An actuator is connected to an operating element of a manual user interface and outputs a haptic feedback signal to the user via the operating element. The operating element includes a sensor that detects a sensor signal generated by a manual input of the user as a reaction to the emitted feedback signal. The user's reaction time is determined. If the reaction time is shorter than a predetermined limit value, future haptic feedback signals are adjusted in such a way that their haptically perceptible intensity is reduced, and if the reaction time is greater than the predetermined limit value, future haptic feedback signals are changed in such a way that their haptically perceptible intensity is increased.