Running Experiments 2: Response Hardware

We have the tools, we have the talent!
– Winston Zeddemore

In the last post I talked about timing of experiments in general, and mentioned that timing of responses is critical for success in a reaction time-type experiment. This topic will discuss some of the options for hardware that’s available for collecting responses.

The simplest solution is just to use the human-interface hardware which you’ve already got i.e. get your participants to click a mouse button, or a key on the keyboard as their response. However, there are several reasons why this will generally be undesirable. Most mice and keyboards on modern computers connect via USB, and this introduces a slight lag. The computer ‘looks at’ or ‘polls’ its USB-connected peripherals at a standard rate of 125Hz (meaning 125 times per second, or every 8 milliseconds). This means that if you make a response, there may be a (variable) lag of anywhere between 0 and 8 ms between the response and the computer actually ‘seeing’ it. USB-keyboards have a similar problem. In addition, mice (and keyboards) have a lot of internal circuitry which can also introduce timing lags of variable durations. This paper (Plant et al., 2003) presents the results of some bench-testing of a set of mice. The best mouse they tested had a minimum button-down latency of just 0.4ms, whereas the worst one showed a minimum lag of 48ms! Running timing-critical experiments with the latter mouse (where the effect you might be chasing could be in the range of 20-30ms) would clearly be disastrous.