The masking phenomenon has been used to investigate cochlear excitation patterns and has even motivated audio coding formats for compression and speech processing. For example, cochlear implants rely on masking estimates to filter incoming sound signals onto an array. Historically, the critical band theory has been the mainstay of psychoacoustic theory. However, masked threshold shifts in cochlear implant users show a discrepancy between the observed critical bandwidths, suggesting separate roles for place location and temporal firing patterns. In this chapter, we will compare discrimination tasks in the spectral domain (e.g., power spectrum models) and the temporal domain (e.g., temporal envelope) to introduce new concepts such as profile analysis, temporal critical bands, and transition bandwidths. These recent findings violate the fundamental assumptions of the critical band theory and could explain why the masking curves of cochlear implant users display spatial and temporal characteristics that are quite unlike that of acoustic stimulation. To provide further insight, we also describe a novel analytic tool based on deep neural networks. This deep learning system can simulate many aspects of the auditory system, and will be used to compute the efficiency of spectral filterbanks (referred to as “FBANK”) and temporal filterbanks (referred to as “TBANK”).
Part of the book: Advances in Clinical Audiology
Implantable microelectrodes are useful for monitoring neural response patterns in the auditory cortex, however chronic neural recordings can often deteriorate with time (e.g. impedance measures across electrode arrays generally increase monotonically over the first 7 days post-implant). This problem is caused by the increasing spatial distribution of reactive tissue responses (corresponding to changes in impedance spectra along the electrode-tissue-interface). Therefore, the design of microelectrode probes must ensure that the neuronal ensembles lie within a cylindrical radius of the recording electrodes. In this chapter, chronic neural recording failure is examined via cortical spike patterns, histological analyses, indentation experiments, and finite element models. Next, the microfabrication of the “Utah” electrode array and the “Michigan” probe is compared to determine how their size, shape, and geometry address: (1) the spatial distribution of neurons (as related to recording quality); (2) the initial penetrating profile (as related to insertion killzones); (3) the reactive cell responses (as related to glial encapsulation); (4) the anchoring of the probe’s position in the tissue (as related to micromotions) and (5) the embedding of various bioactive reagents (ex: growth factors, anti-inflammatory drugs, etc.). Finally, a novel hydrogel “Dropping Method” is proposed for controlling the biocompatibility and mechanical properties at the electrode-tissue-interface.
Part of the book: An Excursus into Hearing Loss