Electrical Stimulation of the Midbrain for Hearing Restoration: Insight into the Functional Organization of the Human Central Auditory System

Abstract

The cochlear implant can restore speech perception in patients with sensorineural hearing loss. However, it is ineffective for those without an implantable cochlea or a functional auditory nerve. These patients can be implanted with the auditory brainstem implant (ABI), which stimulates the surface of the cochlear nucleus. Unfortunately, the ABI has achieved limited success in its main patient group [i.e., those with neurofibromatosis type 2 (NF2)] and requires a difficult surgical procedure. These limitations have motivated us to develop a new hearing prosthesis that stimulates the midbrain with a penetrating electrode array. We recently implanted three patients with the auditory midbrain implant (AMI), and it has proven to be safe with minimal movement over time. The AMI provides loudness, pitch, temporal, and directional cues, features that have shown to be important for speech perception and more complex sound processing. Thus far, all three patients obtain enhancements in lip reading capabilities and environmental awareness and some improvements in speech perception comparable with that of NF2 ABI patients. Considering that our midbrain target is more surgically exposable than the cochlear nucleus, this argues for the use of the AMI as an alternative to the ABI. Fortunately, we were able to stimulate different midbrain regions in our patients and investigate the functional organization of the human central auditory system. These findings provide some insight into how we may need to stimulate the midbrain to improve hearing performance with the AMI.

Enhanced cognitive flexibility in reversal learning induced by removal of the extracellular matrix in auditory cortex

“…Our study now implicates that controlled ECM modulation might even bear potential to guide neuroplastic changes and support reprogramming and relearning of impaired cortical processing by improved experience-dependent fine-tuning without interfering with remaining functions. Invasive therapeutic approaches based on plastic reorganizations, as for instance in specific forms of deep brain stimulation (45) or auditory prostheses (20,46), might, henceforth, include local activation or supply of ECM-modifying enzymes promoting synaptic rearrangements and stabilization for longlasting successful treatment.…”

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

“…Again the penetrating electrode showed the advantages of low electrical charge, and some improvement was observed regarding loudness, pitch, and temporal and directional cues, but open-set speech perception was not obtained until now. 35 As of this writing, this target has not yielded any superior outcome, and altogether the inferior colliculus has not led to any signifi-cant improvement, either by superficial or by penetrating electrodes. 7,35 The lack of responsiveness of the colliculus is speculated to be caused by cochlear nucleus compression.…”

“…35 As of this writing, this target has not yielded any superior outcome, and altogether the inferior colliculus has not led to any signifi-cant improvement, either by superficial or by penetrating electrodes. 7,35 The lack of responsiveness of the colliculus is speculated to be caused by cochlear nucleus compression. 40 Experimental data by acute cochlear nucleus compression suggest immediate decrease of function in the inferior colliculus and recovery after nucleus release.…”

“…To date the functional results in trials with inferior colliculus implants are less favorable than previously expected; 30,[33][34][35] this may be attributed to proximal degeneration, due in part to the complexity of the structure and to difficulties in exact electrode placement.…”

“…Recently, a penetrating electrode model for the inferior colliculus was designed and tried out experimentally and clinically by Lenarz and coworkers. 28,30,35 In view of the tonotopic gradient within the inferior colliculus, it was presumed to be an ideal target for such electrodes. Again the penetrating electrode showed the advantages of low electrical charge, and some improvement was observed regarding loudness, pitch, and temporal and directional cues, but open-set speech perception was not obtained until now.…”