SIGCHI Conference Paper Format

Session: Paper Presentation

CHI PLAY 2018, October 28–31, 2018, Melbourne, VIC, Australia

Don’t Sweat the Small Stuff: The Effect of Challenge-Skill Manipulation on Electrodermal Activity Madison Klarkowski, Daniel Johnson, Peta Wyeth, Cody Phillips Queensland University of Technology Brisbane, Australia m.klarkowski@, dm.johnson@, peta.wyeth@, [email protected]

Simon Smith University of Queensland Brisbane, Australia [email protected]

ABSTRACT

INTRODUCTION

Challenge plays a critical role in enabling an enjoyable and successful player experience, but not all dimensions of challenge are well understood. A more nuanced understanding of challenge and its role in the player experience is possible through assessing player psychophysiology. The psychophysiology of challenge (i.e. what occurs physiologically during experiences of video game challenge) has been the focus of some player experience research, but consensus as to the physiological markers of challenge has not been reached. To further explore the psychophysiological impact of challenge, three video game conditions – varying by degree of challenge – were developed and deployed within a large-scale psychophysiological study (n = 90). Results show decreased electrodermal activity (EDA) in the low-challenge video game condition compared to the medium- and highchallenge conditions, with a statistically non-significant but consistent pattern found between the medium- and highchallenge conditions. Overall, these results suggest electrodermal response increases with challenge. Despite the intuitiveness of some of these conclusions, the results do not align with extant literature. Possible explanations for the incongruence with the literature are discussed. Ultimately, with this work we hope to both enable a more complete understanding of challenge in the player experience, and contribute to a more granular understanding of the psychophysiological experience of play.

Challenge is a core element of the video game play experience: it is both intrinsic to ensuring successful video game design [33], and essential in promoting optimal player experiences [9, 3]. Research in the psychology of motivation indicates that personal successes in tasks are met with feelings of pride and joy, emphasising the role of challenge in the generation of positive experiences [24]. Owing to its critical role in the design of video games and the player experience, challenge manifests in games in a variety of ways – for instance, through logic puzzles (platform and puzzle games), tasks that require a high degree of fine motor control and reaction speed (firstperson shooters, action games), and resource management (strategy games). In video games, challenge-skill balance – that is, ensuring the demands of the game neither overwhelm nor underwhelm the ability of the player – is of special interest. Challenge-skill balance is thought to either enable, represent, or robustly predict flow [9, 35, 14], a positive psychological state synonymous with the optimal experience [9]. A more nuanced conception of challenge and challenge-skill balance, its role within the player experience, and the granularity of its impact on the player represents a vital step towards a more complete understanding of the player experience. Research is needed to better understand how to design, manipulate, and measure challenge in video games.

CCS CONCEPTS

One promising avenue towards expanding contemporary understanding of video game challenge is the employment of psychophysiological analysis. Psychophysiology represents a real-time, quantitative, and arguably objective approach in a space that often utilises post-hoc, qualitative, and subjective evaluations of experiences [27]. Of particular benefit is the near-immediate nature of physiological response. The psychophysiological analysis of challenge in video games allows for an understanding of the impact of challenge as it occurs, without interruption of the play experience (as with surveys or interviews). Psychophysiological analysis is also complementary to post-hoc understandings granted by self-report measures within extant literature. While the benefits of the psychophysiological evaluation of challenge have already been realised in contemporary research [22], results are often contradictory (that is, suggesting both increased and

• Applied computing → Computers in other domains → Personal computers and PC applications → Computer games Author Keywords

Psychophysiology; video games; electrodermal activity; challenge; player experience. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]. CHI PLAY '18, October 28–31, 2018, Melbourne, VIC, Australia © 2018 Association for Computing Machinery. ACM ISBN 978-1-4503-5624-4/18/10…$15.00 https://doi.org/10.1145/3242671.3242714

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CHI PLAY 2018, October 28–31, 2018, Melbourne, VIC, Australia RELATED WORK

decreased psychophysiological arousal to challenge) and limited by methodology, experimental design, or small sample sizes. As such, the aim of the current study was to robustly explore the influence of challenge on the psychophysiological state using a large sample size: specifically, whether challenge affects physiological arousal as measured through electrodermal activity. To do this, three video game conditions were created to represent different levels of challenge-skill balance and imbalance, so that the impact of low challenge (‘Boredom’), medium challenge (‘Balance’) and high challenge (‘Overload’) could be examined. Of interest was to what extent, if any, electrodermal activity differs between three disparate challenge experiences. By employing psychophysiology to investigate experiential phenomena associated with the player experience, a novel understanding of the player experience can be gained and some clarification of current contradictions in the literature may be provided.

Challenge

Challenge is central to the video game play experience, as supported by Andrade et al.’s survey of game features revealing it as one of the key critical factors of a successful game [3]. The wellbeing effects of challenge are also positive, with successful completion of appropriately challenging tasks in game environments generating a sense of greater self-efficacy and accomplishment for players [26]. Research establishes challenge-based game play as an intrinsically motivating activity, suggesting that simply undertaking optimally challenging activities - rather than just the experience of succeeding at the task - is an enjoyable experience in itself [9]. Following this, much of game design is rooted in the concept that players seek, and are driven by, challenge [26]. Challenge-Skill Balance

Challenge-skill balance occurs within a video game when: a) the challenges of the game do not outstrip the abilities of the player, and b) the abilities of the player do not outstrip the challenges of the game. The role of challenge‒skill balance in ensuring an optimal player experience is championed by Przybylski, Rigby and Ryan [33], who describe this balance as critical:

Results show greater electrodermal activity associated with increased challenge. Electrodermal activity was greater in the Overload condition than in the Boredom condition, and in the Balance condition than in the Boredom condition (though it should be noted that the pairwise comparison between the Balance and Overload conditions did not reach statistical significance). These findings, although somewhat intuitive, misalign with much of the existing literature within this space.

