Reaction Time and Game Design

Have you ever felt like a video game was just... unfair?  Maybe an enemy seemed to come out of nowhere or went at a speed that a normal human shouldn't be expected to keep up with.  Nowadays, game designers usually err towards making things easier on the player, but in the early days of gaming, this was not the case.  Arcade game developers, in particular, didn't want individuals monopolizing the machine, so there was a financial incentive to keep games difficult.

There are a lot of ways in which a game can seem unfair, but here I'm going to focus on action games that demand too much of your reflexes.  Of course, it's reasonable to challenge the player, but if a game is frequently demanding superhuman response times, there might be something amiss.

How Long Do We Need?

Human Reaction Time


The ability of human beings to respond to sights and sounds, whether in video games or elsewhere, depends upon the speed at which we can translate sensory stimuli into actions.  This, in turn, depends upon a wide variety of factors, like the person's age and health or the complexity of the response required.  

Let's look at a simple case, where you need to push a button the moment an enemy appears on the screen.  This scenario is fairly common in early video games; Space Zap or Cosmic Ark, for example, make it a central part of the gameplay.  It's also a common scenario in scientific research, and you can actually test it for yourself on various websites (here and here, for example).  These tests typically find response times of around 200 - 300 milliseconds for a simple visual cue (auditory cues take slightly less time).

What does this correspond to in gaming terms?  Usually, gamers express time delays in terms of frames.  If we assume a frame rate of 60 frames per second (fps), as is common in early video games, 200 - 300 milliseconds corresponds to 12 - 18 frames of action.  This means that a typical gamer will need on the order of 15 frames to make simple game-related decisions.  More complex decisions, like choosing a path, will tend to take longer, but I'll treat 15 frames as a baseline.

Input Lag

This isn't the end of the story, however.  Even if it only takes 15 frames for you to press the button on the controller, you still need to account for the time it takes for the game to process your command.  And what if there was a delay in the game sending updates to your display?  Both of these things are casually referred to as "input lag", and it's a very common concern among serious action gamers.  

Unfortunately, the input lag can vary wildly from one system to the next, so this can get complicated fast.  Some classic console systems show minimal lag; for example, the Atari 2600 had 0-2 frames of lag at 60 fps in test runs (see Code Dojo).  Modern gaming systems typically have a bit more -- 4 - 8 frames of lag at 120 fps -- while an arcade-like setup was found to have ~5 frames of lag at 60 fps (for Street Fighter II Turbo).

If you're using an emulator and/or non-native display hardware, the situation can get even worse.  In the case of MAME, the amount of input lag will depend on your build and video settings.  Shmupmame might be a good choice if you're trying to play with minimal lag in shoot 'em ups, and GroovyMAME has been shown to have minimal input lag in specific cases, but there is no magic bullet.  And if you're concerned about lags in your display, I suggest the DisplayLag database.

As you can see, this is a complex subject and not one I'm equipped to address in any detail, but clearly there will be an additional delay added on to the 15 frames of human response.

What Does This Imply for Game Design?

A brief section of gameplay from the arcade game, Wizard of Wor, where the player is getting run through by a monster.

Our first instinct might be to fault any game that demands more than about 20 frames of response time for any player action, but that would be going to far.  Video games are an art, and as in any art, there are no rules that govern what makes good or bad game design.  Sometimes "unfair" situations have their own aesthetic appeal.  If you're making a survival horror game, for example, it might make good sense to have moments when the player feels totally helpless. 

That being said, I think in the majority of cases where a game places unreasonable demands on a player's reflexes, it's just lazy game design.  Ideally, a game will provide opportunities for improvement to players of all levels -- the more you play, the more you improve and the more immersed you become in the game.  If, on the other hand, all of your in-game failures are hopeless situations, there's not much opportunity for growth.

For example, compare two games that I've already covered in depth: Pac-Man and Phoenix.  In Pac-Man, the player is given many opportunities for improvement; for example, by identifying safe patterns through the maze or learning to recognize ghost configurations.


Most of the enemy movements follow simple, fixed rules, with only the post-power-pellet ghost scatter having any randomness.  It's possible to find yourself in hopeless situations in Pac-Man, but you can learn to avoid them from experience, much as a chess player learns to avoid getting their king trapped.

Phoenix, by contrast, employs a lot more moment-to-moment randomness.  The bird flight patterns appear to be randomly drawn from a set of complex shapes, like the ones below.


The small loops in these patterns will sometimes be executed in less than 15 frames of action, even on the first stage of the game!  Learning to recognize and anticipate patterns like this seems unrealistic for all but the most experienced players, meaning it's unlikely you'll see much improvement in your performance without having played the game a lot.  Until that happens, the enemy movements will appear random. 

Galaxian used a similar game design as Phoenix, but with much simpler enemy dive patterns.

The flight paths of the three different alien types in the arcade game, Galaxian.

I think this simplicity played a big part in Galaxian's success.  Players can learn to anticipate enemy movements, even when several are attacking at once.  Much as in Pac-Man, you can end up in a hopeless situation, but only after failing to account for the enemy movements appropriately.

As we move forward in gaming history, action games will increasingly provide players with opportunities to replenish themselves through power-ups or recharging damage meters.  In these games, the question of "unfair" gets increasingly blurred because a player can get away with choices that minimize the chance of damage rather than eliminate it.  However, the basic limits of human response still apply, and there should be a reasonable opportunity for the player to succeed other than sheer luck.

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