Visual layout modulates Fitts’s law: The importance of first and last positions

Abstract

When a target in the last position of a structured visual array is aimed for, movement times (MTs) are shorter than predicted by Fitts's law (Adam, Mol, Pratt, & Fischer, 2006). That study, however, confounded relative target position with absolute target location. To determine whether target position does, indeed, produce changes in the speed-accuracy trade-off function, the present experiment manipulated relative target position (e.g., first or last) independently of absolute target location (e.g., nearest or farthest). This was accomplished by presenting connected placeholders at three adjacent locations from a set of five possible locations (i.e., the middle location could be the first, middle, or last placeholder position in an array). The results of a speeded manual-pointing task showed that relative position is important for Fitts's law; when absolute location was held constant, shorter MTs were found for last-position than for middle-position targets. In addition, a similar effect was found for first-position targets. These results suggest that Fitts's law holds within, but not between, relative target positions in visible structured arrays.

Comment on “Visual layout modulates Fitts’s law: The importance of first and last positions”

“…According to the criterion developed by Gan and Hoffmann (1988), all movements should have required visual control. Pratt et al (2007) obtained the results shown in Fig. 2.…”

“…The present case of the Pratt et al (2007) data is different for the first and last targets, since there was no constraining target on one side of these targets. In the Pratt et al study, the distance between adjacent targets (17 -11.2 = 5.8 mm) was smaller than any reasonable finger width (see below for finger width discussion).…”

“…In the following, a model developed for movement times on keypads, which are an array of targets similar to those studied by Pratt et al (2007), is used to show that a single explanation can be given for the movement time data for all target locations. It appears that, with the correct interpretation of the available target sizes, all data will lie along a single line described by Fitts's law.…”

“…The experiment of Pratt, Adam, and Fischer (2007) Pratt et al (2007) used the experimental arrangement shown in Fig. 1 (extracted from their article), with targets presented on a touch screen and the finger used as the test probe (a stylus was not used).…”

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Abstract Pratt, Adam, and Fischer (2007) investigated the effect of surrounding targets on the time it took to move to an individual target and found that the movement time to a central target was above the Fitts's law line related to the first and last targets. They explained their results in terms of a "visuomotor hypothesis."

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