“The pacing of challenges was designed so players could continually experience enhanced competence as they progressed in the game, with challenges increasing apace with player ability. This balancing of game difficulty and player skill was critical to the success of arcade games …”

Our findings allow for a better understanding of the player experience associated with differing levels of challengeskill balance, and point to electrodermal activity as useful for the evaluation and comparison of differing experiences of challenge (via identifying increases in EDA as responsive to increases in video game challenge). We present implications for game designers seeking to build optimal game experiences, especially with regard to opportunities for the deployment of EDA in biofeedback– driven dynamic difficulty adjustment. The identification of electrodermal activity as a potential measure of differing challenge is also useful for researchers and developers aiming to build, test, and evaluate games. Our method for altering the challenge-skill variable in a complex game environment may also provide a template for challengeskill manipulations in future experimental work. The largescale psychophysiological assessment of challenge conducted on a successful commercial video game, undertaken within a methodology informed by standard psychophysiological practice, represents an additional clarification and point of comparison within extant literature on the psychophysiology of the player experience. Finally, challenge-skill balance is relevant to a breadth of activities (e.g. health, pedagogy); as such, the broader research community may benefit from these results by considering our findings in software contexts where difficulty in navigating the software is under investigation.

Challenge-skill balance is often pointed to as a precursor to (or robustly associated with) flow, an indicator of optimal player experience with associated positive impacts on wellbeing [37]. Should a game be too difficult or too easy for the player, there will be a challenge-skill imbalance, and the player experience is assumed negatively impacted [2]. One addendum to this is that the distinction between ‘boredom’ and ‘relaxation’ is not always clear, particularly within flow research [13]. To this end, Csikszentmihalyi proposed a renaming of the ‘Boredom quadrant’ within flow visualisations to the ‘Relaxation quadrant’ [10]. As such, the potential conflation of ‘relaxation’ and ‘boredom’ presents an additional hurdle when designing for an optimal player experience. In terms of game design, challenge-skill balance is sometimes promoted through the use of dynamic difficulty adjustment (DDA). DDA provides real-time adjustment of the game’s difficulty in response to player performance, ensuring a similar play experience regardless of individual ability or expertise [18]. DDA is often used in video games as a solution to challenge-skill imbalance [3], ensuring that the play experience is neither too easy nor too difficult. Psychophysiology

Psychophysiology is the study of the relationship between psychological manipulations and the associated physiological response [4]. This data is collected through

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instruments placed on or within the body that record physiological response to stimuli. In terms of player experience, the psychophysiological method is valuable in its provision of a covert, direct indication of emotional response to the video game stimuli [30]. Other qualities of psychophysiological analysis beneficial for player experience evaluation include the potential for high measurement stability and fidelity, the ability for continuous and non-obstructive data collection and the resulting potential for close synchrony to game stimuli, and the reproducibility of the psychophysiological method. Since 2004, researchers have published multiple papers exploring the relationship between specific psychophysiological indices and player experience. These papers have shown the value of psychophysiological measures as a means of quantitatively monitoring the realtime player experience [27, 28, 30]. While this research has explored a number of topics through the lens of psychophysiology – including, but not limited to, affect, flow, immersion, violence, addiction, and mood [22, 30, 31, 34] – challenge is of particular interest to this study. As such, the psychophysiological impact of challenge on the player experience will be explored within.

scaffolded by a robust experimental methodology and, when reporting, a concession that other (potentially unknowable) psychological processes may be contributing to the physiological response should be made. Within psychophysiological literature, increases in electrodermal activity have been associated with increased task difficulty, mental effort expended during a task, and increased cognitive load, [8, 15, 29, 32] as well as negative feedback associated with performance in a difficult task [8]. Overall, the responsiveness of electrodermal activity to stress, cognitive load, and mental effort positions it as a robust measure for the analysis of task difficulty. EDA and Game Challenge

Due to the robust relationship between electrodermal response and arousing stimuli, as well as its relative ease of deployment and interpretation, EDA has featured prominently in much psychophysiological research undertaken within the player experience space [29]. As a measure of arousal, EDA has been employed within psychophysiological player experience studies that investigate challenge as either a primary or secondary focus of the research. Despite this, the influence of challenge on EDA is still irresolute: despite psychophysiological literature positioning EDA as responsive to task difficulty, results within player experience literature so far have been varied, inconclusive, and, occasionally, contradictory.

EDA

EDA - previously known as galvanic skin response (GSR), amongst other terms - is one of the most widely used measures of psychophysiology, owing largely to its relative ease of deployment and quantification, as well as its sensitivity to psychological stimuli [11]. Primarily a measure of arousal, EDA is the study of electrical activity of the skin (recorded by cutaneous electrodes). Variation in electrical activity is generated by an interaction between the sympathetic nervous system (SNS) and local processes in the skin that provoke sweating from eccrine sweat glands in the dermis [5, 11]. While the primary function of most eccrine sweat glands is that of thermoregulation, those found on the palms of the hand (palmar) and soles of the feet (plantar) are more strongly associated with ‘grasping behaviour’ and are thus more responsive to emotional rather than thermal stimuli [11]. This is most noticeable in the phenomenon of ‘clammy hands’ in stressful or adverse contexts, regardless of the external temperature.

