Physiological Basis for Cochlear and Auditory Brainstem Implants

Abstract

Cochlear implants bypass functions of the cochlea that have been regarded to be fundamental for discrimination of the frequency (or spectrum). Frequency discrimination is essential for discrimination of sounds, including speech sounds, and the normal auditory system is assumed to make use of both (power) spectral and temporal information for frequency discrimination. Spectral information is represented by the place on the basilar membrane that generates the largest amplitude of vibration on the basilar membrane. Evidence has been presented that the temporal representation of frequency is more robust than the place representation and thus regarded more important for speech discrimination. The fact that some cochlear implants provide good speech discrimination using only information about the energy in a few spectral bands seems to contradict these studies. In that way, frequency discrimination may be similar to trichromatic color vision, which is based on the energy in only three different spectral bands of light, accomplished by different color-sensitive pigments in the cones of the retina. Cochlear nucleus implants (ABIs) also bypass the auditory nerve, which does not perform any processing. Therefore, it may be expected that ABIs are equally efficient as cochlear implants. However, experience from the use of ABIs in patients with bilateral vestibular schwannoma has not been encouraging, but recent studies of the use of ABIs in patients with other causes of injuries to the auditory nerve have shown similar speech discrimination as achieved with modern cochlear implants. Cochlear implants and ABIs are successful in providing speech discrimination because of redundancy in the processing in the ear, redundancy of the speech signal and because the auditory nervous system has a high degree of plasticity. Expression of neural plasticity makes the auditory nervous system adapt to the change in demands of processing of the information provided by cochlear implants.

Auditory brainstem implant outcomes and MAP parameters: Report of experiences in adults and children

“…Instead, was frequency were simply mapped in reversed order (electrode 1 set for the stimulation of the lowest frequencies), in the children. It is assumed that in children with little or no auditory input before implantation, the pitch perception can be organized by electrical stimulation through the expression of neural plasticity [29].…”

Auditory brainstem implants in neurofibromatosis Type 2: is open speech perception feasible?

“…17,49 Electrode side effects are a common finding in ABI programming. 41,57,60,63 Up to 42% of activated electrodes have been reported to cause unintended side effects. There are 2 groups of such electrodes; those causing only nonauditory side effects and those with a combination of auditory and nonauditory effects.…”

“…The first case is simple to solve by programming out. 41 In the second situation this programming out may have disadvantageous consequences. 60 Different sites of stimulation may lead to different pitch percepts and code for different frequencies.…”

“…Colletti et al 8 speculate that the cochlear nucleus contains a group of neurons for "a separate pathway of auditory processing that is specialized for modulated sounds," which they presume to be critical for speech understanding and which might be damaged in NF2. In his analysis of the physiology of the auditory pathway Møller 41 outlines the functional anatomy of the cochlear nucleus, with 3 divisions of each auditory neuron performing some kind of parallel processing of incoming information. Whether a VS or its removal causes the degeneration of 1 group of these neurons is open to speculation.…”

An anatomical assessment of the supracerebellar midline and paramedian approaches to the inferior colliculus for auditory midbrain implants using a neuronavigation model on cadaveric specimens

“…[1][2][3][4][5] To date, the only treatment solution for patients with these conditions has been the auditory brainstem implant (ABI), a central auditory prosthesis that stimulates the auditory pathway proximal to the damaged nerve. 2,[5][6][7][8] However, in patients with NF2, the achievable degree of hearing using a two-dimensional electrode array is rarely more than an augmentation of lip-reading capacity and the perception of environmental sounds; a result that compares to single-channel CI. 3,[9][10][11] Only a few patients will achieve open-set speech recognition without lip reading.…”