms and 150 ms. Thus, the results show that higher latency values tend to correspond to higher final SSQ scores.

The results furthermore show significant lower SSQ score before the users put on the HMD (0 ms) and after they experienced end-to-end latency of 75 ms (p < .001). Thus, these findings indicate that users will experience sufficient discomfort at end-to-end latency above 75 ms. As it can be seen in Table 2, the reported final SSQ score at 75 ms is already > 22. Stanney et al. [START_REF] Stanney | Cybersickness is Not Simulator Sickness[END_REF] also complement these findings, stating that the final SSQ scores of > 20 indicate that such a system is troublesome.

Table 2: Final SSQ scores prior the users put on the HMD (0 ms) and after each end-to-end latency condition (50 -150 ms). Moreover, Stanney et al. [START_REF] Stanney | Cybersickness is Not Simulator Sickness[END_REF] report that users who report symptoms which are already higher than 15 on the SSQ are experiencing sufficient cybersickness. Comparing this value with the measurement results, the users reported increased symptoms at end-to-end latency above 63 ms (see Table 2). Thus, to minimize cybersickness symptoms, end-to-end latency of below 58 ms (< 15 on SSQ) is required.

User Performance Results

To evaluate user performance, we measure the time the users need to complete the task successfully. Figure 3 shows the average time needed to reach a cube while users were exposed to different end-to-end latencies. Error bars represent 95% confidence intervals of the mean. Results of the Friedman test show that higher end-to-end latency has a significant effect on time, needed to complete the task (χ 2 (2) = 68.71, p ≤ .001, Kendall's W = .658). The internal reliability of the scale was high (Cronbach's α = .872.) Post-hoc analysis with Conover's test and α = .05 results in significant differences between 50 ms and ≥ 69 ms, 54 ms and ≥ 92 ms, 58 ms and ≥ 92 ms, 63 ms and ≥ 104 ms, 69 ms and ≥ 121 ms, 75 ms and ≥ 121 ms, 83 ms and ≥ 104 ms, 92 ms and ≥ 121 ms, and 104 ms and 150 ms. In other words, with the minimum end-to-end latency (i.e., 50 ms), users could complete the task faster than with added delay. Thus, the results show that higher latency values tend to require more time to complete the task. By observing Figure 3, we can see that higher delay worsen user performance. Looking at the findings from the post-hoc test indicate that user performance drops at 69 ms. The data in Table 3 shows that mean needed time values at endto-end latency of 69 ms and 104 ms are nearly the same. Thus, user performance gets worse with increased delay and most users need significant longer to fulfill the task at end-to-end latency above 69 ms.

Even though the users were allowed to touch the virtual cubes only once, we could not identify any mistakes. We believe that the reaching task is very simple to complete and that more complex tasks would lead to higher chances of performance errors.

Embodiment Results

We conducted a user study to furthermore measure the sense of presence, agency, and body ownership at different end-to-end latency conditions. We calculated the median (MED) and Inter-Quartile Range (IQR) for each question, as it can be seen in Table 4 and Figure 4. Each question was assessed on a seven-point Likert scale: -3 for "Strongly Disagree" and 3 for "Strongly Agree."

The internal reliabilities for embodiment measurements are high. Cronbach's α for embodiment ranges between .706 (impaired side), .869 (simultaneity), .953 (sense of agency), .988 (sense of presence), and .992 (sense of body ownership). For the sense of agency (MED = 3) and for the sense of presence (MED = 2), we could not find any significant differences. These findings indicate that end-to-end latency of up to 250 ms does not cause a significantly lower sense of agency or presence.

However, results also show that end-to-end latency above 101 ms will decrease the sense of body ownership (see Figure 4a). Indeed, body ownership significant decreases for end-to-end latency above 192 ms. Thus, the sense of body ownership decreases with increasing end-to-end latency. Furthermore, with end-to-end latency above 67 ms, one user already noticed different latencies between the left and right side and also correctly specified the impaired side. As expected, with higher end-to-end latency the users not only perceived increased latency but also correctly specified the side (either left or right body side) which was influenced by additional delay (see Figure 4b). At end-to-end latency of 250 ms, almost half of the users (n = 10) perceived the delay and could correctly specify the impaired side. Similarly, with an end-to-end latency of 116 ms and ≥ 158 ms, the users perceived the movements of the avatar not simultaneously to their movements.

Discussion

With the significant results of the SSQ comparing pre-and post-conditions, we accept hypothesis H1. Thus, increased end-to-end latency will evoke cybersickness symptoms. Stauffert et al. [START_REF] Stauffert | Effects of Latency Jitter on Simulator Sickness in a Search Task[END_REF] expressed a similar view. The researchers evaluated latency jitter and found that the disorientation sub-scale was higher in the affected group, although nearly half of the users did not notice the repeated lag in the tracking of the HMD. In contrast to Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF], we found significant results at the end-to-end latency of 75 ms. However, as suggested by Stanney et al. [START_REF] Stanney | Cybersickness is Not Simulator Sickness[END_REF], we want to satisft the desire of keeping the value beneath 15 on the SSQ. Therefore, we recommend that the researchers and developers should keep end-to-end latency well below 58 ms (see Section 5.2). Our guidelines also confirm those proposed by Attig et al. [START_REF] Attig | System Latency Guidelines Then and Now -Is Zero Latency Really Considered Necessary[END_REF], according to which the lower limit of 100 ms seems to be outdated. Moreover, we indeed recommend keeping the end-to-end latency below 20 ms as suggested by DiZio and Lackner [START_REF] Dizio | Motion sickness side effects and aftereffects of immersive virtual environments created with helmet-mounted visual displays[END_REF]. Furthermore, with the significant results for user performance, we also accept hypothesis H2. Thus, with higher end-to-end latency, the user performance will decrease. In particular, users will need more time to complete the task. Compared to Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF], stating that motor performance is affected by latency above 75 ms, we obtain similar results. The results show that the higher delays worsen user performance (see Section 5.3). Although the users typically need more time to complete the reaching task with increased end-to-end latency, we could not identify significant differences until 69 ms.

Hypothesis H3 cannot be accepted. We found no significant results for the sense of agency and presence. However, with increased end-to-end latency, body ownership and simultaneity perception will decrease. As already noted by Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF], the delay affects body ownership significantly later, between 125 ms and 210 ms. Our results show that the sense of body ownership decreases earlier, already at 101 ms; however, we could not find significant differences until 192 ms. Similarly, Waltemate et al. [START_REF] Waltemate | The Impact of Latency on Perceptual Judgments and Motor Performance in Closed-loop Interaction in Virtual Reality[END_REF] stated that simultaneity perception was affected by latency above 75 ms. Our results show that simultaneity perception dropped at latency values higher than 116 ms.