In an early example of psychophysiological player experience evaluation, Mandryk et al. [28] employed multiple psychophysiological measures, including EDA, to explore associations between a player’s physiological states, events occurring during the game experience, and the subjective reported experience. Eight participants played three versions of a hockey game: easy, medium and difficult. No main effects of difficulty level were found on any of the physiological measures. The researchers later determined that the participants were responding inconsistently to experimental manipulations (no differences in perceived difficulty were found between the medium and difficult levels). Additionally, the researchers discovered potential methodological limitations in their work, finding that interviews undertaken between each play session continued to influence psychophysiological response during the play sessions.

EDA is responsive to a breadth of stimuli, including ‘stimulus novelty, surprisingness, intensity, emotional content, and significance’ [11]. This stimuli sensitivity emphasises the necessity for a controlled experimental paradigm to restrict electrodermal response to the investigated variables, although it is often difficult to completely avoid the influence of spontaneous activity. This is further warranted by the complex nature of many-toone and one-to-many domain relationships between the psychological and physiological, which suggest that the same physiological response can be indicative of separate – and occasionally contrasting – psychological experiences [7]. As such, the direct interpretation of EDA as symptomatic of a certain psychological state must be

Nacke and Lindley [31] employed EDA, amongst other psychophysiological measures, in an investigation of flow and immersion within a first-person shooter (FPS) environment. Twenty-five participants played three versions of a custom FPS game: a ‘boring’ low immersion condition, in which the skill of the players outstripped the challenges of the game and the graphics were altered to be non-immersive; a ‘flow’ condition, which introduced generally progressive difficulty; and an ‘immersion’ condition, designed to create a more immersive experience through visuals, sound, and narrative. Results pointed to

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significantly greater EDA in the ‘flow’ condition than the ‘boredom’ condition.

challenge and EDA. In other instances, the impact of challenge-skill balance is partly hidden by the examination of additional variables [25, 31]. While Nacke and Lindley found a difference between conditions, in their study challenge co-varied with immersion. In contrast, Kneer and colleagues – whose study co-varied challenge with violence – found no impact of challenge on EDA. Additionally, there is value in further study of the physiological response to challenge outside of puzzle games. In sum, a single unified examination of the impact of video game challenge on EDA – sans additional variables – would represent a step towards improving current understanding of physiological response to challenge. A focus on a well-received commercial action video game would also improve the generalisability of these results to the contemporary titles, and separately assess the suitability of EDA in measuring more complex and varied player experiences. This would, in turn, assist in a more dimensional understanding of the player experience, and possibly better situate the potential and usefulness of EDA as a tool for measuring it.

A study of 32 participants by Kivikangas [21] does not support Nacke and Lindley’s psychophysiological findings, showing no relationship between psychophysiological arousal, as measured by EDA, and flow, as manipulated on challenge-skill balance. The author suggests that this may be because of the length of the play sessions in their experiment design (approximately 40 minutes). There is a possibility that averaging over long periods may have reduced the associations between flow and psychophysiological response due to the potential for physiological habituation - wherein psychophysiological response is reduced after exposure to the continued presentation of, or interaction with, the same stimulus [36]. Drachen et al. [12] employed EDA in their assessment of physiological correlates of player experiences from 16 participants amongst three first-person shooter (FPS) games. In their approach, the authors explored several psychological constructs of play, including challenge, within a single subjective scale. No significant correlation was found between EDA and self-reported level of challenge. Drachen et al. propose the absence of results for EDA and challenge as a function of imprecise wording within the scale used.

STUDY DESCRIPTION

A within-subjects experiment was designed to measure the effects of challenge-skill balance and imbalance on the play experience. The study made use of three ten-minute game conditions modified to be representative of dimensions of challenge (challenge > skill imbalance, skill > challenge imbalance, and challenge-skill balance). The physiological impact of challenge-skill balance and imbalance on the player experience was evaluated using a variety of psychophysiological measures, with EDA results reported within this paper. The current paper focuses on the following research question:

In an investigation of the effect of violence and difficulty, Kneer, Elson and Knapp [25] employed four modifications of the FPS game Team Fortress 2: a low-difficulty, highviolence condition; a low-difficulty, low-violence condition; a high-difficulty, high-violence condition; and a high-difficulty, low-violence condition, all within the same map. A sample of 90 participants was observed, with each participant playing one of the conditions. The researchers again did not find any significant effects on EDA for difficulty, but offer the explanation that the conditions may not differ sufficiently enough in difficulty to provoke a physiological response.

RQ: What are the differences in electrodermal response between challenge-skill balanced and challenge-skill imbalanced player experiences? Despite the incongruence of results in current player experience literature, literature within psychophysiological spaces suggest a relationship between electrodermal activity and task difficulty/challenge. As such, this research will examine the potential of this relationship within the player experience space specifically.

