Abstract
In 1954, Paul Fitts showed that the time needed for manual movements increases as a logarithmic function of Index of Difficulty (ID; relationship between target size and movement amplitude). Fitts's formula for predicting the movement time from ID has since then also been known as Fitts's Law and has been investigated for a large range of target sizes, movement amplitudes, types of response and has also been applied to ergonomics. In the current study we investigated whether Fitts's Law is also valid for saliency by using a simple visual search task with manual pointing movements. Thus in this experiment, the stimuli only varied in context features (manipulation of saliency), whereas in the original idea of Fitts, local features (size manipulation) of stimuli differed. The factor target size as manipulated in the original experiment was replaced by the factor level of saliency. The task of the participants was to perform a pointing movement as fast as possible to a pop-out target which differed from surrounding distracter items to a varying degree in luminance or orientation. An additive effect was found for both factors, i.e. for highly salient targets and low movement amplitudes, movement time was shorter compared to less salient targets and larger distances to the target. Thus, as predicted by Fitts's Law, the movement time increased with ID, which was in our experiment the different movement amplitudes and the different levels of saliency. This indicates that Fitts's law can also be applied to saliency. The bottom line is that the system can access the saliency information which is persistent throughout the whole movement preparation and the actual movement. Applied to ergonomics it is not sufficient to simply consider target features but as well take context features into account.