5Agent Reaction TimeHow Fast Should an AI React?
Steve Rabin

5.1
5.2
5.3

Introduction
Context Is Key
What Does Cognitive
Research Tell Us?

5.4
Conclusion
References

5.1 Introduction
Modeling human behavior is a core aspect of game AI, which raises a very simple, yet
important question: How fast should an enemy AI react to seeing the player?
Like many questions concerning human behavior, it is difficult to answer precisely andlargely depends on context. Researchers have investigated human response time for over
100 years, trying to time the inner workings of the brain in an area of study known asmental chronometry [Posner 05].
However, if you want the quick and dirty answer to what should be used for an AI’s
reaction time, then let’s just spill the beans. It’s somewhere between 0.2 and 0.4 seconds,
possibly longer depending on context. If you’re curious as to which exact number to use
and where these numbers come from, you’ll need to read further!

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5.2 Context Is Key
There are two primary situations, studied by cognitive psychologists and neuroscientists,
which are commonly applicable to game AI and result in different reaction times:
1. Simple reaction time: An AI agent is aiming at a doorway, expecting an enemy
to come through. When the enemy is finally seen, the agent pulls the trigger andthe gun fires. We want to know the time between the enemy appearing in theagent’s vision and the gun fired. This time period is modeling the time taken for
the agent’s brain to recognize the presence of the stimulus, the time to turn thatrecognition into a decision to fire, the time for the finger muscle to be told to contract, and the time to physically pull the trigger until the gun fires.
2. Recognition or go/no-go time: An AI agent is aiming at a doorway, expecting
either a fellow teammate or an enemy to come through. When someone is seenin the doorway, the AI must recognize if it is a teammate or an enemy, then
only pull the trigger if it is an enemy. What makes this more difficult than asimple reaction time is that the agent must discern between two different possible stimuli. We want to know the time between the enemy appearing in theagent’s vision and the gun fired. This time period is modeling the time taken
for the agent’s brain to see the stimulus, the time to recognize it is an enemy,
the time to turn that recognition into a decision to fire, the time for the finger
muscle to be told to contract, and the time to physically pull the trigger until
the gun fires.
It was recognized by Donders, a Dutch physiologist and ophthalmologist, in 1868 thata simple reaction time is faster than a go/no-go reaction time [Donders 69]. It has also
been found that the weaker the intensity of the stimulus, the slower the reaction time
[Luce 86].
A further complication with context is whether the AI agent is experiencing a momentary attentional lapse, which is common in humans and would cause much more variability in reaction times.

5.3 What Does Cognitive Research Tell Us?
Fortunately, researchers have determined average times for our two primary situations.
5.3.1 Simple Reaction TimeIn the context of a simple reaction time, the mean time of college-age individuals for anauditory stimulus is 0.16 s and the mean time for a visual stimulus is 0.19 s [Kosinski 13].
Other research puts the mean time for a visual stimulus at 0.22 s [Laming 68]. This time
is typically determined by telling the human subject to tap a button when a light stimuluschanges state (for example, turns from off to on or from green to red). You can try this test
yourself at http://www.humanbenchmark.com/.

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These simple reaction time studies are most similar to an AI agent firing a gun when
they are focused, expecting an enemy, and aiming directly at the enemy when it is seen.
However, there are three caveats that would increase the time beyond ~0.2 s:
1. If the stimulus is weak, then the reaction time is slower [Luce 86], for example,seeing an enemy far away versus seeing an enemy close up.
2. If the AI agent must aim after seeing the enemy, then that added time must be
factored in.
3. A lapse in focused attention, which is common in humans, would cause the reaction time to be much higher.
5.3.2 Recognition or Go/No-Go TimeIn the context of having to differentiate between two different stimuli and only acting on
one of them, the mean time for a visual stimulus is 0.38 s [Laming 68]. The stimuli typically used in this test are two-digit numbers, where the subject must push a button when
the number is above 50 and not push the button when it is below 50. You can try a versionof the go/no-go test yourself at http://cognitivefun.net/test/17.
Go/no-go time studies are most similar to an AI agent firing a gun when they are focused,expecting either an enemy or a friend, and aiming directly at the subject when it is seen. Aswith simple reaction time, the caveats that would increase reaction time also apply here.
5.3.3 Complex Cognitive Task Reaction TimeAs you would expect, the more complex the cognitive task, the more time it typicallytakes. When faced with a choice between several items, choosing the best one will takelonger than a go/no-go reaction time [Donders 69]. However, to get reasonable responsetime estimates for any unique complex task, a careful experiment with human subjects
would need to be constructed and executed.

5.4 Conclusion
As you program your AI behaviors, you’ll be forced to choose a reaction time for youragents. However, based on the context, you should use different reaction times.
The good news is that we are fairly confident that a simple reaction time should bearound 0.2 s, while a go/no-go reaction time is around 0.4 s. From there, you’ll need to
add additional time to account for weak stimuli, more complex cognitive recognitions,additional movement time, or lapses in attention. As for picking the ideal reaction timefor your particular game, it ultimately depends on the exact context and what feels rightto the player.

References[Donders 69] Donders, F. C. 1969. Over de snelheid van psychische processen [On thespeed of psychological processes]. In Attention and Performance: II, ed. Koster, W.
(Original work published 1868). Amsterdam, the Netherlands: North-Holland.

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[Kosinski 13] Kosinski, R. 2013. A literature review on reaction time. Clemson, SC:
Clemson University. http://biae.clemson.edu/bpc/bp/Lab/110/reaction.htm (accessedSeptember 10, 2014).
[Laming 68] Laming, D. R. J. 1968. Information Theory of Choice-Reaction Times. London,U.K.: Academic Press.
[Luce 86] Luce, R. D. 1986. Response Times: Their Role in Inferring Elementary MentalOrganization. New York: Oxford University Press.
[Posner 05] Posner, M. 2005. Timing the brain: Mental chronometry as a tool in neuroscience. PLoS Biol 3(2): e51. doi:10.1371/journal.pbio.0030051. http://www.plosbiology.
org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0030051#pbio-0030051-b18(accessed September 10, 2014).

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