The effect of electrode configuration and duration of deafness on threshold and selectivity of responses to intracochlear electrical stimulation

Abstract

This report examines the effects of intracochlear electrode configuration and mode of stimulation (bipolar or monopolar) on neural threshold and spatial selectivity in the inferior colliculus (IC) of the cat. Single and multiunit IC recordings were made in three groups of animals; acutely deafened adults (controls), neonatally deafened animals studied at 6 to 18 months of age and neonatally deafened cats studied at 2.5 to 6.5 years. Response thresholds were plotted versus IC depth to measure the spatial distribution of responses. The response selectivity for each stimulating configuration was defined as the width of the resulting spatial tuning curve (STC) measured at 6 dB above threshold. Spiral ganglion cell (SG) survival was examined histologically in all neonatally deafened animals and correlated with physiological results. Animals studied at less than 1.5 years had SG densities of 23.5%-64.4% of normal (mean=42.7%) while animals studied at greater than 2.5 years had densities of 5.1%-18.3% of normal (mean=9.9%). Electrophysiological results include the following. (1) Monopolar thresholds were 7-8 dB lower than bipolar thresholds in the same animals. (2) Varying the configuration of bipolar contacts (measured as radial, offset radial and longitudinal pairs) did not systematically affect IC threshold in either controls or short-term neonatally deafened animals. In contrast, the long-term neonatally deafened animals showed a difference in threshold with each configuration. (3) The spatial distributions (Q(6 dB)) of responses to bipolar stimulation were approximately 40% more restricted than those for monopolar stimulation. (4) The spatial selectivity of neonatally deafened animals studied at ages up to 1.5 years was equal to that of control animals with normal auditory experience. However, selectivity was degraded in the older animals. (5) Selectivity was decreased in some animals with the longitudinal bipolar configuration and multiple response peaks were seen in several cases using this stimulus configuration.

Speech perception skills of deaf infants following cochlear implantation: a first report

“…It is possible that auditory deprivation may affect auditory acuity post-co-chlear implantation. Leake et al investigated this possibility using cats that were deafened for different lengths of time and then given cochlear implants [46,47]. They found that spatial selectivity of electrode impulses in the inferior colliculus was affected by the length of deafness and the amount of degeneration of the spiral ganglion cells.…”

Design and fabrication of multichannel cochlear implants for animal research

“…These patients often describe perceptual distortions that may be the result of channel interaction, an inability to discriminate electrode channels or a very limited useful dynamic range for some or all channels. Previous animal experiments using model cochlear implant devices have shown that the design of the implanted electrode array and its location in the scala tympani directly affect these limitations (Shepherd et al, 1993;Xu et al, 1993;Rebscher et al, 2001). Specifically, the location of the stimulation sites on the insulating carrier, the configuration of these sites (radial, longitudinal or diagonal geometries) and the mode of stimulation used (monopolar or bipolar) measurably affected threshold, dynamic range and specificity of responses to electrical stimuli.…”

“…These experiments have included systematic evaluation of the effects of stimulating contact configurations (Rebscher et al, 2001;Snyder et al, 2004), the long-term effects of intracochlear stimulation on the survival of peripheral neurons and central nervous system reorganization (Snyder et al, 1990;Leake et al, 2000;Moore et al, 2002;Snyder et al, 2004), the temporal response characteristics of IC and cortical neurons to acute and chronic electrical stimulation Snyder et al, 1995;Schreiner and Raggio, 1996;Vollmer et al, 1999;, the effects of chronically administered neurotrophins on the peripheral and central nervous system and studies documenting how input signals presented at different intracochlear locations inhibit, mask or excite neuronal responses. The scientific study of the interaction between multiple electrical stimuli, or between electrical and acoustic stimuli, and how these stimuli are processed in the central nervous system is in its infancy.…”

“…Auditory neural activation patterns in response to electrical stimuli have been reported in cats, guinea pigs and primates at the levels of the auditory nerve, the inferior colliculus and the auditory cortex (van den Honert and Stypulkowski, 1987;Snyder et al, 1990;Shepherd et al, 1993;Xu et al, 1993;Raggio and Schreiner, 1994;Hartmann and Klinke, 1995;Pfingst et al, 1995;Schreiner and Raggio, 1996;Kral et al, 1998;Shepherd et al, 1999;Leake et al, 2000;Rebscher et al, 2001;Bierer and Middlebrooks, 2002;Middlebrooks and Bierer, 2002;Moore et al, 2002;Snyder et al, 2004). These studies provide a basic understanding of how the auditory system responds to electrical stimuli and a framework for the development of intracochlear electrode arrays and signal processing strategies for use in human cochlear implant systems.…”

“…These previous physiology studies used intracochlear electrode arrays that modeled human cochlear implants in use during the 1970s and 1980s (Walsh et al, 1981;Shepherd et al, 1983;Leake et al, 1985;Snyder et al, 1990;Xu et al, 1993;Pfingst et al, 1995;Rebscher et al, 2001). More recently, two perimodiolar electrode arrays, the HiFocus™ and Contour™ models (Advanced Bionics, Inc., www.advancedbionics.com, and Cochlear Corporation, www.cochlear.com, respectively), were introduced and are now in widespread use throughout the world (Cords et al, 2000;Tykocinski et al, 2000;Tykocinski et al, 2001;Zwolan et al, 2001;Balkany et al, 2002).…”