A system for controlling the operation of a human machine interface (HMI) device located in a vehicle configured to carry a passenger and a driver. The system includes an electrode positioned in a vehicle seat and a signal generating device configured to provide a signal to the electrode. The electrode transmit the signal through the occupant of the seat and the HMI device detects the signal when the occupant contacts the HMI device. The system is configured so that the HMI device operates in a first mode in response to exposure to the signal and in a second mode if the signal is not detected.