Defenseless chinchillas, poisoned by gentamicin and, as a result, suffering from impaired sense of balance, were outfitted with a multichannel vestibular prosthesis that was developed by biomed engineers at Johns Hopkins’ Vestibular Neuroengineering Laboratory. The animals showed an improved sense of balance in a study that was published in the June 2007 edition of the journal I.E.E.E. Transactions on Biomedical Engineering.
The Hopkins study, done in chinchillas because their inner ear function is well studied, “is proof of concept that we can restore three-dimensional sensation of head movement with a multichannel vestibular prosthesis,” says Charles C. Della Santina, M.D., Ph.D., director of the Vestibular Neuroengineering Laboratory at Hopkins.
“While everyone knows about the five senses of sight, smell, taste, touch and hearing, few people think about a possible sixth sense – the sensation of head orientation and movement – until the system fails,” says Della Santina, who has been working on this prosthesis since 2002…
…the Hopkins team showed that a matchbox-size prototype device, weighing less than 3 ounces, effectively mimics the workings of the inner ear’s three semicircular canals by sensing head rotation and transmitting that information to the brain.
Adapting the design of cochlear implants, which restore hearing through electrical stimulation of the cochlear nerve, researchers constructed a circuit that could measure and transmit 3-D balance information to the brain through multiple electrodes connected to the vestibular nerve.
The device, which researchers started testing more than a year ago, consists of a head-mounted, battery-operated box containing the sensors, which are positioned outside the head so that the sensors are parallel to the animal’s actual semicircular canals, where head rotation is normally sensed. The sensors are connected to a microprocessor and up to eight electrodes surgically implanted in the inner ear and separately connected to nerve endings. Each electrode can act as one information channel…
According to Della Santina, an assistant professor of otolaryngology – head and neck surgery and biomedical engineering at The Johns Hopkins University School of Medicine, this is the first implantable device made with multiple sensors and channels of processing that can measure and encode head rotation in all directions.
Each of the three sensors, he notes, can measure the speed of head rotation about one of three axes, or directional planes.
Della Santina says that previous implants developed elsewhere were limited to one functioning sensor and electrode and one plane or axis of rotation, “when in reality, we move in multiple directions.”
Every measurement in the balance device is processed in the implanted central microprocessor unit, using computer software developed by Della Santina and his team.
Once processed, the information is used to tailor timing of brief, electronic pulses through the electrodes implanted near the three branches of the vestibular nerve that respond to changes in head rotation. These branches normally carry signals from the inner ear’s three semicircular canals.
In the chinchilla tests, pulses lasting less than a millisecond were delivered with timing patterns that mimicked normal nerve activity.