In Human Computer Interaction and User Experience there's some oft repeated rules of performance:

  • 0.1 second is about the limit for having the user feel that the system is reacting instantaneously, meaning that no special feedback is necessary except to display the result.
  • 1.0 second is about the limit for the user's flow of thought to stay uninterrupted, even though the user will notice the delay....
  • 10 seconds is about the limit for keeping the user's attention focused on the dialogue...

I'm most interested in this "instantaneous" response. Related is this User Experience Question. A related answer does bring up some sources:

Card, S. K., Robertson, G. G., and Mackinlay, J. D. (1991). The information visualizer: An information workspace. Proc. ACM CHI'91 Conf. (New Orleans, LA, 28 April-2 May), 181-188.

Miller, R. B. (1968). Response time in man-computer conversational transactions. Proc. AFIPS Fall Joint Computer Conference Vol. 33, 267-277.

Myers, B. A. (1985). The importance of percent-done progress indicators for computer-human interfaces. Proc. ACM CHI'85 Conf. (San Francisco, CA, 14-18 April), 11-17.

However I can't access any copies of those articles. Also, frankly, they're all conference papers. Big HCI research often debuts at conferences, but ideally I'd like something robust which has been tested rather than something written 20-40 years ago for a conference.

One thing I've noticed is the "instant response" threshold quoted tends to be around 100ms to 200ms. Typical human reaction time is about 200ms so I'm wondering if there is a relation there. It seems possible; if something happens so quickly you can't react to it, it is practically "instant" in a way.

Approximately what length of time between action and reaction is perceived as instant by humans?

  • 2
    $\begingroup$ This question was inspired by Defining “Instantaneous” as part of usability acceptance criteria over on User Experience $\endgroup$
    – Ben Brocka
    Sep 18, 2012 at 19:16
  • $\begingroup$ My personal research has lead me to discover that quantum physicists seem to think that the information slips out of our dimension/universe travels back in time and tells the brain e.g that you have touched an apple thus appearing to be instant. I appreciate that this is far fetched to say the least - however a haphazard non linear approach to information creation and annihilation seems to be the workings of the universe - eg quantum entanglement state. how do you like them apples? $\endgroup$
    – user2815
    Mar 13, 2013 at 23:19
  • $\begingroup$ Side note: CHI is not like most conferences. They are an "archival" publication and have strict rules about the originality of content. While many researchers treat conferences as a stepping stone to a journal publication, CHI is often the end of the line for research in HCI. $\endgroup$
    – Jeff
    Mar 15, 2013 at 23:33
  • $\begingroup$ Apple has a WWDC video called Designing Fluid User Interfaces with a neat demo of latency in swipe gestures: developer.apple.com/videos/play/wwdc2018-803/?time=352 (you may need a (no-cost) Apple Developer Account to view it). $\endgroup$ Dec 6, 2018 at 18:48

2 Answers 2



This is a very interesting question, that is also somewhat related to my area of research. I know of several related results (which I might add later in an edit), and I thought that with a few minutes of scholar search I'll find a paper dealing with this question exactly. I was surprised to find no such papers. So I decided to conduct an experiment...

Answer (preliminary. hopefully, will be edited)

As with many cases in psychology, the threshold is completely non-existant: there is no delay up to which the reaction is perceived as instantaneous, and after which it is perceived as delayed. You can measure the probability of the subject perceiving a delay, as a function of the delay time. You can choose any probability you want, arbitrarily, and call it the 'threshold'. If we decide on 50% (the chance of responding there was a delay is identical to the chance of responding it was instantaneous) then for me it is about 110ms.

results of preliminary experiment on 1 subject, myself

The results presented here are based on 100 trials, performed by myself. Hardly counts as good study, but if a few more people run the test, I promise to process the results and post them here (and who knows, maybe even publish it somewhere :))

To participate in the experiment yourself

  • Sign up to ofri.webfactional.com. (It is a site I developed for web-based experiments conducted in my lab. It's still in beta, which is why its under a domain with my name, and not under huji.ac.il. I promise we do not use your email for anything except for identifying you. No spam ever).
  • In the Group Name field in the registration, enter instant.
  • Login, and follow the on-screen instructions. Its really simple.
  • After running more than 60 trials another button will appear with the text 'show results'. Clicking it will draw a graph similar to the one presented here, based on your personal results.
  • $\begingroup$ You are testing, what I called the subjective threshold in my comment below. Regarding this subjective threshold, I agree with you completely. But there is an absolute physiological threshold as well. See my other comment. $\endgroup$
    – user1196
    Sep 21, 2012 at 23:34

Interesting question. One (theoretical) answer would be: instantaneous is when people cannot percieve a delay, so the threshold would be the "speed" of perception. But subjectively people might consider something instantaneous, even if they can slowly see it coming, because from their understanding it is "amazing quick" compared to other experiences (for example the media wrote about "instant page load" when DSL was introduced and began to replace dial-in modems). So the question is what exactly you want: do you want subjective user experience, or a physiological limit?

If you look for the physiological limit, look for articles by Rolf Ulrich from Tübingen, Germany, who (among other things) researches how well humans can discern visual or auditory stimuli that are given quickly one after another. His articles give the times. Here is his web page with a list of publications: http://www.uni-tuebingen.de/en/faculties/faculty-of-science/departments/psychologie/research-groups/cognition-and-perception/research-group/rolf-ulrich/publikationen.html (some of them can be downloaded as PDFs from that page).

  • $\begingroup$ Welcome to the site. Is there any chance that you could summarise the relevant findings from Ulrich? $\endgroup$ Sep 21, 2012 at 1:08
  • $\begingroup$ No, I'd have to wade through the articles myself. But I can explain what Ulrich does. His research focus is cognition of time. One typical experiment would present the proband with a light that blinks twice in differing intervals. The subject has to decide wether the interval was shorter or longer than a reference stimulus. That way the average (or rather: probable) limit of discernability is determined. I cannot summarize Ulrich's results, but he is a good starting point if anyone wants to begin research into this area. $\endgroup$
    – user1196
    Sep 21, 2012 at 11:07
  • $\begingroup$ There is a difference between the question "what is the smallest time interval between 2 stimuli we can discriminate" and "what is the smallest time interval between action and response that we can discriminate". If we perform an action that is expected to have an immediate response, we are much more likely to perceive the response as immediate. We don't have such predisposition for 2 stimuli. So I would assume we perceive as instantaneous much larger time delay for action-response than the time interval needed to tell 2 arbitrary stimuli apart. $\endgroup$
    – Ofri Raviv
    Sep 21, 2012 at 23:11
  • $\begingroup$ For the brain an action is a stimulus. Because: How do we know that we acted? By perceiving the feedback from our external (seeing the arm move) and internal (feeling the arm move) senses. So action-reaction is also stimulus-stimulus. The only difference is the knowledge of the causes of the stimuli. This knowledge certainly influences the judgments made regarding the stimuli, and – as I wrote in my answer – people might regard time spans as instantaneous that are above their perception threshold, but this knowledge does not change the perception threshold. (contd.) $\endgroup$
    – user1196
    Sep 21, 2012 at 23:31
  • $\begingroup$ (contd.) And timespans shorter than this threshold MUST be felt as instantaneous. So, I would conclude that we have TWO thresholds: an absolute physiological threshold (which, by the way, varies with the type and characteristics of the stimulus), and a subjective threshold (which you tested in your experiment described in your answer above) that is probabilistic in nature and dependent on "knowledge". I like it, that you reply with an experiment, by the way :-) $\endgroup$
    – user1196
    Sep 21, 2012 at 23:32

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