In contrast to work from Kondo et al. [START_REF] Kondo | Illusory Body Ownership of an Invisible Body Interpolated Between Virtual Hands and Feet via Visual-Motor Synchronicity[END_REF], not all our participants were able to identify the impaired side even not in the 250 ms end-to-end latency condition. Nevertheless, with the increased delay, more and more users identified the impaired side correctly. With end-to-end latency of 67 ms only one user and with an end-to-end latency of 250 ms total of ten users were able to specify the impaired side correctly.

Conclusion

This study aimed to gain insight into the effect of latency on user experience and performance in IVR by artificially increasing the end-to-end latency. We designed three different tasks to evaluate which impact increased latency has in IVR. For quantitative results, we used the Friedman test with Conover's test of multiple comparisons as a post-hoc test. We found significant differences when comparing final SSQ scores between several end-to-end latency conditions. Increased latency negatively affects user experience, e.g., users reported significant cybersickness symptoms with end-to-end latency above 63 ms. Thus, to ensure final SSQ scores of < 15 and minimize cybersickness symptoms, the researchers and developers should ensure end-to-end latency far below 58 ms.

Furthermore, also user performance decreases with higher end-to-end latency. The results show that latency above 69 ms causes significant longer execution time. The results suggest that the sense of body ownership does accept higher latency values than user performance and cybersickness before significant results can be found. Sense of body ownership decreases with end-to-end latency above 101 ms; however, significant results were not found until 192 ms. We could not find any significant results for the sense of presence and agency. In general, with increasing end-to-end latency, user experience decreases and user performance worsens. We conclude that latency between perceived and physical head movements does not only contribute strongly to cybersickness but also decrease user performance and sense of embodiment.

In future work, we will focus on physiological data, such as ECG to measure the electrical activity of the heart. We believe that this will detect the cybersickness symptoms easier than subjective measurements of a SSQ. Furthermore, the task complexity and duration should be increased and breaks should be included between latency conditions, to avoid carry-over effects.

Fig. 1 :

 1 Fig.1:The virtual environment, used for our experiment. (a) The search task consists of three corridors, where the users have to find platforms, spawned on the ground in one of three corridors. (b) In the reaching tasks, the users sit on a stool and have to touch virtual targets, spawned around them. (c) In the embodiment task, the users have to observe themselves in a virtual mirror, to find the impaired side. Note that the player can see the virtual body from the first-person view.

Fig. 2 :

 2 Fig. 2: Final SSQ scores at different end-to-end latency conditions. Error bars show 95% confidence interval of the mean. The values above the Friedman threshold contribute to the significant cybersickness symptoms. Indeed, values above Stanney et al. threshold already indicate too high SSQ scores.

Fig. 3 :

 3 Fig.3: Mean needed time for different end-to-end latency conditions. Error bars show 95% confidence interval of the mean. At values above the Friedman threshold, users need significantly longer to complete the reaching task.

Fig. 4 :

 4 Fig. 4: Median responses for the embodiment questionnaire at different end-toend latency conditions.

Table 3 :

 3 Mean needed time for each end-to-end latency condition.

	End-to-end Latency [ms]	50	54	58	63	69	75	83	92	104 121 150
	Mean [ms]	16.5 16.6 16.9 17	17.3 16.9 16.9 17	17.2 17.5 17.7
	SD [ms]	.7	.7	1.5	1.9	1.9	.8	.9	1.2	.9	1.3	1.1

Table 4 :

 4 The table depicts the median (MED) answers for embodiment questionnaire for each end-to-end latency condition. Inter-Quartile Range (IQR) values furthermore indicate the variability of the questionnaire responses.

	End-to-end Latency [ms]	50	58	67	77	88	101 116 137 158 192 250
	MEDagency		3	3	3		3	3	3	3	3	3	3	3
	IQR agency		1	1	1		1	1	1	1	1	1	1	1
	MEDpresence		2	2	2		2	2	2	2	2	2	2	2
	IQR presence		2.5 2	2		2.5 2	2	2	2.5 2	2.5 4
	MED body ownership 2	2	2		2	2	1	1	1	1	1	0
	IQR body ownership 2.5 3	3.5 3	3	3	3.5 4	4	3	3
	MED simultaneity	2	2	2		2	2	2	1	2	1	0	0
	IQR simultaneity	1	1	1		1	1	1	1.5 0	1	2	1
	MED impaired side -3	-3	-3	-3	-3	-2	-3	-2	0	-2	2
	IQR impaired side	1.5 1	1		1.5 1	2.5 1.5 3	4	5.5 5.5
		3
		2
	7-point Likert scale	-1 0 1
		-2
		-3
		50 58 67 77 88	101	116	134	158		192			250
							End-to-end Latency [ms]
			Body Ownershi p	Agency	Presence	Simultaneity	Impaired Side
	(a) Sense of body ownership, agency, presence, simultaneity, and im-
	paired side.
		10
		8
	Numer of users	4 6
		2
		0
		58 67 77 88	101	116	134	158		192			250
							End-to-end Latency [ms]
					Correct	Not-Sure	Wrong
	(b) Number of users who identified the impaired side correctly, wrong
	or were unsure.

https://www.vive.com/us/support/vive-pro-hmd/category howto/ what-are-the-system-requirements.html


Manuella Vieira
email: manuellavr7@gmail.com

AutomataMind: A Serious Game Proposal for the Automata Theory Learning

Keywords: Game based learning, Automata theory, Serious games

Digital games facilitate the teaching-learning process, arousing the interest of students in various areas of competence. This paper introduces AutomataMind, a game that seeks to stimulate the player logical thinking skills, while presenting the concepts of Computer Theory with emphasis on Automata Theory in a playful and challenging way.

Introduction

Educational software is a significant element in the construction of a knowledge base for students [START_REF] Bernardi | Utilização de softwares educacionais nos processos de alfabetização, de ensino e aprendizagem com uma visão psicopedagógica[END_REF]. In addition, digital games have been designed for teaching purposes to engage and motivate the players into learning activities [START_REF] Klopfer | Moving learning games forward: Obstacles, opportunities & openness: An educational arcade paper[END_REF].

Studies have shown that many difficulties experienced by students when learning Theory of Computation are related to the lack of logical thinking and problem-solving skills [START_REF] Pillay | Learning difficulties experienced by students in a course on formal languages and automata theory[END_REF], as well as a sense of monotony [START_REF] Silva | Automata defense 2.0: reedição de um jogo educacional para apoio em linguagens formais e autômatos[END_REF] that is caused by the great number of complex and abstract concepts involved [START_REF] Devedzic | An intelligent tutoring system for teaching formal languages[END_REF].

As a gamified approach to introduce concepts related to the Automata Theory in a playful way, this paper presents "AutomataMind ", an educational game based on the Mastermind board game that seeks to stimulate the player logical reasoning skills through proposed automata challenges.

