Computer aided surgery. Surgeon using the tongue display unit (TDU), a surgical guidance system. The surgeon locates the target point in the patient with an ultrasound scan, and the entry point for the procedure with an optical locater. A computer then calculates the best trajectory between the two. The surgeon places the matrix of electrodes, which is attached to an electronic ribbon (white), on his tongue, and proceeds with the operation. The surgeon's moves are monitored and if he strays, even slightly, from the planned trajectory the electrodes on his tongue produce impulses that direct him in the right direction.