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Chia-Chien Wu, Brian Schnitzer, Eileen Kowler, Zygmunt Pizlo; Fitts's Law and the optimal planning of sequences of saccades. Journal of Vision 2008;8(6):934. doi: 10.1167/8.6.934.
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© ARVO (1962-2015); The Authors (2016-present)
Speed/accuracy tradeoffs in motor tasks obey Fitts's Law, which states that movement time depends on distance and the required level of precision. We investigated whether Fitts's Law applies to sequences of saccades.
Saccades were made in sequence to 4 target circles (diameter 15′–180′) located at the corners of an imaginary square (corner separation 90′–360′). Following convention, Fitts's “index of difficulty” (ID) was defined as Log2(2S/D), where S=separation; D=diameter.
The time to complete the sequences increased with ID, showing that Fitts's Law can apply to sequences of saccades. For small targets and large separations, the latency of the primary saccade was short and corrective saccades were frequent. For larger targets and smaller separations, the latency of the primary saccade increased, and corrective saccades were rare.
These patterns suggest that saccadic planning takes the expected variability of landing positions into account. When landing variability is large relative to required precision (small targets; large eccentricities), short-latency primary saccades are followed by secondary saccades to correct errors. When variability is small relative to required precision (large targets; small eccentricities), a single, longer latency primary saccade is sufficient. Meyer et al. (1988) observed comparable results for a manual task and proposed that planning reflects a compromise between latency and precision to achieve optimal performance.
Conclusions: (1) The applicability of Fitts's Law shows that transformations to visual arrays that do not alter the ID (such as changes in viewing distance) will not affect the time to perform the saccadic sequence. This ties scanning time to the properties of the scene itself. (2) Saccadic reaction time and the occurrence of corrective saccades are typically attributed to structural aspects of the stimulus or task. Our results suggest that saccadic timing and corrections are the result of strategies designed to achieve required accuracy in the shortest time.
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