Related Work

As an example of digital game related to the Automata Theory learning, Automata Defense 2.0 [START_REF] Silva | Automata defense 2.0: reedição de um jogo educacional para apoio em linguagens formais e autômatos[END_REF] is a tower defense game whose objective is to stop a variety of creatures, each one associated with a word, from reaching the map exit. To achieve this goal, the player must design an automaton and assign it to a tower. Each creature that carries a word accepted by a tower automaton is attacked when it moves inside of that tower coverage area. The player is able to position towers in a strategic way, so that they can either attack or slow the creatures movements.

Another game example is the "Chomsky's Mountain" [START_REF] Leite | Montanha de chomsky: jogo tutor para auxılio no ensino de teoria da computaç ao[END_REF], where the player goal is to reach the top of a mountain by solving problems that represent the "enemies" that the user must defeat. To do this, the player must build a computing model that matches the theme of the current level of the game. Each level refers to languages associated with two types of grammar from Chomky's Hierarchy: Regular Languages and Context Free Languages.

Regarding the Mastermind game, whose game dynamics will be applied in AutomataMind game, it is a code-breaking game that challenges the players by prompting them to guess a specific pattern -also referred to as "the code"usually composed of a combination of pegs with different colors. Every time the player takes a guess about the code, a feedback is provided showing how close the player's attempt to the actual code was.

The AutomataMind Game

AutomataMind was designed by integrating Mastermind game play aspects with Automata Theory concepts. The game was structured in the following manner: each level of the game starts with an automaton being shown on the screen. Then, a string -a sequence of characters -that can be accepted by that automaton is generated. This sequence remains hidden and represents the code that should be "broken" by the player.

The player must try to guess what the secret sequence is, based on the automaton that is being presented on the current level screen. Every time the player takes a guess on what the code might be, a certain amount of information regarding the current guess is provided, so that the player next guess can be improved based on that feedback.

There is a limited number of guesses that can be made during a level. Therefore, if the player reaches the limit without having guessed the correct sequence, the stage restarts from the beginning, with a new code that may or may not be similar to the prior one (the sequence selection process is random). The game main menu (Fig. 1) was designed as a graphic representation of a finite automaton that has a lock as its final state, implying that in order to find the correct code, the player guesses must necessarily be able to reach the automaton accept state. For the menu buttons, three options are provided to the player: Read the instructions, Play and Exit the game.

Fig. 2 illustrates one game level, where the automaton at the top of the screen is used as the player reference to discover the hidden sequence. The pegs in the right side of the screen are used to provide feedback about the player guess, where a red peg is placed for each character that is in the same position as a similar character on the secret code. The white peg indicates the presence of a matching character -a character that can be found in both the guess and the secret code -but that is placed in the wrong position. The green confirmation indicates that the string inserted by the player is accepted by the automaton (meaning it ends in a final state) even though it might not be the correct code. AutomataMind is also divided into levels with increasing degrees of difficulty, ranging from: number of characters used in the secret code; number of symbols belonging to the automaton alphabet; and number of states and transitions the automaton has. Initially, only the first level of the game is available to the player, and to unlock a specific level, the player must successfully complete the previous one (Fig. 3).

The score mechanism takes into account the number of red pegs the player obtained while playing a level, as well as a multiplier that is inversely proportional to the amount of guesses made. Thus, a greater amount of points is given when the player takes a less amount of guesses to get to the correct sequence.

Conclusion and Future Work

This paper presented the AutomataMind, an educational game capable of helping students to develop logical reasoning skills while learning about the Theory of Computation. The developed game is a simple solution that has the possibility of contributing to the scenario of games aimed at teaching topics related to the theoretical aspects of computing.

As AutomataMind is an on-going project (available at https://github.com/ lenda-uefs), an usability evaluation with undergraduates related to computer science will be performed in the future. The inclusion of Pushdown automata and Turing machines topics in future levels of the game will also be performed for challenging and learning purposes.

Fig. 1 .

 1 Fig. 1. Title screen of the AutomataMind game.

Fig. 2 .

 2 Fig. 2. Game play screen of the AutomataMind game.

Fig. 3 .

 3 Fig. 3. Level selection screen of the AutomataMind game.


Murilo C Camargo

Rodolfo M Barros
email: rodolfomdebarros@gmail.com

Jacques D Brancher
email: jacques@uel.br

Vanessa T O Barros
email: vanessa@uel.br

Matheus Santana

Designing Gamified Interventions for Autism Spectrum Disorder: A Systematic Review

Keywords: Gamification, Autism Spectrum Disorder, Systematic Review

Serious games and gamified interventions have become increasingly popular among researchers and therapists dealing with the autistic audience. The number of studies on technology for autism has multiplied, with the aim to foster independence and improve learning outcomes. Nevertheless, designing interventions for Autism Spectrum Disorder is challenging, due to the complex clinical conditions and the broad range of symptoms covered by the disturbance. Thus, this systematic review investigates the current status of gamification resources for autism, with a special interest in the gamification elements and the User Interface design. We describe the planning and the searching procedures and present the data extracted from 30 primary sources. The studies analyzed show a multitude of gamification elements and a plethora of methods and strategies to support decision-making and improve accessibility in the development of autism-specific software. It is concluded that the existence of methodological gaps related to the definition of the target audience and the conduction of testing may impose additional challenges to the development process, whilst the combination of gamification elements is generally positive.

Introduction

According to the Diagnostic and Statistical Manual of Mental Disorders -DSM-5 [START_REF]Diagnostic and Statistical Manual of Mental Disorders, DSM-5. Artmed[END_REF], autism is characterized by persistent limitation in social communication/interaction, restrictive and repetitive behavioral patterns, and (verbal and non-verbal) communication deficits. As the autism diagnosis evolved to cover a broader spectrum of symptoms, there is emerging evidence of increasing epidemic rates. Numbers may vary depending on the metrics used for evaluation, but there are studies reporting on rates as high as 1 in 150 children being diagnosed with Autism Spectrum Disorder (ASD) [START_REF] Venkatesh | Cognitive intervention in autism using multimedia stimulus[END_REF]. Accessibility has played an important role in providing autistic individuals with opportunities to overcome their limitations by fostering independence and assisting them in social relationships [START_REF]Accessibility and development: mainstreaming disability in the post-2015 development agenda[END_REF]. Despite the fact that much information on autism etiology remains to be uncovered, accessibility advocates (researchers, psychologists, therapists, teachers, among others) have been thriving to propose feasible and interesting intervention solutions, which can be confirmed by the growing number of computer-based resources published and available to the community.

Serious games and gamification strategies have been largely used in the treatment of psychiatric conditions, such as depression, eating disorders, substance use, dementia, and, also, Autism Spectrum Disorder [START_REF] Chandran | Gamifying education and mental health[END_REF]. Gamification is a powerful resource to increase motivation and engage participants, thus favoring the teaching-learning process or training for specific situations. Design elements of serious games and/or gamified interfaces often include storylines, mid-term and long-term goals, increasing level of difficulty, feedback and/or rewards, and provision of choice [START_REF] Whyte | Designing serious game interventions for individuals with autism[END_REF].

