Incorporating Biometric Data Into FPS Games
How can PC games interface with the player’s body? How do we leverage human biology and cognition to influence game states?
Bloodline is a single-player, first-person shooter that follows a futuristic criminal as he escapes levels of a prison. The game uses the player's heartbeat to unlock new mechanics, creating an organic source of tension across play sessions.
I created Bloodline after leaving the Five Nights at Freddy’s: Security Breach team to reconcile the sensational, serendipitous design I practiced professionally and the competitive, analytical design that ignited my love of games.
June 2021 — August 2021
Unreal Engine 4
Arduino Pulse Sensor
Audio & Dialogue
HOW CAN VIDEO GAMES INTERFACE WITH HUMAN BIOLOGY?
When we think about human-computer interfaces, we often limit our discussion to wearable technology rather than commercial games. The Fitbit on your wrist or the Health app on your iPhone are great examples of how machines can gamify our relationship with our bodies. But what if I told you that all games interface with our biology?
To see where I'm coming from, let's first look at the communication point between games and biology. Games are fundamentally emotional experiences—they make us feel things. If you don't believe me, I would recommend joining a Ranked League of Legends game and insta-locking Teemo. Try not to FF at 15—the late game is well worth the wait.
Emotion is a tricky subject for neuroscientists—myself included—because it's far more nebulous than we'd like. I like to use the Cannon-Bard Theory, which I've illustrated below.
A Side Note
Bloodline focuses on the biological basis of emotion, but what about cognition? Imagine that you've just settled into your calculus class, and that philosophy major with the half-moon glasses sits behind you. Now, imagine that you've encountered a venomous snake on your weekend hike. Lovesick college kids and thrill-seeking nature fanatics may come to different conclusions about how they feel, but they exhibit the same suite of biological responses—heart pounding, knees locking, hands shaking. That's because biological responses are nonspecific; we use cognition to contextualize those responses and select an appropriate label for the combined experience.
Emotions arise from a combination of biological and psychological responses. Think about your biological responses to playing games—your hands sweat, your heart races, your peripheral vision subsides. We can conceptualize these responses as metaphors for tension between the player and the game systems.
We can measure a number of biological responses to games, but I chose to record heartbeat due to the cultural significance of heart rate as a proxy for emotions (thanks, Freud). I used an Arduino Pulse Sensor connected to an Arduino Uno to collect the player's heartbeat directly from their right-hand radial pulse. The sensor can be used on either hand, but I'm electing to use my right hand to leave my left hand free to use the keyboard.
The timing of each heartbeat feeds into the Arduino Integrated Development Environment, which outputs the heart rate. The following C++ code shows one method of calculating BPM.
The Arduino setup connects to a computer using a USB A/B cable. Information from the COM port feeds into Unreal Engine 4 through the UE4Duino plugin.
HOW DO SUPPORTING GAME SYSTEMS CONTRIBUTE TO EMOTIONAL PAYOFF?
Do Games Have a Heartbeat?
When thinking about how to incorporate the player's heartbeat into Bloodline, I often relied on the design considerations behind Five Nights at Freddy's: Security Breach, where emotional payoff served as a core goal of the franchise. Sensational design operates on a framework called emotional architecture, for which designers tend to evaluate changes in the player's emotional state over time rather than moment-to-moment gameplay.
We can think about emotional architecture in terms of a real heartbeat, shown to the right. Critical friction drives the uptick from Q to R; it's the set of conditions within which the player's decisions become meaningful. Often, critical friction arises around a threat—should you counter your opponent's Queen's Gambit with a Sicilian defense or Albin Counter-Gambit? Do you drop in Pleasant Park or Colossal Coliseum? Emotional payoff does not arise from friction itself, but rather its release—the catharsis following a jumpscare in Five Nights at Freddy's or the satisfaction of a headshot in VALORANT.
