Tongue Display Unit research. Functional magnetic resonance imaging (fMRI) scans of the brain functions (orange and yellow) of a person using a Tongue Display Unit (TDU). The scans are saggital (upper left), coronal (upper right) and axial (lower left). 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 areas of the brain that are stimulated are different from the normal visual centres. TDUs can be used as a pressure sensor for amputees and paraplegics, by surgeons carrying out operations, and by blind people.