The concepts at work behind electrotactile stimulation for sensory substitution are complex, and the mechanics of implementation are no less so. The idea is to communicate non-tactile information via electrical stimulation of the sense of touch. In practice, this typically means that an array of electrodes receiving input from a non-tactile information source (a camera, for instance) applies small, controlled, painless currents (some subjects report it feeling something like soda bubbles) to the skin at precise locations according to an encoded pattern. The encoding of the electrical pattern essentially attempts to mimic the input that would normally be received by the non-functioning sense. So patterns of light picked up by a camera to form an image, replacing the perception of the eyes, are converted into electrical pulses that represent those patterns of light. When the encoded pulses are applied to the skin, the skin is actually receiving image data. According to Dr. Kurt Kaczmarek, BrainPort technology co-inventor and Senior Scientist with the University of Wisconsin Department of Orthopedics and Rehabilitation Medicine, what happens next is that "the electric field thus generated in subcutaneous tissue directly excites the afferent nerve fibers responsible for normal, mechanical touch sensations." Those nerve fibers forward their image-encoded touch signals to the tactile-sensory area of the cerebral cortex, the parietal lobe.
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Mouse-over the part labels of the brain to see where those parts are located.
Under normal circumstances, the parietal lobe receives touch information,
the temporal lobe receives auditory information, the occipital lobe receives
vision information and the cerebellum receives balance information.
(The frontal lobe is responsible for all sorts of higher brain functions,
and the brain stem connects the brain to the spinal cord.)
Within this system, arrays of electrodes can be used to communicate non-touch information through pathways to the brain normally used for touch-related impulses. It's a fairly popular area of study right now, and researchers are looking at endless ways to utilize the apparent willingness of the brain to adapt to cross-sensory input. Scientists are studying how to use electrotactile stimulation to provide sensory information to the vision impaired, the hearing impaired, the balance impaired and those who have lost the sense of touch in certain skin areas due to nerve damage. One particularly fascinating aspect of the research focuses on how to quantify certain sensory information in terms of electrical parameters -- in other words, how to convey "tactile red" using the characteristics of electricity.
This is a field of scientific study that has been around for nearly a century, but it has picked up steam in the last few decades. The miniaturization of electronics and increasingly powerful computers have made this type of system a marketable reality instead of just a really impressive laboratory demonstration. Enter BrainPort, a device that uses electrotactile stimulation to transmit non-tactile sensory information to the brain. BrainPort uses the tongue as a substitute sensory channel. In the next section, we'll get inside BrainPort.