Fitts' Law with transmission delay

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Abstract
It is shown that a modified form of Fitts’ Law applies when there is a transmission delay between control movements and feedback of system response to the operator. The model predicts, and experiments verify, that a multiplicative relationship holds between movement time, Index of Difficulty (ID), and transmission delay. Movement time at constant ID is shown to increase linearly with delay, and linearly with ID at constant delay. The model indicates that lines of constant delay meet at a single point on the MT vs ID graph; experimental data show the same behaviour. The data indicate that a change in control strategy occurs at a delay time of about 0·7, possibly from a more continuous form of control to a wait-and-move strategy. The model predictions fit the data in both regions, but with different coefficients.