The retina of the eye converts the optical image into electrical patterns (acts like an encoder) and passes them along the optic nerve fibers. The brain then re-creates and perceives the image from these impulse patterns. This is how we normally see.
The eye is only a middle-man in this visual process, and if the image encoded nerve patterns can be generated in any other way, the brain can adapt to the change in source of the input, and still produce vision!
Paul Bach-y-Rita created a system where the tongue is stimulated by an array of 400 electrodes, which receives input from a camera attached to the subject’s head. The electrodes represent the image data as differences in pulse characteristics such as frequency, amplitude and duration. The subject could then move his/her head to direct the camera, like as though he/she was looking around with his/her eyes.
It was found that after receiving training, blind people using these systems could recognize shapes and track motion. Subjects could perceive the motion of a rolling ball. Others could carry out an assembly-line task at an electronics plant, which required them to recognize glass tubes lacking solder and then to deposit some solder into those tubes.
They had depth perception and had the ability to judge distance and absolute size. A few were even able to recognize faces!
The main limitation to this is the technology. It simply cannot rival the complexity of the eye in terms of the resolution offered with just 400 or so electrodes (apparently the resolution of the human eye is 576 Mega-Pixels). Or even the brightness ranges for producing contrast. However, technology will improve with time and so will the utility of this sensory substitution.
While having obvious applications to help disabled people, it can also be used by the military. For example, an IR camera can be used with electro-tactile stimulation to improve combat vision.
As to whether the subjects actually attain visual perception or it is still just tactile perception, I do not know for sure, though I understand that vision centers of the subjects’ brains lit up when visual information was sent to the brain through the tongue.
I’ll just conclude with Bach-y-Rita’s quote:
“If a subject without functioning eyes can perceive detailed information in space, correctly localize it subjectively, and respond to it in a manner comparable to the response of a normally sighted person, I feel justified in applying the term ‘vision’ “.