Richard E. Cytowic M.D.

The Fallible Mind

See With Your Tongue!

Substituting one sense for another is not as crazy as it sounds.

Posted Jul 30, 2011

Blind mountain climber

Blind mountain climber Erik Weihenmayer sees via video inuput fed to his tongue.

Take a gander with your tongue??? I must be joking, right? Except that I'm not.

You can see with your tongue. No kidding. Here's how, and here's why the tongue is a fantastic brain-machine interface with many real-world applications.

Rather than being a futuristic promise of pie-in-the-sky, for some time now Navy divers have been able to "see" in murky, black waters when sonar signals are fed into their tongue interface. Likewise, battlefield soldiers gain the advantage of 360-degree night vision thanks to data beamed to their tongues from infrared sensors mounted on their helmets. What makes these scenarios possible is a technique called "sensory substitution" originated by the late Paul Bach-y-Rita, a rehabilitation physician at the University of Wisconsin Medical School. His studies stemmed from a trait that every brain has called plasticity—meaning "capable of molding or shaping," and referring to the brain's inherent ability to reorganize itself.

Weihenmayer wall climbing with aid of the tongue-display unit.

Weihenmayer wall climbing with aid of the tongue-display unit.

The fetal brain is the most plastic, making 2 million synapses per second. Neuronal shaping slows down around age two, and then has another burst around puberty that finally settles down to adult levels around age 25. (This is why teenagers have all the drive and none of the judgment compared to adults: their brains are literally works-in-progress, with executive functions the last to mature.)

Surprisingly, adult brains are still able to reorganize themselves. Consider Braille, for instance, which shows how latent cross-sensory connections exist in everybody. When newly blind individuals learn Braille, the brain area corresponding to their reading finger greatly expands. Now blind, their visual cortex (V1) is unused. Plasticity lets new working connections reach out to sites in unused visual cortex to switch its functional assignment from seeing, to feeling Braille and subsequently "reading" it. Not many years ago orthodoxy declared such changes flatly impossible.

Brain's visual area senses touch and sound

After two days the visual cortex of blindfolded volunteers responds to touch, sound, and even words.

What surprises most people is that this kind of plasticity happens in normally-sighted individuals. For example, blindfolded for only two days, a volunteer's visual cortex activates when he feels with his fingers, or hears tones or words. Two days is far too short for new synapses to grow from the brain's touch and hearing areas into V1. Furthermore, removing the blindfold for a mere twelve hours reverts V1 so that it once again responds solely to visual input. One explanation for such astonishing malleability is that the brain's sudden, reversible ability to "see" with the fingers and ears depends upon dormant connections from other senses that are already there but never used so long as the eyes input a signal. The above kinds of experiments show that all of us harbor untapped multi-sensory potential.

So, how did Dr. Bach-y-Rita hit upon the tongue, of all things, as a brain-machine interface? Initially he experimented by transforming camera input into an electrode grid that made a tingling pattern on a patch of skin. He was interested in whether the tactile pattern would be comprehensible in terms of a visual pattern. One morning when he was busy and needed a free hand, he stuck the electrode grid in his mouth. What he felt changed the direction of neuroscience and our understanding of the degree to which the brain can reorganize itself. Although we usually think of the tongue in terms of taste, it is loaded with touch receptors (which is why texture and temperature discrimination are a crucial part of what we call flavor).

In one set-up, the experimenter strikes a hand posture—two fingers up, say, in a victory sign—in front of a camera. Software transforms the camera's visual recording into electrical impulses that travel to the tongue array. With no training required, the subject effortlessly replicates the hand posture, sensing through touch-in-the-mouth qualities that are usually ascribed to vision, such as distance, shape, directional movement, and size. The demonstration reminds us we don't see with our eyes, but with our brain.

Sighted surgeons gain enhanced spatial control when operating in tight areas

Sighted surgeons gain enhanced spatial control when operating in tight areas

Dr. Bach-y-Rita's technique was initially developed to help blind individuals, but it also helps sighted individuals with inner-ear disease who have lost their sense of balance. Neurosurgeons operating in tiny, hard-to-reach spaces where one millimeter can mean the difference between success or death gain considerable enhancement of their dexterity when they use the tongue array. Aside from the applications mentioned above for feeding sonar or infrared signals to the tongues of divers and soldiers, or NASA efforts to let astronauts feel things on the outside of their spacesuits, today's cochlear implants owe much to these earlier efforts in sensory substitution.

tongue display unit

Size of earlier prototype tongue array.

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