Tongue Display Units. The TDU, first developed in 1999, stimulates the tongue with input from an external sensor, and is an example of a tactile vision subsitution device. The first model (at left) is just a plastic tongue stimulator and grid of electrodes, with the external electronics not seen. The later models (centre and right) still include the grid of stimulating electrodes, but the electronics are now miniaturised. The latest models (as of 2007) include wireless communications and plastic surrounds moulded to the palate, designed to allow it to be easily worn in the mouth. TDUs can be used as pressure sensors for amputees and paraplegics, by surgeons carrying out operations, and by blind people. Photographed in France.