Cochlear implants matching the prosthesis to the brain and facilitating desired plastic changes in brain function

Abstract

The cochlear implant (CI) is one of the great success stories of modern medicine. A high level of function is provided for most patients. However, some patients still do not achieve excellent or even good results using the present-day devices. Accumulating evidence is pointing to differences in the processing abilities of the "auditory brain" among patients as a principal contributor to this remaining and still large variability in outcomes. In this chapter, we describe a new approach to the design of CIs that takes these differences into account and thereby may improve outcomes for patients with compromised auditory brains.

Fig. 1

Block diagram of the continuous interleaved sampling (CIS) strategy. The input to the strategy is indicated by the filled circle in the left-most part of the diagram. Next, a pre-emphasis filter (Pre-emp.) is used to attenuate strong components in speech below 1.2 kHz. This filter is followed by multiple channels of processing. Each channel includes band-pass filtering (BPF), envelope detection, compression, and modulation. The envelope detectors typically use a rectifier (Rect.) followed by a low-pass filter (LPF). A Hilbert transform or a half-wave rectifier without the LPF also may be used. Carrier waveforms for two of the modulators are shown immediately below the two corresponding multiplier blocks (the circles with an “X” within them). The outputs of the multipliers are directed to intracochlear electrodes (EL-1 to EL-n), as illustrated by the X-ray micrograph in the inset. (Block diagram is adapted from Wilson et al. (1991) with the permission of the Nature Publishing Group. Inset is reproduced from Hüttenbrink et al. (2002) with the permission of Lippincott Williams and Wilkins.)

Fig. 2

Hierarchy or cascade of electrophysiological measures (event-related potential (ERP) components) to assess the functional integrity of the auditory pathways and cortices. Abbreviations in the figure include BERs for brainstem evoked responses; MMN for mismatch negativity responses; Cort. feat. discrim. for Cortical feature discrimination; and Lang./Sem. for Language and Semantic processing. Measures of other auditory ERP components could also be included for additional noninvasive assessment of the functional processing in the auditory pathways. The thick arrows coming in across the top of the figure reflect the potential influence of top-down factors, such as attention, on these various processing stages.