Recent studies suggest improvements in handling disorder-related symptoms through the use of serious games or gamified interventions [START_REF] Lau | Serious games for mental health: are they accessible, feasible, and effective? A systematic review and meta-analysis[END_REF]. The same applies to autism: gamification may be a potentially effective approach due to the high visual processing skills the ASD individuals possess. On the other hand, developers may face challenges. According to [START_REF] Whyte | Designing serious game interventions for individuals with autism[END_REF], little evidence exists that ASD individuals subjected to computer-based interventions are able to apply such learning to real life. This can be partly explained by the lack of fundamental gamification elements during the design process.

In light of this situation, there is an undeniable need to understand the current status of serious gaming targeted to Autism Spectrum Disorder. Through a systematic review, this paper collects data on serious games and/or gamified interventions for autism, with a focus on the gamification elements used and their visual representation in terms of the User Interface design. We believe that this study may help uncover best practices used and shed light on the graphic design approach to deliver them at the same time.

This paper is organized as follows: in section 2, the review protocol is presented, including research questions, inclusion criteria, search strings, and databases; in section 3, results from the review are described and discussed; finally, section 4 brings conclusions and future perspectives regarding the design of gamified interventions for the autistic audience.

Review Protocol

This systematic review applies a three-phase process, including the design of the review protocol, the conducting of the search, and the data extraction [START_REF] Kitchenham | Procedures for performing systematic reviews[END_REF]. We search, identify, analyze, and summarize evidence on computer-based gamification resources targeted to the ASD audience. In order to prevent biased or mixed results, the scope is limited to resources in which the User Interface is the primary output channel. Robotics, hardware-only ubicomp solutions, or any interface-less devices are not included in this study, because their examination would require a completely alternative approach.

Preliminary studies and consultations with field experts helped pilot the review protocol. The protocol was designed to provide comprehensive guidance through the review process, with respect to the inclusion criteria and the nature of the results. It intends to promote the identification of all relevant primary sources and assist the data extraction process at the same time. The research questions are: what is/are the learning objectives? What is the development process/methodology/approach employed? Who is the target audience? What gamification elements are used to improve learning skills? What representation methods are applied in the User Interface design?

Then, a list of keywords was defined to support the searching process. Keywords should cover three aspects of major importance, namely: (1) the target audience, that is, autistic individuals; (2) the object of study -software development; and (3) the intervention approach -computer-based serious games or gamified interventions. Similar keywords were grouped to form a search string, using Boolean "ANDs" and "ORs": ("Autism spectrum disorder" OR "autistic individuals" OR "autism" OR "high-functioning autism") AND ("computer-assisted learning" OR "computerassisted intervention" OR "computer-enhanced learning" OR "therapy software") AND ("serious games" OR "gamification" OR "gamified resources").

In order to be included, papers had to adhere to the sensitivity inclusion criteria: describe a novel contribution to computer-based serious games for ASD; have the software development as the primary object of study; provide relevant information on the gamification elements used during development; and rely on the User Interface as the main output channel. Secondary studies and hardware-only interventions were not considered. Also, publications older than 10 years were discarded, thus prioritizing the recent literature.

Six sources were selected in this study: (1) IEEE Xplore Digital Library, (2) ACM Digital Library, (3) SpringerLink, (4) SCOPUS, (5) ScienceDirect, and (6) Google Scholar. Together, they grant access to a large collection of relevant resources for Software Engineering. Besides, important journals on autism research can be accessed through these sources, such as the Journal of Autism and Developmental Disorders (Springer) and the Autism (SCOPUS).

Searching was performed using a combination of the search string described previously. Multiple trials of the keywords were performed in order to reach as many primary studies as possible. The search resulted in a combined total of 2,995 papers, which were downloaded. After the exclusion of duplicated papers, the total narrowed down to 2,642 studies. Then, two assessors examined titles and abstracts to determine the relevance of the subject matter. Papers that failed to meet the inclusion criteria were eventually withdrawn from this review, resulting in 30 eligible studies.

Results and Discussion

All 30 selected papers were carefully examined according to the review protocol. Data extracted was compiled in quality reports and double-checked whenever assessors had conflicting recommendations. Quality reports included data on the learning objectives, the team organization, the software development process applied, the target audience, gamification elements, and the User Interface design.

The selected papers present a variety of learning objectives. Many aim to improve communication skills (46.6%), mostly by teaching new vocabulary and phrases while improving semantic meaningfulness. Other learning objectives are social interaction (13.3%), basic education (10%), and daily activities (10%). Two studies focused on facial recognition [START_REF] Tan | Can you CopyMe?: an expression mimicking serious game[END_REF][START_REF] Washington | SuperpowerGlass: A wearable aid for the at-home therapy of children with autism[END_REF] and only one aimed at refining motor skills [START_REF] Finkelstein | Astrojumper: Motivating children with autism to exercise using a VR game[END_REF]. Six of them (20%) presented multiple learning purposes [START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Konstantinidis | Using affective avatars and rich multimedia content for education of children with autism[END_REF][START_REF] Hourcade | Multitouch tablet applications and activities to enhance the social skills of children with autism spectrum disorders[END_REF][START_REF] Venkatesh | TOBY: Early intervention in autism through technology[END_REF][START_REF] Yan | A sunny day: Ann and Ron's world an iPad application for children with autism[END_REF][START_REF] Malinverni | An inclusive design approach for developing video games for children with autism spectrum disorder[END_REF]. All of them involved the major impairments faced by ASD individuals: difficulty of socialization, imagination impairment, communication deficit, and behavioral issues [START_REF] Silva | Star: Speech therapy with augmented reality for children with autism spectrum disorders[END_REF]. Popular development approaches are the Participatory Design [START_REF] Wadhwa | Collaborative tablet applications to enhance language skills of children with autism spectrum disorder[END_REF][START_REF] Hourcade | Multitouch tablet applications and activities to enhance the social skills of children with autism spectrum disorders[END_REF][START_REF] Hayes | Interactive visual supports for children with autism[END_REF][START_REF] Malinverni | An inclusive design approach for developing video games for children with autism spectrum disorder[END_REF] and the User-Centered Design [START_REF] Alvarado | Valpodijo: Developing a software that supports the teaching of Chilean idioms to children with autism spectrum disorders[END_REF][START_REF] De Leo | A smart-phone application and a companion website for the improvement of the communication skills of children with autism: Clinical rationale, technical development and preliminary results[END_REF][START_REF] Winoto | A highly customizable parent-child wordlearning mobile game for Chinese children with autism[END_REF][START_REF] Signore | You talk! -you vs[END_REF]. Both are techniques that aim to involve external help (experts, parents, teachers, and/or end-users) to assist the development process. Other approaches mentioned are the Applied Behavior Analysis [START_REF] Silva | Star: Speech therapy with augmented reality for children with autism spectrum disorders[END_REF], the ADDIE approach (Analysis, Design, Development, Implementation, Evaluation) [START_REF] Kurniawan | Development of game for self-help toilet learning for children with autism[END_REF], and the Extreme Programming [START_REF] Fotjik | Agile methodology and development of software for users with specific disorders[END_REF]. Some studies reported on the use of an "iterative" [START_REF] Konstantinidis | Using affective avatars and rich multimedia content for education of children with autism[END_REF][START_REF] Tan | Can you CopyMe?: an expression mimicking serious game[END_REF][START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF][START_REF] Shminan | AutiPECS: Mobile based learning of picture exchange communication intervention for caregivers of autistic children[END_REF][START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF][START_REF] Zaffke | iCanLearn: A mobile application for creating flashcards and social stories for children with autism[END_REF][START_REF] Cabielles-Hernández | Specialized intervention using tablet devices for communication deficits in children with autism spectrum disorders[END_REF] and/or "agile" approach [START_REF] Shminan | AutiPECS: Mobile based learning of picture exchange communication intervention for caregivers of autistic children[END_REF][START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF][START_REF] Barros | Interactive and customizable use of mobile technology in the cognitive development process of children with autistic spectrum disorder[END_REF], although they did not provide detailed information on it.

