Threshold Levels of Dual Electrode Stimulation in Cochlear Implants

Abstract

Simultaneous stimulation on two contacts (current steering) creates intermediate pitches between the physical contacts in cochlear implants. All recent studies on current steering have focused on Most Comfortable Loudness levels and not at low stimulation levels. This study investigates the efficacy of dual electrode stimulation at lower levels, thereby focusing on the requirements to correct for threshold variations. With a current steered signal, threshold levels were determined on 4 different electrode pairs for 7 different current steering coefficients (α). This was done psychophysically in twelve postlingually deafened cochlear implant (HiRes90K, HiFocus1J) users and, in a computer model, which made use of three different neural morphologies. The analysis on the psychophysical data taking all subjects into account showed that in all conditions there was no significant difference between the threshold level of the physical contacts and the intermediate created percepts, eliminating the need for current corrections at these very low levels. The model data showed unexpected drops in threshold in the middle of the two physical contacts (both contacts equal current). Results consistent with this prediction were obtained for a subset of 5 subjects for the apical pair with wider spacing (2.2 mm). Further analysis showed that this decrease was only observed in subjects with a long duration of deafness. For current steering on adjacent contacts, the results from the psychophysical experiments were in line with the results from computational modelling. However, the dip in the threshold profile could only be replicated in the computational model with surviving peripheral processes without an unmyelinated terminal. On the basis of this result, we put forward that the majority of the surviving spiral ganglion cells in the cochlea in humans with a long duration of deafness still retain peripheral processes, but have lost their unmyelinated terminals.

Constructing a three‐dimensional electrical model of a living cochlear implant user's cochlea

“…The relationship between neural threshold levels and perceptual T‐levels is not known precisely. Snel‐Bongers, et al used a 1‐mm region of excitation as an indication of perceptual threshold levels, and based on their findings, single‐fiber threshold levels may be expected to be lower than mapped T‐levels. However, the present model predicts these to be around 25 dB higher than the T‐levels in the maps of the specific users whose cochleae were modeled.…”

The effect of the resistive properties of bone on neural excitation and electric fields in cochlear implant models

“…The stimulus level at which this occurs is referred to as the probe threshold and was defined as the level at which a neuron population over a width of 1 mm along the length of the basilar membrane is stimulated (Snel-Bongers et al, 2013). For the probe to be heard above the masker, a section of at least 1 mm of neural excitation needs to extend beyond the masker's neural response.…”

The history and future of neural modeling for cochlear implants

“…The morphology of the nerve, its diameter, the position and thickness of the myelin shell, and, not least, its trajectory in the electric field affect spike generation and are increasingly refined by anatomical knowledge in more recent models. Degeneration of the nerve is a critical aspect in CI users and compartmental models have been used to investigate the effects of various stages of degeneration, from the dying of the peripheral dendrite to demyelination (Snel-Bongers et al 2013).…”