Overall, within the player experience literature, investigation of psychophysiological impact often occurs in response to puzzle games (most often Tetris), which has the advantage of a more controlled environment (e.g., relatively small differences in muscle activity required as difficulty increases) [17, 19, 20, 38]. While only one of these studies utilised EDA (finding increases in EDA in response to challenging or frustrating tasks [38]), there exists within psychophysiological player experience literature a predominance of studies assessing player experience of puzzle games.

Video Game Design

The game chosen was Valve Corporation’s Left 4 Dead 2, a post-apocalyptic zombie FPS. This was due to its high production standards, popularity within the gaming community, and intuitive gameplay. The game was also chosen due to its native inclusion of DDA in the form of an entity known as the ‘AI Director’, which was assistive in the creation of the challenge-skill balance condition. The three modified game conditions all took place within a map named ‘The Port’. In this level, participants explored an industrial complex to retrieve 16 gas canisters with the help of three companion AI. Once all 16 gas canisters were emptied into a fuel tank, a bridge would lower, granting access to a car and finishing the level.

Gap

The previous section summarises literature showing both the importance of challenge in the player experience and the current uncertainty regarding the influence of challenge on electrodermal activity. In some instances [12, 28], researchers point to methodological limitations as explanations for the absence of a relationship between

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that other players did not (for example, killing five ‘Hunter’ type special enemies).

Manipulations

Game conditions that only differed on the challenge-skill variable were required for the study. To achieve this, three conditions were designed towards manipulating challengeskill balance and imbalance: ‘balance’ (medium challenge), ‘boredom’ (low challenge), and ‘overload’ (high challenge). The game level chosen featured two key challenges: successfully collecting and delivering sixteen gas canisters, and surviving combat with enemy agents (zombies). The challenge of gas canister collection was dependent on the challenge of enemy agents - canisters were clearly marked and highlighted in the map, and canister collection was only inhibited through the presence of enemy AI. To this end, combat was the only feature of the game directly altered.

 Sprays, player-chosen images that can be ‘pasted’ into the game world by pressing a button, were also disabled.  Players were restricted to only one of four playable characters (‘Nick’).  Weapon choice was eliminated from the game. In the default game, players may choose to play with a sniper rifle, machete, assault rifle, chainsaw, and so on. To ensure a similar experience across all conditions and experiments, only the assault rifle as the primary weapon and pistol as the secondary weapon were enabled.  In all conditions, the ‘Witch’ - a rare special enemy type with the ability to instantly kill a novice player and their AI teammates - was removed.

Left 4 Dead 2 features two types of enemy agents: ‘common’ and ‘special’. Common enemy zombies are slow-moving, attack through melee hits only, often move in ‘herds’, and have low health. Special enemy zombies have unique abilities (such as spitting pools of acid or pouncing on the player), move alone, require special tactics to eliminate, and have lower spawn rates than common enemy zombies. Both these enemy types were modified for each game condition. In boredom, they were removed entirely; in balance, they appeared as they would in standard Left 4 Dead 2 and were influenced by DDA; finally, in overload, spawn rates, health, responsiveness, herd size, and damage dealt were radically increased. For challenge manipulations across all conditions, please refer to Table 1. Balance

Boredom

Overload

Common enemy agents

Standard

None

Extreme

Special enemy agents

Standard

None

Extreme

Collecting canisters

Standard

Standard

Standard

DDA

Enabled

Disabled

Disabled

Balance

The balance condition was designed for challenge-skill balance. This condition remained almost identical to the standard version of Left 4 Dead 2, with the exception of the alterations made for all conditions as detailed previously. The ‘AI director’, or DDA, was enabled to create challenge-skill balance for each participant. To this end, game difficulty was set to ‘normal’ in the map editor. Common enemy zombies had 50 points of health and spawned in herd sizes respective to the player performance as judged by the AI director. The player and their AIcontrolled teammates had 100 points of health each, and took two damage points per hit to their front and one damage point per hit to their back. Refer to Figure 1 for a screenshot of the typical play experience for the balance condition, featuring a reasonable number of enemy zombies consistent with the demands of the game.

Table 1. Challenge manipulations.

The conditions were developed to be different enough that players of any skill level would experience challenge > skill in the Overload condition, skill > challenge in the Boredom condition, and a match of skill and challenge in the Balance condition. Previous research confirming the likely success of the conditions in achieving this has been published [23]. Some features of Left 4 Dead 2 were altered for all conditions for the preservation of experimental integrity and inhibition of potential confounds. These alterations include:

Figure 1. Screenshot of 'Balance' condition Boredom

The boredom was designed for challenge-skill imbalance, where the skill of the player far exceeded the challenge of the game. In this condition, enemies were removed entirely from the game. The gameplay in this condition consisted entirely of retrieving the gas canisters scattered throughout the map. As the gas canisters are highlighted in the map in all conditions, finding the canisters did not represent a notable challenge; additionally, the large size of the map and the distribution of the canisters ensured a repetitive experience. Despite the removal of combat altogether, the

 The achievement system was disabled to ensure no players experienced a reward for achieving something

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CHI PLAY 2018, October 28–31, 2018, Melbourne, VIC, Australia Participants

inclusion of canister collection and travel in the virtual world ensures the game remains sufficiently game-like. Refer to Figure 2 for a screenshot of the typical play experience for the boredom condition, featuring zero enemy presence (note the player holding the canister as part of the fetch task).

