A Test of Fitts' Law in a Dual-Task Paradigm

Shehab, Randa L.; Schlegel, Robert E. · 1993 · Crossref

DOI: 10.1177/154193129303700909

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Summary

This study investigates the validity of Fitts’ Law in predicting aimed hand movements under dual-task conditions, addressing a limitation in prior research that primarily examined single-task scenarios. The authors utilized a simulated automobile driving environment to assess how visual and perceptual-motor loading affect movement time (MT) and reaction time (RT) when reaching for instrument panel controls. The primary aim was to determine if Fitts’ Index of Difficulty (ID), derived from movement amplitude and target size, remains a robust predictor of performance when concurrent tasks are introduced. Ten subjects performed an aiming task involving four touch-sensitive response plates arranged in a 2x2 array, varying in distance (27–53 cm) and size (0.64 cm or 1.27 cm). This created seven unique ID levels. The aiming task was performed alone and in combination with two loading tasks: a Display Monitoring task to induce visual loading and an Unstable Tracking task to induce perceptual-motor loading. Subjects completed three replications of six task conditions (three loading levels × two target sizes) after extensive training. Results indicated that Fitts’ ID significantly affected both RT and MT across all loading conditions. However, the specific type of loading influenced performance differently: visual loading consistently produced the longest reaction times, while perceptual-motor loading resulted in the longest movement times. While simple linear regressions of MT on ID yielded moderate predictive power ($R^2$ values of 0.66 to 0.82), multiple linear regressions using target size and movement amplitude as separate predictors provided superior accuracy ($R^2$ values of 0.90 to 0.93). The analysis revealed that the effect of target size on MT became more pronounced as task difficulty increased, whereas the impact of distance remained relatively stable across loading conditions. An anomaly was noted where combinations with identical ID values but different amplitude/size ratios produced discrepant results, suggesting that location-specific factors may overshadow pure ID calculations. The study concludes that while Fitts’ Law generally applies to dual-task aiming, separate consideration of target size and movement amplitude offers more precise predictions for human factors design. The derived regression equations allow designers to estimate response time differences based on control size and location. These findings are applicable to the design of control panels in various human-machine systems, suggesting that prospective designs should be evaluated using dual-task simulations to optimize layout and minimize performance interference.

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• dual task performance

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• Empirical Findings: behavioral performance data