Elsevier

Acta Psychologica

Volume 137, Issue 1, May 2011, Pages 101-105
Acta Psychologica

Modulating Fitts's Law: Perceiving targets at the last placeholder

https://doi.org/10.1016/j.actpsy.2011.03.005Get rights and content

Abstract

Fitts's Law predicts increasing movement times (MTs) with increasing movement amplitudes; however, when targets are placed in a structured perceptual array containing placeholders, MTs to targets in the last position are shorter than predicted. We conducted three experiments to determine if this modulation has a perceptual cause. Experiment 1, which used extremely diminished (three pixel) placeholders, showed that the modulation is not due to perceptual interference from neighboring placeholders. Experiment 2, which measured reaction times using a target detection task, showed that the modulation does not result from speeded perceptual processing at the last position of the array. Experiment 3, which measured accuracy using a masked letter-discrimination task, showed that the modulation does not result from the increased quality of perceptual representation at the last position of the array. Overall, these findings suggest that the changes in effectiveness of visual processing (less interference, speeded processing, and increased quality) at the last position in the perceptual array do not drive the modulation. Thus, while the locus of the Fitts's Law modulation appears to be in the movement planning stage, it is likely not due to perceptual mechanisms.

Research Highlights

► Is the modulation of Fitts's Law with placeholders is due to perceptual factors? ► Three experiments examined both perceptual interference and facilitation. ► The modulation of Fitts's Law appears to be in the motor, not perceptual, system.

Section snippets

Experiment 1: Minimal placeholders

In most previous experiments showing the modulation of Fitts's Law, the placeholders were 10 × 10 mm outline boxes (25 by 25 pixels)separated by 10 mm. This means that the edge of one placeholder was quite close to the edge of its neighbors. When a target is surrounded by placeholders indicating other potential target locations, there is competition from those neighboring placeholders for the response (Tipper et al., 1992, Keulen et al., 2002), and thus movements to middle targets may be biased

Experiment 2: Target detection

In Experiment 2 we tested whether or not the modulation of Fitts's Law results from a speeding of perceptual processing at the final position in perceptual arrays. While measures of response speed (i.e., RT) were collected in previous Fitts's modulation studies, these previous studies are not well suited to assess changes in perceptual processing. First, in these prior studies responses were not speeded: subjects were told to move as quickly and accurately as possible, but could take as long as

Experiment 3: Target identification

Although Experiment 2 showed that the speed with which stimuli are detected at the last position in the array is not enhanced by placeholders there is still the possibility that the target is perceived more accurately when it is presented at the last position, and this results in a more effective target representation for movement planning. To test this hypothesis we created an object recognition task by replacing the previously used target, a green square, with one of two possible letters, and

General discussion

In the current study we examined possible perceptual contributions to the Fitts's Law modulation, according to which the last position in a visually structured array produces shorter than predicted MTs. Experiment 1, which used minimally visible placeholders, replicated the basic modulation with MT, showing that possible interference from the other placeholders in the previous studies (e.g., Adam et al., 2006, Pratt et al., 2007) was not the cause of the modulation. Experiment 2, which measured

Acknowledgments

This study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada to Jay Pratt. We would like to thank Digby Elliott and an anonymous reviewer for comments regarding this paper.

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