The team organization is either cross-functional or participatory. Cross-functional teams are composed by the development staff and stakeholders, who are deeply involved in the development lifecycle. On the other hand, participatory teams also engage stakeholders, but they are involved only in specific moments during the process (requirements specification or testing, for example). In any case, this reveals an effort to understand the needs and preferences of the end-user. Two studies did not provide any information on the methodological approach or the team organization [START_REF] Finkelstein | Astrojumper: Motivating children with autism to exercise using a VR game[END_REF][START_REF] Tsiopela | Pre-vocational skills laboratory: designing interventions to improve employment skills for students with autism spectrum disorders[END_REF].

The vast majority of studies analyzed (76.6%) refer to their target audience simply as "autistic children" or some variation of the term. Only four studies report on the development of software for severity-specific audiences -that is, high-functioning and/or low-functioning autism [START_REF] Winoto | A highly customizable parent-child wordlearning mobile game for Chinese children with autism[END_REF][START_REF] Iradah | EduTism: An assistive educational system for the treatment of autism children with intelligent approach[END_REF][START_REF] Malinverni | An inclusive design approach for developing video games for children with autism spectrum disorder[END_REF][START_REF] Barros | Interactive and customizable use of mobile technology in the cognitive development process of children with autistic spectrum disorder[END_REF]. Two studies (6.6%) referred to the audience as "autistic individuals", claiming that the proposed intervention would serve for ASD adolescents or adults as well [START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF][START_REF] Tsiopela | Pre-vocational skills laboratory: designing interventions to improve employment skills for students with autism spectrum disorders[END_REF]. There is also one study [START_REF] Fotjik | Agile methodology and development of software for users with specific disorders[END_REF] that designed an intervention for children with "specific disorders", such as ASD, Down syndrome, mental retardation, etc. According to the author, this is possible due to the

✔ ✔ ✔ ✔ ✔ ✔ C [12] ✔ ✔ ✔ ✔ ✔ ✔ ✔ [15] ✔ ✔ ✔ ✔ ✔ ✔ [16] ✔ ✔ ✔ ✔ ✔ [20] ✔ ✔ ✔ ✔ ✔ ✔ ✔ [21] ✔ ✔ ✔ ✔ ✔ ✔ [22] ✔ ✔ ✔ ✔ ✔ ✔ [23] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [24] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [26] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [29] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [30] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [31] ✔ ✔ ✔ ✔ ✔ ✔ [35] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [9] ✔ ✔ ✔ ✔ ✔ ✔ M [13] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [17] ✔ ✔ ✔ ✔ ✔ ✔ ✔ [19] ✔ ✔ ✔ ✔ ✔ ✔ [25] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [32] ✔ ✔ ✔ ✔ ✔ ✔ [16] ✔ ✔ ✔ ✔ ✔ SI [26] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [27] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [34] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [8] ✔ ✔ ✔ ✔ ✔ ✔ BE [28] ✔ ✔ ✔ ✔ ✔ ✔ [37] ✔ ✔ ✔ ✔ ✔ ✔ [30] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ DA [33] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [36] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [14] ✔ ✔ ✔ ✔ ✔ ✔ ✔ FR [18] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ [10] ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ MS
Table 1 summarizes data on the gamification elements applied in the software and the User Interface design, that is, what visual representation methods were used to deliver the interface. Also, it clusters the primary studies according to the learning objective(s) they aim to achieve. "C" stands for Communication; "M" is Multiple, meaning that the study intended to cover more than one learning objective at the same time; "SI" means Social Interaction; "BE" means Basic Education; "DA" stands for Daily Activities; "FR" means Facial Recognition; and "MS" is an abbreviation for Motor Skills.

Gamification Elements

According to Table 1, many of the studies use feedback (50%) and/or rewarding systems (33%) to appeal to the audience. Those can be either virtual output stimuli, such as visual cues or auditory feedback, or real-life tokens (a fruit, a game, or anything that the user would associate with having positively accomplished a task). Rewards have been largely used in ASD-specific interventions, often applied immediately after the task completion, in order to reinforce certain behavioral patterns [START_REF] Barros | Interactive and customizable use of mobile technology in the cognitive development process of children with autistic spectrum disorder[END_REF].

Four studies applied Virtual Reality environments within the software [START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Finkelstein | Astrojumper: Motivating children with autism to exercise using a VR game[END_REF][START_REF] Winoto | Look to remove: A virtual reality application on word learning for Chinese children with autism[END_REF][START_REF] Silva | Star: Speech therapy with augmented reality for children with autism spectrum disorders[END_REF]. There has been increasing interest in using VR for autistic intervention, since this technology provides an immersion environment that improves interventionist control and promotes engagement [START_REF] Finkelstein | Astrojumper: Motivating children with autism to exercise using a VR game[END_REF]. In addition to that, many studies (60%) increased the level of difficulty as the user progresses. This feature includes either mid-term or long-term goals, and is used to foster motivation and interest in using the software.

