This paper describes the contribution of the closed-loop control of the motion platform (six degrees of freedom: longitudinal, lateral, and vertical displacements; pitch, roll, yaw) and motion platform’s three-dimensional (3D) displacement scale factor (SF) (0.2 and 1.0) on eye pupil diameter (PD) as an objective measure of driver cognitive load. Longitudinal, lateral, and vertical accelerations as well as longitudinal, lateral, and vertical positions from the center of gravity (CG) of the vehicle were registered through the driving simulation software SCANeRstudio® from OKTAL. Closed-loop control decreases the driver mental load. This type of closed-loop control can be used to decrease the driver mental load.