Everything surrounding the core game loop from Q-S provides a sense of apprehension, which is crucial in determining a game's pace. If Five Nights at Freddy's had too little apprehension—as in, the player was jumpscared every few seconds— the game would become too predictable to be scary. If it had too much apprehension, the player would progress through large portions of the game without any emotional payoff.
To determine my source of critical friction and Bloodline's genre, I analyzed games that met the following three criteria.
Combat-driven - I chose this criterion purely due to my curiosity as a designer
First-person - I wanted to emphasize the feeling of being embodied rather than controlling an embodied character
Elicits drastic changes in BPM - Due to hardware limitations, the pulse sensor requires large changes in heart rate
- The use of standard FPS controls facilitated high learnability
- The match-based structure created inconsistent spikes in heart rate rather than phasic changes
- Semi-transparent HUD is easy to parse and imposes a low cognitive load
Mirror's Edge Notes
- Low learnability, but forgiving checkpoint system thwarted frustration from not immediately understanding the controls
- Parkour system of movement felt organic; however, the pacing was consistently fast rather than phasic
- Combat was largely optional, infrequent, and weak; it did not fulfill the power fantasy I derive from combat-driven games
- Similar forgiving checkpoint system to Mirror's Edge
- More intuitive and fantasy-like parkour movement than Mirror's Edge; grappling hook allowed players to exit tough situations
- "Sensory boost" mechanic facilitated phasic pacing by allowing players to quickly clear enemies
- Uncomfortable hand position; default controls required players to alternate their pinky between left Shift and left Ctrl keys
Genre Selection and Movement System
Borrowing from the above three games, I chose to create an FPS due to the high standardization of the controls. Drawing from the themes in Ghostrunner, the player—controlling a criminal protagonist—must escape a prison by killing all of the guards in a given time frame. I found that the parkour movement in Ghostrunner and Mirror's Edge facilitated more long-term changes in my heart rate than VALORANT, and the organic nature of the movement system paired well with the biological theme.
I built an analogous parkour system to Mirror's Edge by adding crouch, slide, sprint, wall run, mantle, and ledge grab to the default FPS controls. I noted that the input design of both Mirror's and Ghostrunner felt uncomfortable due to reliance on both the left Shift and left Ctrl keys. Therefore, I removed the use of the Shift key by making sprint, wall run, mantle, and ledge grab trigger automatically based on proximity to walls.
Mirror's Edge's (left) and Bloodline's (right) gray box parkour system
Emotional payoff is difficult to harness in parkour systems because interruptions—such as missing a wall run in Mirror's Edge—are disruptive to the game's pacing. In our heartbeat analogy, think about having an extremely high heart rate before flatlining; it's an unpleasant shift that can break the player's immersion.
Ghostrunnner's pace feels more like a sine wave than a binary function, which I attribute to the "sensory boost" mechanic. While the player is suspended in air, they can trigger an effect that slows down time and highlights enemies. For a brief period, the player can gauge their positioning and queue their next moves. This ability provides a relief valve during high-friction situations and enhances the player's sense of mastery over their environment.
Sensory boost in Ghostrunner
I mimicked Ghostrunner's sensory boost with the "Bloodrush" ability by triggering a time dilation when the player's heart rate reaches a certain threshold. Moreover, the enemies and prediction lines from their guns become highlighted to provide the player with more information to evaluate the situation. To limit the usage of this ability, the player can only trigger Bloodrush once every ten seconds.
The lower HUD elements show the player's health (left) and a custom heart rate metric (right). Most measures of BPM utilize an aggregate average over ten-second intervals, causing slower rate fluctuations. My metric shows real-time beat counts and resets every ten seconds, evaluating to faster rate changes. Bloodrush is triggered when the player's heart beats over ten times in ten seconds.
HOW CAN WE FURTHER EMBED HEARTBEAT IN THE CORE GAME LOOP?