Most of the studies also include some degree of personalization to the software they propose. This allows the child (or teachers, or parents) to customize the graphics and/or the content according to their particular needs and preferences. On the top level, it may allow for changes in the color scheme, graphics size, and layout arrangement. Besides that, it may include changes in content presentation: photographs instead of cartoon-like illustrations, text messages or picture-only content, background music on and off, etc. A few studies combine personalization with a multimedia approach, including text, audio and video options [START_REF] Kamaruzaman | Developing user interface design application for children with autism[END_REF][START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Winoto | Look to remove: A virtual reality application on word learning for Chinese children with autism[END_REF][START_REF] Konstantinidis | Using affective avatars and rich multimedia content for education of children with autism[END_REF][START_REF] Wadhwa | Collaborative tablet applications to enhance language skills of children with autism spectrum disorder[END_REF][START_REF] Hulusic | LEFCA: Learning framework for children with autism[END_REF]. These features are particularly interesting if we take into account that each ASD individual has unique interests and preferences, and that the autism covers a broad range of symptoms [START_REF] Kamaruzaman | Developing user interface design application for children with autism[END_REF].

In terms of monitoring, many studies apply strategies to customize learning outcomes and provide parental control (56.6% combined). This is often reached by synchronizing child-parent / child-teacher devices or having multiple interface profiles. These studies report on the use of an integrated system that allows teachers, therapists or parents to monitor their children progress, and select custom learning objectives depending on individual cognitive responses. Again, ASD individuals can benefit from having tasks tailored specifically for each of them and, at the same time, teachers and parents are entitled with more control over the learning outcomes.

A few studies also reported on the use of storytelling [START_REF] Wadhwa | Collaborative tablet applications to enhance language skills of children with autism spectrum disorder[END_REF][START_REF] Hourcade | Multitouch tablet applications and activities to enhance the social skills of children with autism spectrum disorders[END_REF][START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF][START_REF] Zaffke | iCanLearn: A mobile application for creating flashcards and social stories for children with autism[END_REF][START_REF] Kurniawan | Development of game for self-help toilet learning for children with autism[END_REF]. This method uses multiple media formats (text, images, sounds, animation, and video) in order to encourage students to create their own narratives. In ASD children, this may help improve their communication skills and the symbolic function, thus promoting an autonomous attitude [START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF]. Other features found on the studies include familiarization (pictures of the surroundings and/or voice recording) [START_REF] Washington | SuperpowerGlass: A wearable aid for the at-home therapy of children with autism[END_REF][START_REF] Tsiopela | Pre-vocational skills laboratory: designing interventions to improve employment skills for students with autism spectrum disorders[END_REF], information sharing [START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF], use of virtual avatars [START_REF] Konstantinidis | Using affective avatars and rich multimedia content for education of children with autism[END_REF], active experimentation [START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF], and remote control [START_REF] Zaffke | iCanLearn: A mobile application for creating flashcards and social stories for children with autism[END_REF].

By clustering the information in relation to the game objective, a few more insights emerge. When dealing with communication, there is a preference to work with personalization, monitoring, custom learning, and increasing level of difficulty. This may be explained by the communication process itself, which requires a certain vocabulary level in order to make sentences, and so on. The same applies to Social Interaction, since it involves cognitive skills rather than physical abilities. In fact, objectives that cope with physical development, such as Daily Activities and Motor Skills, tend to apply feedback, rewards, and also personalization strategies, which provides a more immediate response to the user. In turn, games that included multiple objectives often used a wide variety of strategies (although rewards and level of difficulty are more frequent), perhaps because a single approach would not be sufficient to comply with all the objectives at once.

User Interface Design

The User Interface design methods reported by the studies include visual presentation (colors, illustration, photographs, video), textual elements (words or phrases) and even sounds, voice/narration, and virtual 3D environments. The most popular approaches, however, are the use of illustrations (76.6%), words/text (80%), and colors (73.3%) to express feelings and guide the user through the interface. The items checkmarked (refer to Table 1) represent the topics described in more detail in the papers, which does not mean unmarked items were not covered by the study.

In relation to illustrations, there are multiple approaches available. Reports range from the use of simple, sketchy, basic shapes (squares, triangles, and circles) [START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF][START_REF] Hourcade | Multitouch tablet applications and activities to enhance the social skills of children with autism spectrum disorders[END_REF][START_REF] Alvarado | Valpodijo: Developing a software that supports the teaching of Chilean idioms to children with autism spectrum disorders[END_REF][START_REF] Hulusic | LEFCA: Learning framework for children with autism[END_REF] to more sophisticated illustrations [START_REF] Yan | A sunny day: Ann and Ron's world an iPad application for children with autism[END_REF][START_REF] Wojciechowski | Assistive technology application for enhancing social and language skills of young children with autism[END_REF][START_REF] Signore | You talk! -you vs[END_REF]. None of them report on the use of realistic drawings. Instead, whenever the authors wanted a more realistic style, they would rely on real-life pictures or photographs. The main explanation provided in those cases is that real-life pictures would be easier to recognize, especially if the parents/teachers use the built-in camera to upload their own images. It would also look more familiar to the user, and may, in some cases, be more effective [START_REF] Harrold | Towards and expression recognition game to assist the emotional development of children with autism spectrum disorders[END_REF].

The use of colors appeared as one of the major concerns in the development process. Again, there are various approaches. Some of the studies inform on the use of more vibrant colors [START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF][START_REF] Alvarado | Valpodijo: Developing a software that supports the teaching of Chilean idioms to children with autism spectrum disorders[END_REF][START_REF] Yan | A sunny day: Ann and Ron's world an iPad application for children with autism[END_REF][START_REF] Iradah | EduTism: An assistive educational system for the treatment of autism children with intelligent approach[END_REF], while others suggest a calm and half-toned color scheme [START_REF] De Leo | A smart-phone application and a companion website for the improvement of the communication skills of children with autism: Clinical rationale, technical development and preliminary results[END_REF][START_REF] Barros | Interactive and customizable use of mobile technology in the cognitive development process of children with autistic spectrum disorder[END_REF]. There is no apparent revaluation of colors based on users' feedback. A similar situation arises from the use of words/text: there is not much information on why authors chose one strategy over another. The differences in communication deficits experienced by ASD individuals impose a challenge because some of them are able to read while others are not [START_REF]Diagnostic and Statistical Manual of Mental Disorders, DSM-5. Artmed[END_REF]. Thus, using an amount of written information that fits the communication skills of the target audience is of major importance to achieve success.

The use of sounds is also reported in many of the studies (56.6%), though voice recording features or narration are much scarcer (10%). Sounds, in this case, include background music and/or other sound effects (clicking a button, for instance). Sounds can be interesting decorative strategies to increase motivation and reinforce learning activities [39]. A few studies report on the use of voice recording or narration in order to add customization attributes or improve familiarization [START_REF] Tan | Can you CopyMe?: an expression mimicking serious game[END_REF][START_REF] Winoto | A highly customizable parent-child wordlearning mobile game for Chinese children with autism[END_REF][START_REF] Hayes | Interactive visual supports for children with autism[END_REF]. Around 30% of the studies also include video (self-recorded or not) and virtual environments as means to promote immersion experiences.

