Overview of Hearing Loss, Hearing Aids, & Cochlear Implants
Hearing loss is usually caused by the tiny hairs in the spiral shaped Cochlea breaking off. They can break off due to certain diseases such as Rubella, exposure to loud sounds, certain medicines, heredity, or age.
Hearing aids rely on the residual hearing in your ear. They make sounds louder, to shake the remaining hairs harder, so that you can still hear. But at some point, you can make sounds very loud and they still can not be heard. There are no hairs left to shake.
When the hairs wiggle, they fire neurons at their base. And even after the hairs break off, those neurons are usually still healthy and can be made to fire with an Electric Field. This is what a Cochlear Implant (CI) does.
An electrode is inserted inside the cochlea with 16 to 20 metal buttons that can create electric fields and excite the neurons. A picture is included: CI System.jpg. In addition to the electrode, a small computer that sends current to the desired electrode button, and an antenna to receive instructions from the external sound processor also get implanted inside your head. The external sound processor has a microphone, audio filters, battery, and the algorithms that determine which electrode buttons get fired and when.
While most folks with CIs can understand speech, and a few enjoy music, there is still disagreement on how the ear works, and whether a CI can provide aspects of normal hearing.
The spiral shaped cochlea is organized with high pitches at one end and low pitches at the other, with thousands of Hairs along the length. And with only 16 electrode buttons, you’d think you might only get 16 different pitches with a CI. Fortunately, English speech is quite redundant and can be understood even if distorted and imperfectly presented. The brain over weeks and months learns to recognize CI patterns as speech – but has a much harder time with music.
Some folks when they first get their CI describe speech as sounding like someone ringing bells, tinnitus with a pattern, or Donald Duck, or Darth Vader, or Mickey Mouse. Currently CIs are not “Pitch Matched” to your ear. Frequencies are shifted, and harmonics, chords and overtones, are not preserved. But over time, folks say speech sounds “normal”. So this implies that pitch is plastic – that the brain can tweak pitch in over time.
Until recently, only one electrode was fired at a time. If you fire two adjacent electrodes simultaneously, the electric field will maximize between the two buttons and provide a pitch intermediate between the two. And depending on the ratio of the currents in the two buttons, you can get many intermediate pitches – so in addition to physical electrodes, you now have Virtual Electrodes. This technique is called Beam Steering – a familiar radar concept.
But the tones produced are fuzzy & noisy – far from a pure tone. Folks with CIs can not determine when a note in a melody is mistuned. Radar again comes to the rescue. By firing 3 or 4 electrodes simultaneously, it’s possible to not only steer the beam, but to shape it. In July of 2007, we demonstrated that 5 of 6 subjects heard significantly purer tones when using the beam shaping technique.
The three button beam shaping configuration (tripolar) has a problem with pitch gaps – not all pitches can be generated. So our current work uses the 4 simultaneous electrode firing technique and we’re currently determining its usefulness & limitations. I tried a 5 electrode pattern and it had poor tonal quality. I'm guessing that over 5 buttons, there is too much curvature in the cochlea, which prevents the beam shaping from working as expected.