The time to react can be termed a response time composed of:

1. a reaction component which will vary depending on many factors including the type of stimulus, the complexity of recognition and the complexity of decision making and
2. a movement component.

In this guide, the term reaction time is used for the total. The movement time is expected to be low for pressing a touch pad or a button.

The diagram above shows some of the processing in a monkey brain with an estimation of the connectivity and minimum-average latencies in milliseconds (ms). This diagram may be familiar from the Stadia Streaming Tech: A Deep Dive (Google I/O'19) which takes it and misrepresents it as human and changes the meaning and value of the times.

Movement Time

A prototype of the Reaction Timer used a touch pad for input. A high-speed video revealed that a typical finger movement took 20-30ms. This may be partly because a cautious user needs to carefully hover about 3-4mm to avoid getting too close and accidentally triggering the capacitive input.

The input was changed to the right button to allow the user to rest their finger on the button and reduce the travel distance.

Measurement Platform

There are many reaction timing programs including ones that run in the browser. A typical, modern desktop computer is unfortunately not the best platform for measuring reaction times. Dan Luu has studied this and published a list with a great write-up on his computer latency page. His tests measure the time from a keyboard press to the character appearing on screen. The Apple IIe from 1983 is the current winner! The page explains the elements of this latency from the keyboard to the operating system (o/s) to the screen. There is also another page dedicated to keyboard latency which discusses travel time and debouncing.

For basic reaction timing, this explains why the cheapest microcontroller board with an LED is superior to an expensive computer with a keyboard, general purpose operating system and an LCD screen.

When CRT computer displays were superseded by TFT LCD panels the refresh rate standardised from around 50-90Hz to 60Hz. This means that a modern desktop computer only displays a new image every 16.7ms and many tablets and smartphones have a similar limitation. Television models can vary in their refresh rates but often have added latency from additional image processing. This can sometimes be minimised with a "game mode" setting.

LCD displays also have a significant response time. The small PyGamer's screen can be seen below taking around 6 frames (26ms) to reach full brightness for a red letter A.

Most computer displays would be slightly better than the PyGamer's screen but there is no standard way to measure and summarise the response time. Some caution is required when comparing manufacturers' published numbers. Blur Busters have some interesting techniques to visually compare displays.

Latency in all its forms is an important factor in games. particularly fast-paced, distributed, multi-player games. Linus Tech Tips conducted a set of tests on 60, 144 and 240Hz refresh rate displays to see how frame rate affects the gaming experience. This also included some simple reaction time tests using a mouse and reacting to a (visual) red/green banner. The summary of their results is shown below.

In the video there is mention of "run them until we see statistical convergence" - for a basic reaction time test, this sounds like cherry picking of the test results. This explains the low but plausible mean (AVG) reaction times. The standard deviation (sd) numbers look less plausible based on calculating sd from the low values actually shown in the video (sd=15.7ms) and from trying to reproduce low sd results for a visual stimulus.

On a similar theme, the reaction times for the finalist sprinters from the World Athletics Championships are shown below. These particular results are highly selected in two ways: the elite nature of the athletes; and their success in reaching the final. The graph starts at 0.1 seconds (100ms) because IAAF rules regard lower values as impossible, i.e. false starts. These results cannot be directly compared to the previous visual results as athletics uses sound to start the race (from equidistant speakers) and pressure from the starting blocks.

Some results from the Reaction Timer in this guide are shown on the next page.