Ninety people participated in the study, but a single participant was removed from the final sample due to computer hardware failure prematurely terminating the experiment. Of the final sample size of 89 participants, 77.5% were male and 22.5% were female, all between the ages of 17 and 38 (mean = 23.41, SD = 4.53). On a Likert scale of 1–7, with ‘7’ representing ‘extremely experienced’ and ‘1’ ‘not at all experienced’, participants self-rated with a mean of 5.96 (SD = 1.33) for ‘general experience with video games’ and 5.16 (SD = 1.72) for ‘experience with first-person shooters’. Participants reported between 0 and 70 hours spent playing video games per week, with a mean of 21.79 hours (median = 20 hours, SD = 16.04). In terms of familiarity with the Left 4 Dead franchise, participants were asked how many hours they had spent playing both Left 4 Dead 1 and Left 4 Dead 2. Forty-five participants had never played Left 4 Dead 1 before, with the remaining 44 having played between 1 and 120 hours (mean = 25.61 hours, median = 10 hours, SD = 36.48). As for Left 4 Dead 2, 34 participants had never played before, and the remaining 55 had played between 1 and 200 hours (mean = 32.47, median = 15 hours, SD = 43.43). Overall, 69.66% of participants had previously played a Left 4 Dead title.

Figure 2. Screenshot of 'Boredom' condition Overload

Similar to the boredom condition, the overload condition was also designed for challenge-skill imbalance. In this condition, the challenge-skill imbalance was accomplished through challenge outstripping player skill. This was achieved through extra enemy health, drastically increased enemy count, extra damage taken from enemy hits and friendly fire, and reduced friendly AI line-of-sight range. The high spawn rate of enemy AI also reduced the potential for ‘cooldown spots’ [31] that allow players time to heal, regroup, and restock on ammunition. To this end, game difficulty was set to ‘expert’ in the map editor. Common enemy health was raised from 50 (in the balance condition) to 1000 points. Players and their AI-controlled teammates had 100 points of health each, and took 20 points of damage per hit to their front and 10 points damage per hit to their back. All special enemies, including the ‘tank’, had their health multiplied by 4; this gave tanks 16,000 health points. In addition to this, zombies were more likely to spawn behind the player, there was no game-enforced limit on how many zombies could be present in the map at a time, and special infected spawn rate was increased. Refer to Figure 3 for a screenshot of the typical play experience for the overload condition.

Study Affordances

The presentation and play of the game conditions was moderated by some affordances undertaken to reduce the risk of potential confounds. These affordances took shape in the form of play session termination before victory, and the use of a semi-counterbalanced study design. Due to disparate skill levels, it was determined that some expert players would be capable of achieving the victory condition within the 10 minutes of play allotted to the Boredom and Balance play sessions. To ensure a consistent play experience among participants, and to limit the potential impact of the peak end effect [16], gameplay was terminated immediately prior to successful completion of the level within both the Boredom and Balance conditions. Due to the nature and layout of the canister fetch task, termination would occur roughly at or slightly before the 10-minute mark. While the study design employed counterbalancing to control for order effects, the conditions were not fully counterbalanced. This decision was ultimately predicated on the risk of prolonged emotional or physiological response to the Overload condition; early internal playtesting revealed the condition to be frustrating or overwhelming, potentially influencing participant mood and receptiveness to subsequent conditions. Nonetheless, this approach has introduced methodological limitations that are discussed within this paper. Experimental Setting

The experiments took place in a laboratory environment on high-end PCs. The experimenter sat behind a partition in the corner of the laboratory to avoid discomfort related to

Figure 3. Screenshot of 'Overload' condition

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direct observation. Only the participant and one researcher were present in the room during data collection.

Two disposable EL507 snap electrodes were attached to the thenar and hypothenar regions of the palm (see Figure 5) respectively, and secured with medical tape to reduce the risk of movement or detachment throughout the experiment. The real-time EDA recording within the AcqKnowledge recording and analysis software was then visually verified to ensure connection and an uninterrupted EDA trace. A 10minute ‘settle’ time, in accordance with [6], was provided prior to data collection. EDA activity (mS) was captured using a BIOPAC EDA amplifier, and recorded in BIOPAC’s AcqKnowledge 4.2 data acquisition and analysis software.

Figure 4. Laboratory in which experiments took place Measures

This study employed the use of a variety of psychophysiological measures (EDA, EMG, EEG, and ECG) as well as self-reported surveys that evaluated various experiential phenomena associated with play. This paper reports the EDA results. Process

Each experiment session took place between 10 am and 6 pm, with most sessions occurring on weekdays. After providing informed consent, participants were lead to a sink within the laboratory and asked to wash their hands in preparation for the application of EDA electrodes. Upon drying their hands, participants were seated at a PC in a corner booth. The electrodes and instruments for the psychophysiological measures (including EDA) were then applied over a duration of approximately 30 minutes. Participants were directed to answer questions regarding their demographics and previous experience with both video games in general, and with Left 4 Dead 2 specifically. Participants played a four-minute structured tutorial for the game that exposed them to the mechanics and requirements necessary for gameplay. They then played three 10-minuteand-30-second game sessions in semi-counterbalanced order (Boredom / Balance / Overload, or Balance / Boredom/ Overload). Custom software automatically delivered a two-minute baseline at the start of the experiment, in between in each play session, and after all the play sessions were completed. Once the experiment was completed, the psychophysiological sensors were removed from the participant. Finally, participants were verbally debriefed and thanked.

