The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the ossicular chain, excessive cerumen, or a malformed tympanic membrane. Mild conductive hearing losses can be treated with hearing aids, stronger losses may require a middle ear surgery or a bone-anchored hearing aid.
Sensorineural hearing loss, on the other hand, is primarily caused by the absence or destruction of the outer hair cells on the basilar membrane. There are rare cases in which sensorineural hearing loss is caused by a malfunction of the vestibulocochlear nerve or even the central processing system. To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the major part of the ear by presenting electrical stimulation directly to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function. Cochlear implants are typically capable of providing high-frequency information up to 8 kHz.
There is a certain group of people that has some degree of residual hearing in the low frequencies (e.g., below 1 kHz) and a severe hearing loss in the high frequencies (e.g., above 1 kHz). These people cannot benefit from traditional amplification because of the severity of the hearing loss in the high frequencies. Nor are they classic cochlear implant candidates, because of their mostly intact low frequency residual hearing.
For this group of people, various electro-acoustic stimulation (“EAS”) systems have been developed that provide such patients with the ability to perceive both low and high frequencies. Electro-acoustic stimulation refers to the use of a hearing aid and a cochlear implant together in the same ear. The hearing aid acoustically amplifies the low frequencies while the cochlear implant electrically stimulates the high frequencies. The auditory nerve combines the acoustic and electric stimuli to one auditory signal. Results of various studies have shown a highly synergistic effect between hearing aid and cochlear implant technology, particularly evident in speech understanding, pitch discrimination, and music appreciation.
However, electro-acoustic stimulation systems suffer from the problem that the perceptual sensitivity of a patient to acoustic stimulation is quite different than the perceptual sensitivity of the patient to electrical stimulation. For example, sensitivity of a patient to electrical stimulation often changes throughout the day and in accordance the health state of the patient while the sensitivity of the patient to acoustic stimulation is typically more consistent. Hence, it would be desirable for the patient to be able to independently adjust a volume associated with the acoustic stimulation and a volume associated with the electrical stimulation. Unfortunately, current electro-acoustic stimulation systems balance the two types of stimulation during the fitting process only and do to provide a control for the user to change the balance throughout the day.