Once again, clustering the information according to the objectives leads to other insights. Games with communication objectives largely apply illustration, photographs, and words/text. Surprisingly, however, not many of the studies use sounds, voice recording or video modeling, which could potentially help users become familiar with the sounds of the words and practice pronunciation. The same applies to Social Interaction and Basic Education -they could greatly benefit from having sound reinforcing positive behavior. In turn, video modeling and Virtual Reality appears in all of the studies handling Facial Recognition and Motor Skills, probably because those strategies offer opportunities to simulate real situations -which could also be applied to Daily Activities.

Conclusions and Future Perspectives

The objective of this review was to investigate the current status of research on ASDspecific gamified interventions, emphasizing gamification elements and design representation methods applied in the software development process. Data were also collected on the target audience, the learning objectives, and the software development process. As results show, there are a good number of studies addressing the topic, which suggests more awareness of the disturbance and a willingness to move toward accessibility.

Clinically, the causes of autism are unclear, and as medicine advances more complexity is added to its diagnosis, treatment, and prognostic. On the one hand, parents find it difficult to handle autism inside their homes, because many times they have little or no assistance on how to behave with their autistic children. On the other, accessibility is often seen as unnecessary and expensive in the corporate environment. That is precisely why it became so important: development teams endure the task to develop effective solutions for autism and, at the same time, prove they are feasible from a productive, financial, and corporate perspective.

In this context, this review shows a wide collection of gamification strategies to cope with the development of autism-specific software. Despite that, many challenges emerge from dealing with the ASD audience. Studies included in this review share some common characteristics. Overall, they provide information on the development process, although the procedures are often not described in full detail (with only a few exceptions). We believe this does not mean the development process was neglected, but rather that the authors decided to highlight some of the aspects they assumed to be the most relevant. However, the lack of information is disappointing, since a more detailed description would add consistency to the studies, and also help readers understand and replicate the methods and strategies used.

The target audience is, most of the cases, referred to simply as "autistic children" or "autistic individuals". As mentioned before, autism covers a broad spectrum, that is, different symptoms appear in different severity levels in ASD individuals [START_REF]Diagnostic and Statistical Manual of Mental Disorders, DSM-5. Artmed[END_REF]. For this reason, it is hard to assume a single software product would be equally effective to the entire spectrum. Having a specific target audience (including the comprehensive description of symptoms) would potentially help optimize the development process and also serve as guidance for therapists in managing the software. Furthermore, this has a negative impact on testing, since results may reflect mixed ASD profiles.

In relation to the development process / design approach, there is a clear focus on applying methodologies that prioritize the end-user. On the one hand, cross-functional teams often present a better performance, since their particular expertise helps improving the software product in terms of usability. On the other, involving external agents (teachers, parents, and the ASD users) is fundamental to collect feedback and confirm its efficiency on a User Experience level. Even though most studies did not present the design process comprehensively, they surely reveal an effort to meet the end-user's needs and preferences.

Despite the issues with the process detailing, all of the studies show a good use of multiple techniques (gamification elements and UI design strategies). The combination of such methods for the development of serious gaming applications often results in better usability and acceptance levels [START_REF] Hayes | Interactive visual supports for children with autism[END_REF]. We should remember, once more, that ASD individuals have complex clinical conditions. Thus, combining multiple strategies and forms of presentation may have a huge impact on how they use and benefit from the software.

Unfortunately, most of the studies analyzed did not provide consistent testing procedures or clear results achieved with the games. Many studies used indirect forms of assessment, which is valuable, but hardly represents the end-user realistically. In cases when authors did test the software with autistic children [START_REF] Kamaruzaman | Developing user interface design application for children with autism[END_REF][START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Winoto | Look to remove: A virtual reality application on word learning for Chinese children with autism[END_REF][START_REF] Silva | Star: Speech therapy with augmented reality for children with autism spectrum disorders[END_REF][START_REF] Alvarado | Valpodijo: Developing a software that supports the teaching of Chilean idioms to children with autism spectrum disorders[END_REF][START_REF] De Leo | A smart-phone application and a companion website for the improvement of the communication skills of children with autism: Clinical rationale, technical development and preliminary results[END_REF][START_REF] Yan | A sunny day: Ann and Ron's world an iPad application for children with autism[END_REF][START_REF] Chatzara | Digital storytelling for children with autism: Software development and pilot application[END_REF][START_REF] Iradah | EduTism: An assistive educational system for the treatment of autism children with intelligent approach[END_REF], samples were often small, revealing fragility in the results achieved. In addition to that, none of the studies reported having used control groups within the testing procedures. This leaves us with no reference for comparison on how the audience would have behaved with and without the serious game. Of course, the testing efforts are important but, most of the cases, it is hard to assume they are unquestionable evidence of success.

Results, in turn, are generally presented in a superficial manner. Mostly, they are described as "good/not good" or "positive/negative", without providing further information on the analysis criteria, reference metrics, or a comprehensive review process. Only a small number of studies inform on the specific positive/negative feedback received, as well as how they would apply the suggestions on later improvements [START_REF] Cai | Design and development of a virtual dolphinarium for children with autism[END_REF][START_REF] Konstantinidis | Using affective avatars and rich multimedia content for education of children with autism[END_REF][START_REF] Zhang | Design and evaluation of a collaborative virtual environment (CoMove) for autism spectrum disorder intervention[END_REF][START_REF] Hourcade | Multitouch tablet applications and activities to enhance the social skills of children with autism spectrum disorders[END_REF][START_REF] Wojciechowski | Assistive technology application for enhancing social and language skills of young children with autism[END_REF][START_REF] Malinverni | An inclusive design approach for developing video games for children with autism spectrum disorder[END_REF]. Exclusively negative or unsuccessful outcomes have not been found.

The path towards accessibility for autism is obviously not straightforward. Since the clinical literature is still limited, practitioners are also learning from empirical evidence. In this sense, one of the main challenges in developing serious games for autism is to reach out the end-user, in order to understand the complexity of conditions they have. In addition to that, the development process must be scrutinized. As showed in this systematic review, there are issues related to the selection of the target audience (which may lead to a poor definition of learning objectives), and the conduction of testing procedures (which may lead to biased or mixed results). Finally, developers must be aware of the user-centered approaches -involving autism advocates along the process may be a good strategy to prevent usability disasters.

The studies included in this review are undeniably valuable, and we believe the authors did their best with the limitations they might have faced and with the resources they had available at the time. Results achieved and presented in the studies reinforce that. Notwithstanding, scientific and methodological procedures must be taken seriously given the complexity of the subject matter. This review also intends to open future discussions on scientific rigor and integrity involving the development of ASDspecific software. Efforts must continue toward accessibility, effective learning, and social inclusion.

Fig. 1 .

