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Abstract
Fitts' (1954) classic theorem asserts that the movement time (MT) of voluntary reaches is determined by amplitude and width requirements (i.e., index of difficulty: ID). Actions associated with equivalent IDs should elicit equivalent MTs regardless of the amplitude and/ or width requirements. However, contemporary research has reported that amplitude-based contributions to IDs yield larger increases in MTs than width-based contributions. This discrepancy may relate to the presence of augmented terminal feedback in Fitts' original research, which has not been provided in more recent investigations (e.g., Heath et al., 2011). To address this issue, participants performed reaching movements during two sessions wherein feedback regarding terminal accuracy was either provided or withheld. It was hypothesized that the absence of augmented terminal feedback would result in a stereotyped performance across target widths and explain the violation of Fitts' theorem. Yet, the results revealed distinct influences of amplitude- and width-based manipulations on MT, which also persisted across feedback conditions. This finding supports the assertion that the unitary nature of Fitts' theorem does not account for a continuous range of movement amplitudes and target widths. A secondary analysis was competed in an attempt to further investigate the violation of Fitts' Law. Based on error rates, participants were segregated into accuracy- and speed-prone groups. Additionally, target's IDs were recalculated based on each participant's performance using the effective target width (i.e., IDWe) instead of the nominal target width. When using MT data from the accuracy-prone group with this IDWe, the aforementioned violation was alleviated. Overall, augmented terminal feedback did not explain the violation of Fitts' theorem, although one should consider using the effective target width and participant's strategy in future investigations.
Highlights
Fitts (1954) applied Shannon’s Information Theory (Shannon and Weaver, 1949) to forward a mathematical quantification of the speed-accuracy trade-off associated with goal directed reaching actions
We examined if the discrepant movement time (MT)/index of difficulty (ID) slopes associated with amplitude- and width-based manipulations used by Heath et al (2011) were related to the absence of augmented spatial terminal feedback
Movement Time Regarding MT data, a main effect was found for Strategy [F(1, 17) = 9.67, p = 0.006, η2G = 0.30]
Summary
Fitts (1954) applied Shannon’s Information Theory (Shannon and Weaver, 1949) to forward a mathematical quantification of the speed-accuracy trade-off associated with goal directed reaching actions. Fitts examined how manipulating movement amplitude (A) and target width (W) influenced the movement times (MT) of goal-directed actions. Fitts found that MT was Frontiers in Psychology | www.frontiersin.org de Grosbois et al. Fitts’ Law feedback linearly related to a target’s index of difficulty (ID), which was defined as the log2(2A/W). Since its inception, Fitts’ theorem has been extensively examined and promulgated as a law-based measure of human motor function (e.g., Accot and Zhai, 1997; Crossman and Goodeve, 1983; Guiard and Beaudouin-Lafon, 2004)
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