Resource management is one of the more subtle themes in Bloodline's gameplay; it emerges only when Bloodrush is on cooldown, forcing the player to rely on other systems before the ability becomes available. However, the player's unlimited supply of bullets leaves little reliance on Bloodrush.
To enhance the theme of resource management and thus add a new source of tension, future iterations of Bloodline will leverage heartbeat counts as currency. At its current state, Bloodrush is triggered regardless of the player's desires; players may feel that they wasted the ability during periods out of combat or wish to save the ability for another time. With a currency system, the player may spend a single beat on a bullet or ten beats to use Bloodrush. Not only will this condition provide players with more opportunities for skill expression, but it will also necessitate more frequent usage of the parkour movement system, which emphases satisfying traversal and drives heart rate changes.
Opacity vs. Depth
During one particular session, I explained the metrics behind Bloodrush and its threshold to one of my playtesters. The next time he played, he began holding his breath to increase his heart rate above the threshold. Though I initially conceptualized Bloodrush as an organic relief valve to high-tension scenarios, he demonstrated that some players want to control the game's pacing rather than allow it to be dictated by external systems.
Opacity surrounding Bloodrush engendered a unique sense of discovery; the player can feel when it should be triggered, but they are not told precisely when or what it does. However, that sensation wears off after the first-time experience. Systems that enable strategizing and skill expression—such as resource management—permit greater depth and longevity than short-term sensational experiences.
Bloodline now utilizes a currency system, in which the player may spend one heartbeat on a bullet or ten heartbeats on a Bloodrush effect. This modification forces the player to find new avenues for skill expression through the parkour system. It additionally includes updated UI and alerts to notify the player when Bloodrush becomes available, which enhances learnability without compromising the player's sense of discovery.
Bloodline's protagonist offers a high-agility, high-fragility power fantasy that facilitates rapid decision-making and frequent reruns. However, early feedback from playtesters suggested that the damage system was too unforgiving. I implemented a 5-cost shield that blocks all incoming damage for ten seconds to allow the player to experience longer sessions of fast-paced gameplay.
The shield, mapped to the E key, is indicated by a honeycomb overlay
Progression systems create a positive feedback loop between the player's and avatar's mastery. Player progression—the player improving their knowledge of the game—often facilitates abstracted progression—in-game statistics that evolve a character's abilities. Think about snowballing in League of Legends; the autotelic reward of landing Yasuo's wombo-combo provides more gold to launch even deadlier attacks. Once you hit your 0/7/0 power spike, much of the apprehension of the early game subsides, producing a faster pacing and more frequent opportunities for emotional payoff.
Bloodline takes a different approach to progression systems by eliminating abstracted progression. The heartbeat system demands a consistent difficulty and pacing within levels; if the player finds a level too easy, their sense of urgency dissipates, their heart rate slows, and they run out of bullets. If the player finds a level too hard, their sense of urgency skyrockets, their heart produces an infinite supply of bullets, and we're back to square one. Therefore, Bloodline must deliver the end goal—the feeling of complete mastery over the character—without altering the game's pace.
A Balancing Act
The ten-second shield over-compensated for the unforgiving damage system. The following changes have been implemented to maintain a consistent difficulty level and fast pace.
- Enemy firing period decreased from 2 seconds to 1.25 seconds
- Damage increased from 33 to 40
- Shield cost increased from 5 to 10, and Bloodrush cost increased from 10 to 20
- Shield blocks all damage for 5 seconds, then blocks half of all damage for 5 seconds
- Bloodrush decreases the camera's field of view to improve the player's aim
As an added bonus for the most resourceful Bloodline players, headshots kill enemies instantly.
Bloodline is my first foray into the more analytical approach to design that underlies most of the games I love. Though I have only been making games for one year, the kind of designer I am and the kind of player I am have emerged as very different people. Bloodline represents my attempt to bring those two individuals closer together. I would love to expand the core mechanics of Bloodline into a multiplayer setting, and I invite you to check back on this page in a couple months as I sort out my motivations as a designer.