Instantaneous Human-Computer Interactions: Button Causes and Screen Effects

Many human-computer interactions are highly time-dependent, which means that an effect should follow a cause without delay. In this work, we explore how much time can pass between a cause and its effect without jeopardising the subjective perception of instantaneity. We ran two experiments that involve the same simple interaction: A click of a button causes a spinning disc to change its direction of rotation, following a variable delay. In our adjustment experiment, we asked participants to adjust the delay directly, but without numerical references, using repeated attempts to achieve a value as close to zero as possible. In the discrimination task, participants made judgements on whether the single rotation change happened immediately following the button-click, or after a delay. The derived thresholds revealed a marked difference between the two experimental approaches, participants could adjust delays down to a median of 40 ms, whereas the discrimination mid-point corresponded to 148 ms. This difference could possibly be an artefact of separate strategies adapted by participants for the two tasks. Alternatively, repeated presentations may make people more sensitive to delays, or provide them with additional information to base their judgements on. In either case, we have found that humans are capable of perceiving very short temporal delays, and these empirical results provide useful guidelines for future designs of time-critical interactions.