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Petre Radulescu, Jos Adam, Martin Fisher, Davood Gozli, Greg West, Jay Pratt; Perception and the Fitts's Law Violation: Why is the last one the fastest one?. Journal of Vision 2011;11(11):968. doi: 10.1167/11.11.968.
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© ARVO (1962-2015); The Authors (2016-present)
Fitts's Law (FL) quantifies the tradeoff between speed and accuracy for manual movements, and it predicts that movement time (MT) increases logarithmically as the movement amplitude (i.e., distance) increases when target width is held constant. Replicated hundreds of times over 50 years, FL has proven to be incredibly robust; however, a violation of this law has recently been discovered. When targets of a constant width are placed in a structured perceptual array (e.g., visible placeholders denoting potential target locations), MTs to targets in the last position of the array are much shorter than predicted by FL (often shorter than MTs to targets at the second-to-last position). This violation holds for manual, saccadic, and even imagined, movements. Although it is known that the violation occurs in the movement planning stage, the underlying driving mechanism remains unknown. In the current study, we conducted three experiments to determine if the violation has a perceptual cause. In the first experiment, by measuring MTs to locations demarcated by extremely diminished placeholders (3 pixels long), we show that the violation does not occur due to perceptual interference.Experiment 2, which measured reaction times using a target detection task, showed that subjects are no faster in detecting targets appearing in the last location than they are detecting targets appearing at the other positions. Experiment 3, which measured accuracy using a brief presentation target identification task, showed that targets presented at the last position in the array are identified equally accurately in both placeholder present and absent conditions. Overall, these findings indicate that the changes in effectiveness of visual processing at the last position in the perceptual array do not drive the FL violation. Thus, while the locus of the FL violation appears to be in the movement planning stage, it is not due to perceptual mechanisms.
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