Latency/accuracy trade-offs during sequences of saccades:

For many motor behaviors, the more time devoted to planning a movement, the higher the spatial accuracy. To determine whether trade-offs between planning time and accuracy apply to saccadic eye movements, the present study investigated whether Fitts's Law, which holds that movement time depends on traveled distance and the required level of precision, applies to sequences of saccades. Saccades were made in sequence to 4 stationary target circles. Target eccentricities and target sizes varied. The time to scan the targets increased with Fitts's Index of Difficulty (ID), (defined as log 2 (2S / D), where S is target separation and D is target diameter), showing that Fitts's Law can apply to sequences of saccades. The increase was due mainly to a greater frequency of secondary saccades, rather than to increases in the latency of primary saccades. Analyses of saccadic accuracy showed that subjects used secondary saccades, rather than prolonging saccadic latency, to improve landing accuracy. Even where subjects were explicitly asked to increase saccadic latency (Experiment 2), the spatial distribution of landing positions of primary saccades did not change. These results showed that increasing the time available for saccadic planning did not improve the spatial precision of saccades. Saccades differ from some other motor behaviors, where optimal performance depends on trading off the duration of primary movements with the occurrence of corrections (Meyer et al., 1988). For saccades, the only apparently viable strategy to improve accuracy is to issue more corrections.M.S.Includes bibliographical references (p. 59-61)by Chia-Chien W