Human coordination between eyes and hand is quite complicated. For example, it takes time for us to move the pointer on the screen from one point to another using mouse. The time spend on this procedure is determined by Fitt’s law:

where $D$ is the distance to target and $S$ is size of target. The $240\mathrm{ms}$ is decomposed into three steps:

1. $70\mathrm{ms}$ for the eyes to determine the target;
2. $100\mathrm{ms}$ for the hand to move to the target;
3. $70\mathrm{ms}$ for the eyes to find out the defict between the pointer and the target and decide how to correct it.