Figure 5. EDA placement RESULTS

Once collected, the data were visually inspected for movement artefacts, noise interference and interrupted signals. In all cases, movement artefacts were visually evaluated in windows (timebins) of 10-second epochs. If an artefact was found to influence two or more seconds of the epoch, the full 10-second epoch was removed from analysis. The values corresponding with the 10-minute play sessions, with movement and noise artefacts handled through null value replacement, were then exported into SPSS for analysis. A total mean value of each 10-minute play session was derived per participant. Analysis

As our interest was in states of challenge-skill balance and imbalance rather than responses to individual events, the data was assessed tonically (looking at the averaged EDA across each condition). Analysis of the EDA data was undertaken via a one-way ANOVA to determine the effect of condition (Boredom, Balance, Overload) on psychophysiological response. Box plot assessment revealed three unique univariate outliers for EDA. As no substantive differences were found for within the pattern of results, analysis was performed on all 89 cases. No violations of normality were found for EDA as assessed by the Shapiro-Wilk test (p > .05).

EDA

As discussed in the previous section, all participants washed their hands at a sink provided in the laboratory prior to the attachment of the disposable EDA electrodes. Sink water was heated so to prevent reactionary restriction of capillaries or skin response to cold temperatures; as an additional prevention measure, participants washed with hypoallergenic goat’s milk liquid pump soap out of consideration for both hygiene and minimising the potential for an allergic reaction. Once washed, participants would then dry their hands with paper towel under instruction to ensure that their palms did not remain damp. Participants were then guided back to their chair, whereupon the attachment of all psychophysiological measures occurred.

Mauchly’s Test indicated that the assumption of sphericity had not been violated (W = .965, χ2(2) = 3.095, p = .213) so sphericity was assumed for analysis. A one-way repeatedmeasures ANOVA revealed a significant within-subjects effect of condition on EDA (F(2, 29.999) = 11.222, p < .001, ηp2 = .113).

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Findings

and that increases in challenge may also result in increases in these psychological states.

Post-hoc analysis revealed that for the main effect on EDA, the Overload condition (M = 14.76, SD = 6.79) showed significantly higher levels of activation than the Boredom (M = 13.60, SD = 6.71, p < .001) condition; likewise, the Balance (M = 14.20, SD = 6.79) condition showed significantly higher levels of activation than the Boredom condition (p = .032). See Figure 6 for a visualisation of these results.

The comparatively low levels of EDA in the Boredom condition are unsurprising in the context of the game condition. Whereas both Balance and Overload featured combat, violence, risk of failure and challenge, the undemanding nature of the Boredom condition required only that participants traverse an enemy-free map to fulfil a repetitive fetch task. Dawson et al. [11] report that EDA, primarily a measure of arousal, is associated with stimulus novelty, intensity, surprise, significance and emotional contentment; as most of these experiences are notably absent in the Boredom condition, this plausibly explains the lowered EDA response for the condition. Furthermore, the association between increases in EDA in response to negative feedback, stress, and frustration plausibly explain the relatively elevated levels of EDA in the Overload condition over both the Boredom and Balance conditions. While both Overload and Balance shared experiences of combat, violence, failure, and challenge, the Overload condition leaned more heavily on the negative aspects of these experience through guaranteed failure, overwhelming enemy presence, and impossible combat situations.

Figure 6. EDA results across Boredom, Balance, and Overload

Analysis performed on the pairwise comparisons revealed a low to medium effect size for EDA across all three conditions. While the effect sizes for Balance compared to Boredom (d = .088) and Balance compared to Overload (d = .082) were similar, a greater difference was found for Boredom compared to Overload (d = .17). Notably, despite the absence of a significant difference in post-hoc analysis between the Balance and Overload conditions, this comparison shared a similar effect size with the Balance and Boredom conditions.

Despite the intuitiveness of the result, the finding of decreased EDA in low stress, boring or unchallenging play experiences is not consistently corroborated by other research in the player experience space. In research undertaken by Mandryk et al. [28] comparing beginner, easy, medium and hard difficulties, no main effects of difficulty level were found on any of the physiological measures, including EDA, employed within the study. Mandryk et al. suggest that this may be a consequence of inconsistent participant responses to the difficulty settings, and - as a result of methodology that featured consistent participant‒researcher interviews throughout - response to the experimental situation, rather than to the experimental manipulations. It should be noted that Mandryk et al. also report that only the beginner condition was perceived as significantly less challenging than the remaining conditions; this may indicate a lack of clear challenge distinction between conditions, or be a function of the relatively small sample size of eight.