 1 Fig. 1. Learning objectives extracted from primary studies.

national

  Conference on Wireless Algorithms, Systems and Applications, WASA 2012, pages 33-37. 39. Banire, B., Jomhari, N., and Ahmad, R. (2015). Visual hybrid development learning system (VHDLS) framework for children with autism. Journal of Autism and Developmental Disorders, 45(1):3069-3084. DOI: 10.1007/s10803-015-2469-7

Table 1 .

 1 Gamification elements and User Interface design.

	Communication (46.6%)
	Multiple (20%)
	Social Interaction (13.3%)
	Basic Education (10%)
	Daily Activities (10%)
	Facial Recognition (6.6%)
	Motor Skills (3.3%)


Gabriel Azevedo
email: gabrielsilvadeazevedo@gmail.com

"Aedes vs. Repellents": Applying Tower Defense Dynamics in a Digital Game to Combat the Mosquito

Keywords: Game design, Aedes aegypti, Serious games

Serious games represent a computational resource that can be used as a tool to create new ways of raising awareness of digital natives. In this sense, this paper presents current results with the "Aedes vs. Repellents", a 3D tower defense game that aims to promote the interaction and learning of the population in the prevention and combat of the Aedes aegypti mosquito. For this, mechanics and dynamics of the well-known "Plants vs. Zombies" game will be reuse, being applied in a scenario capable of providing a context of public health learning in the prevention of the mosquito.

Introduction

Aedes aegypti is a mosquito that acts as a vector for the transmission of recurrent diseases in public health with alternating outbreaks and difficult-to-combat epidemics, such as dengue fever, yellow fever, chikungunya and zika virus [START_REF] Kikuti | Distribuição espacial e determinantes ecológicos para dengue em uma comunidade urbana de salvador[END_REF]. As a result, several educational campaigns have been developed and increased over the years in mosquito prevention and control involving science, education and coordinated arts activities with schools, students, parents and teachers [START_REF] Pitta | Estratégias de comunicação frente ao desafio do aedes aegypti no brasil[END_REF].

Educational games, also known as serious games, are types of software primarily aimed at educational purposes [START_REF] Silva | Conheça e utilize software educativo: avaliação e planejamento para a educação básica[END_REF]. It is a computational resource that can be used as a tool to create new forms of dissemination, as well as implement educational actions capable of increasing the public interest in combating the mosquito [START_REF] Silva | Jogos virtuais no ensino: usando a dengue como modelo[END_REF].

However, there are evidences that define serious games as a boring alternative for their players, being focused on learning aspects instead of promoting pleasure and enjoyment for the player [START_REF] Araújo | Processo de desenvolvimento do jogo sério missão aedes: relações entre objetivos pedagógicos, ludicidade e implicações de design[END_REF]. In fact, promoting the relationship between playfulness and learning is a major challenge during the process of designing and building a serious game [START_REF] Araújo | Processo de desenvolvimento do jogo sério missão aedes: relações entre objetivos pedagógicos, ludicidade e implicações de design[END_REF].

In this sense, and considering the reuse of mechanics and dynamics of successful games to be applied in a scenario that provides a health learning context, this article presents the "Aedes vs. Repellents" project. It is a game that seeks to apply available mechanics and dynamics of the "Plants vs. Zombies" [START_REF] Games | Plants vs. zombies[END_REF] game in a context where the player has to protect a house suffering from constant waves of mosquito attacks.

Related Work

Different types of serious games have been developed in the context of combating the Aedes aegypti mosquito. As an example, the Mission Aedes [START_REF] Araújo | Processo de desenvolvimento do jogo sério missão aedes: relações entre objetivos pedagógicos, ludicidade e implicações de design[END_REF] features a 2D platform style game that seeks to: remember which is a mosquito focus; understand and analyze the life cycle of the mosquito; apply knowledge acquired regarding mosquito outbreaks; and evaluate the development cycle of the mosquito. Following the graphic quiz style the Aedes Game [START_REF] Portella | Experiência da unasus/ufcspa no desenvolvimento de jogos educacionais[END_REF] aims to inform the Brazilian population about their social responsibility in the fight against Aedes aegypti, providing a set of questions and answers that seek to inform the player by feedback of the given responses. Finally, the Aedes in the Sights game [START_REF] Moura | Aedes na mira -aplicando realidade virtual no combate a focos de mosquitos[END_REF] uses virtual reality to propitiate an immerse backyard of a house full of dengue spots, where the player must eliminate all the mosquito larvae found.

The "Aedes vs. Repellents" Game

The general objective of the proposed game is to avoid the player house invasion by mosquitoes (Fig. 1). In this sense, as a protection strategy, the player must build defense towers, in this case repellents, able to avoid mosquitoes but with an energy consumption each one. To use energy, the player needs to build solar panels and batteries to generate and store more energy as soon as possible. These resources are necessary to allow the player to be protected at night, when the wave of mosquitoes begins to attack the player residence.

The game difficulty will increase according to the subsequent mosquitoes waves, along with the mosquito variations that can both destroy the towers more easily and also offer more resistance to player damages.

As examples of game modeled characters: the insecticide is the base attack structure (Fig. 2(a)) to kill mosquitoes; the plug repellent and the grid protection work as defense units (Figs. 2(c) and 2(b)) against the mosquito wave; and the special mosquito that does not fly provides a hull that gives a greater resistance to damage promoted by repellents (Fig. 2(d)). The initial scenario of the game play presents a 3D environment, containing the tracks where the elements of the game will be placed to avoid the invasion of the mosquitoes to the residence (Fig. 3). New mosquitoes are applied in each track per night wave, being increased by amount, speed and resistance in each mosquitoes attack, according to the continuous survivor state of the player.

Conclusions and Future Work

This paper presented the "Aedes vs. Repellents", a game that aims to apply digital entertainment in the prevention and combat of the Aedes aegypti mosquito. It is a playful application that aims to use mechanics and dynamics of the Plants vs. Zombies game in a scenario where the player has to protect a house from mosquito attacks using real prevention and combat elements such as repellents, grid protection and insecticide. As future work, in addition to the completion of the game (available at https://github.com/lenda-uefs), it is important to be integrated with social networks, expanding the game reward system to the external and cultural environment of the player. The addition of new characters and multimedia resources, followed by future participation in health campaigns, and the production of educational materials related to the game, will also be performed in the future.

Fig. 1 .

 1 Fig. 1. Initial menu of the "Aedes vs. Repellents" game.

Fig. 2 .

 2 Fig. 2. Designed characters of the "Aedes vs. Repellents" game.

Fig. 3 .

 3 Fig. 3. Initial game play of the "Aedes vs. Repellents" game.

Supported by FAPESB and Kamikan.

Acknowledgments

The authors are grateful for the financial support provided by FAPESB and Kamikan for the development of the "Aedes vs. Repellents" project.