Today’s Daily Telegraph contains a fascinating extract from Norman Doidge’s new book The Brain That Changes Itself, about a woman who feels that she is constantly falling because she has lost her sense of balance as a result of damage to the vestibular system.
Cheryl Schiltz, who is now 50 years old, contracted a bacterial infection following a hysterectomy, and was subsequently treated with gentamicin. Excessive use of this antibiotic damages the semi-circular canals, three joined structures in the inner ear which give us our sense of balance by acting like spirit levels to provide information about the orientation and movements of the body within space.
In Schiltz’s case, the semi-circular canals were damaged severely, and the consequences were dramatic. She awoke one morning to find that she could not stand up without falling, and she would continue to experience this sensation even when she had fallen to the floor.
The remarkable solution to Schiltz’s condition was devised by Paul Bach-y-Rita, a professor in the Biomedical Engineering program at the University of Wisconsin Medical School. It consists of an accelerometer attached to a construction helmet and the BrainPort balance device, a tongue strip containing an array of electrodes. These are both connected to a computer, and together act as a substitute vestibular system.
The tongue strip contains tilt sensors which detect head movements; microprocessors relay this information to the tongue via the electrode array. During repeated short treatments, a new sensory pathway is forged within the brain, such that the information from the tongue strip is conveyed to the vestibular nuclei, which processes balance.
Bach-y-Rita’s contraption may look bizarre, but it worked – after wearing it for just a few minutes, the sensation of perpetual falling that Schiltz had experienced for the past 5 years had disappeared, and she was able to stand upright without falling. The longer she used the device, the longer lasting was the residual effect that she experienced after removing it.
The process by which one sensory system substitutes for another occurs because of the brain’s ability to reorganize itself structurally, a phenomenon known as neural plasticity, which was thought to be impossible until very recently.
The idea that the adult human brain remains plastic was suggested by Bach-y-Rita in the late 1960s. In 1969, he was lead author of a Nature paper describing a “tactile vision” device with which congenitally blind people could “see”.
The device (above) consisted of an old dentist’s chair with 400 vibrating solenoid stimulators incorporated into the back rest. By manipulating a television camera mounted on a tripod, the subject could scan the surroundings. The data from the camera were transmitted to a computer, which processed them and in turn conveyed them to the chair. The stimulators in the back rest acted like pixels, generating a tactile representation of the visual scene that was detailed enough for the subject to discriminate objects in the surroundings:
After being introduced to the mechanisms of operating the apparatus, subjects are trained to discriminate vertical, horizontal, diagonal and curved lines. They then learn to discriminate combinations of lines (circles, squares and triangles) and solid geometrical forms. After approximately 1 h of such training, they are introduced to a “vocabulary” of twenty-five common objects: a telephone, chair, cup, toy horse and others. With repeated presentations, the latency or time-to-recognition of these objects markedly; in the process, the students discover visual concepts such as perspective, shadows, shape distortion as a function of viewpoint and apparent change in size as a function of distance…Our subjects learn to discriminate between individuals, to decide where they are in the room, to describe their posture, movements and individual characteristics such as height, hair length, presence or absence of glasses and so on.
As the blind subjects become more familiar with objects, they learn to recognize them from minimal or partial cues. This skill permits them to describe with accuracy the layout of objects on a table, in depth and in correct relationship, even though the objects may be overlapping and only partially visible. For example, a telephone can be located even though only its cord is showing.
However, Bach-y-Rita’s paper was largely ignored upon its publication. At the time, the idea that the adult brain had a fixed structure was a central dogma of neuroscience. So too was the notion of the localization of cerebral function, which held that specific regions of the cortex specialized for a particular function. But Bach-y-Rita’s thinking was in fact way ahead of its time. It is now well established that the human brain retains the ability to reorganize itself throughout life and, using modern techniques, researchers can now observe how the brain rewires itself after injuries such as stroke.
Bach-y-Rita, P., et al (1969). Vision substitution by tactile image projection. Nature 221: 963-964.