DISCUSSION

The results clearly support that participants experienced greater arousal in the Balance condition compared to the Boredom condition, as well as the Overload condition compared to the Boredom condition. Although the pairwise comparison between Balance and Overload did not reach statistical significance, it might be premature to conclude that electrodermal response did not increase between the Balance and Overload conditions. The overall evidence (significant univariate effect, similar increases in amount of EDA between conditions [as shown in Figure 6], and effect sizes that are similar between Boredom-Balance and Balance-Overload and much larger between BoredomOverload) suggests an increase in arousal across conditions (however it is key to note that these non-significant results cannot support any strong conclusions and are best considered as a worthy path for future research). As such, this tentatively establishes a relationship between increased EDA and increased challenge. The robust relationship between electrodermal response and stress, anxiety, and mental cognition [4] suggests that one or more of these factors may also be true of the experience of challenge –

The findings of the current study also differ from those of Kneer et al. [25], in which no effect on physiological arousal (measured by EDA) as a consequence of difficulty was found. It is possible that the difficulty manipulations used in this study are incomparable to those used by Kneer et al. For example, the Overload condition in this research was developed with the intention of overwhelming the player and rendering the task impossible; while Kneer and colleagues aimed for higher difficulty, it was likely not to the same extent. This could also be true of the Boredom condition in comparison to the low-difficulty condition employed by Kneer et al. As such, the more extreme disparities between conditions reported within this paper may also explain the emergence of the reported pattern of

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results. Another explanation for the disparity in results may be differences in experimental design - play times in the study undertaken by Kneer et al. were 20 minutes in length, compared with the 10-minute-30-second play sessions employed in this study. Therefore, it is possible that participants in Kneer et al.’s study played long enough to physiologically habituate more readily to the high difficulty condition than in the Overload condition used in this research.

Overload conditions. While the pattern of results clearly point to increased electrodermal activity with increased challenge, the absence of a significant difference between Balance and Overload nonetheless warrants discussion. A more conservative interpretation of results (focussing on the lack of statistically significant difference between Balance and Overload, irrespective of effect sizes and the larger pattern of results) would suggest that difference in arousal between Overload and Boredom is less reliable than the difference between Balance and Boredom. This may reflect that for some participants the overload condition was so overwhelming to effectively cease to be challenging. As previously stated, participants experienced greater enemy numbers and higher incidences of player death in the Overload condition than in the Balance condition; following these conclusions, an expectation may be that EDA response would be significantly higher for the Overload condition than the Balance condition. This is supported by considering the larger pattern of results (similar effect sizes – however, if the conservative interpretation is followed, the absence of a significant difference is possibly explained by the potential for detachment or disengagement from the Overload condition, or by physiological habituation.

The results also differ from Drachen et al.’s [12] exploration of the physiological correlates of player experiences in first-person shooter games, in which no significant correlation between EDA and challenge was found. As previously mentioned in ‘Related Work’, Drachen et al. propose that this was a consequence of imprecise wording used in the scale – rather than that challenge does not influence EDA. On a surface level, the findings of the current study align with Nacke & Lindley’s [31] discovery of decreased electrodermal activity in a ‘boring’ condition than in a more challenging condition. However, as discussed in ‘Related Work’, it is important to note that the ‘boredom’ condition created by Nacke & Lindley was also manipulated to provoke a less immersive experience (using repetitive textures, dampened sound, and so on). It is likely the electrodermal response may have also been influenced by the inclusion and absence of immersive features. Following this, Nackle & Lindley’s ‘boring’ condition is not directly comparable to the Boredom condition employed within this study, which varied only on challenge. As such, while this research partially supports Nacke & Lindley’s findings, it also presents an additional result in that electrodermal activity is also responsive to player experiences that differ only in challenge. One conclusion could be that increased EDA indicates greater challenge, but not necessarily a more optimal player experience.

That physiological habituation may have occurred is supported by the repetitive experience of play in the Overload condition - players were almost immediately overwhelmed by the enemy zombies, died and had to respawn and repeat the scenario until the session terminated. Conversely, in terms of study design, the semicounterbalancing of the condition order may have resulted in habituation to the conditions regardless of differences in challenge. As the Overload condition was always placed last for fear of lasting effects on the Boredom and Balance condition, Overload may be more susceptible to habituation - a possible explanation for the non-significant differences in EDA arousal between Balance and Overload. It is also possible that there were still differences in how players of different ability experienced the game; expert players, for example, may have felt more comfortable than novice players even in the Overload condition due to familiarity with the control scheme. This is challenging to avoid in any scenario, and so DDA presents the best option for ensuring a consistent experience.

Further clarification may be found in research undertaken by Ravaja and colleagues, in which increased EDA was found when a player was killed or wounded, or when they killed or wounded an enemy [34]. In the Boredom condition employed within the current study, no enemies were present and player death was impossible; in the Balance condition, some enemies were present, and player death was possible; and finally, in the Overload condition, an excessive number of enemies were present and player death was inevitable. Therefore, increased exposure to the death/wounding of both enemy opponents and the player-character may also be partially responsible for greater EDA in the Balance condition than the Boredom condition, and in the Overload condition than in both the Balance and Boredom conditions. This may also potentially be true of the results reported by Nacke and Lindley [31].

Finally, it is important to acknowledge the possibility that decreased EDA was not necessarily or solely representative of increased boredom. As discussed within ‘Related Work’, the complex domain relationships between the psychological and physiological states suggests that one physiological response (in this example, decreased EDA) may be indicative of multiple or varied psychological states. While we believe, based on condition design, that the decreased arousal likely reflects a boredom state, it is also possible that participants were instead – or additionally – experiencing another low-arousal state, such as relaxation.

The applicability of Ravaja et al.’s conclusions to this study may be complicated by the absence of significant differences in EDA response between the Balance and

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CONTRIBUTIONS

effect to influence the player experience data collected from the Overload condition. One potential impact of the semicounterbalanced approach may be the possibility for physiological drift to influence EDA results. As EDA presents an additive signal, it is possible for the signal to continually increase overtime over the course of the experiment; as such, this could potentially influence the increased EDA signals found in the Overload condition. However, Braithwaite et al. [6] note that drift is primarily only an issue in long-term experiments, and especially long-term ambulatory experiments; furthermore, the impact of potential drift has been somewhat mitigated by the inclusion of baselines between each play session. Despite this, future research may entail further data collection within a fully counter-balanced version of the current study methodology, as well as the use of the baseline data as a calibration point for EDA analysis and investigation of physiological drift. There is also the possibility that greater muscle activity occurred in the more challenging conditions (e.g. more rapid interaction with mouse and keyboard), although it is not possible to know whether such movement was sufficient to influence sweat production.

The results reported within this study point to comparatively increased electrodermal activation alongside increased challenge. While these findings are consistent with current understanding of EDA in psychophysiological literature, they are incongruent with extant player experience literature - highlighting the need for ongoing examination of the psychophysiology of challenge within the player experience space. While other player experience research has co-varied challenge with other features of gameplay (e.g. violence, immersion) or employed a small sample size, this study represents a contribution to the field through the assessment of the impact of challenge with a) a large sample size, b) a successful commercial video game, and c) the isolation of challenge-skill balance in the absence of other factors. This optimistically represents a notable contribution in terms of reducing uncertainty in the field and identifying key foci for future research. These findings have interesting implications for future game development. An establishment of a relationship between physiological arousal, as measured by EDA, and challenge suggests potential avenues for biofeedback: for example, the dynamic difficulty adjustment of a game based on electrodermal activation. Similar implications emerge for play experience evaluation and playtesting – due to the covert and quantitative nature of psychophysiological analysis, EDA has potential for reliable deployment within games testing spaces in both research and industry. If the relationship between increases in EDA and increases in game challenge is reasonably confirmed by future research, this also positions EDA as useful marker of comparative challenge in video games. This is bolstered by the relative (to other psychophysiological measures) ease with which EDA can be deployed.

An additional consideration may be the relationship, or overlap, between challenge and stress. Adams [1] posits that challenge leads to stress in the presence of time pressure, as would be the case in our study. Whether or not the constructs these constructs are disparate, or if EDA can reliably separate them, would represent a useful endeavour for future study. It would also be valuable to explore a more generalisable approach to challenge-skill manipulation. The design of the video game artefacts used within this study were undertaken to ensure both the Boredom and Overload conditions were representative of sub-optimal player experiences for all participants, regardless of ability; a future study may employ a more subtle approach (e.g. a difficult, but reasonably so, condition) to improve ecological validity.

Finally, this study has contributions beyond player experience. That increased challenge is associated with increases in EDA may be useful for HCI spaces in which the user experience is critical - for example, user understanding of novel software. This also represents a promising path forward for both health and teaching software, in which understanding of both task difficulty and user stress is critical in evaluating the ability or success of the program. This finding may also be of interest to general psychophysiological research, as another contribution to the current understanding of the relationship between increases in EDA and increases in task difficulty.

As for the generalisability of results, the video game conditions used for this research program were restricted to a single-player first-person shooter PC game. Future studies in this space may benefit from expanding this research to include additional genres, environments and platforms in the interest of results’ generalisability. Furthermore, as human interaction has been found to have a profound effect on physiological response [20], future research may benefit from a large-scale psychophysiological evaluation of a social play experience.

LIMITATIONS & IMPLICATIONS FOR RESEARCH

As the Overload condition was designed to overwhelm, frustrate or evoke anxiety in participants, a semicounterbalanced design was introduced to mitigate the potential for a long-lasting negative mood induction inadvertently influencing the remaining conditions. Within the semi-counterbalanced design, the Overload condition was placed to always occur last in condition order; both the Boredom and Balance condition were fully counterbalanced. Consequently, potential exists for order

CONCLUSION

This research suggests a relationship between increased video game challenge and greater electrodermal activity, reflecting the link between challenge and EDA established outside of PX research). These results represent a novel contribution to the understanding of the role of challenge within the player experience in terms of being the first to

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11. Michael E. Dawson, Anne M. Schell, and Diane L. Filion. 2000. The Electrodermal System. In Handbook of Psychophysiology (2nd. ed.), John T. Cacioppo, Louis G. Tassinary, and Gary G. Bernson (eds.). Cambridge University Press, New York, NY, 200-223.

show this link in the context of videogame play as well as offering insight regarding how previous PX studies exploring challenge and EDA may relate. These findings suggest EDA may be a useful tool for both researchers and game developers seeking a real-time, non-intrusive measure of challenge. A relationship between electrodermal activity and challenge also presents a potential foundation for the use of biofeedback in game DDA. Despite the shared pattern of results, the absence of a significant difference between the Balance and Overload condition warrants caution in the interpretation of these results - and presents an opportunity for future research investigating the psychophysiological influence of video game challenge.

12. Anders Drachen, Lennart E. Nacke, Georgios Yannakakis, and Anja Lee Pedersen. 2010. Correlation between heart rate, electrodermal activity and player experience in first-person shooter games. In Proceedings of the 5th ACM SIGGRAPH Symposium on Video Games (Sandbox ’10), 49-54. https://doi.org/10.1145/1836135.1836143 13. Stefan Engeser and Falko Rheinberg. 2008. Flow, performance and moderators of challenge-skill balance. Motivation and Emotion 32, 3: 158-172. https://doi.org/10.1007/s11031-008-9102-4

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