Effectiveness of Phantom Stimulation in Shifting the Pitch Percept in Cochlear Implant Users.

OBJECTIVES: Phantom electrode stimulation was developed for cochlear implant (CI) systems to provide a lower pitch percept by stimulating more apical regions of the cochlea, without inserting the electrode array deeper into the cochlea. Phantom stimulation involves simultaneously stimulating a primary and a compensating electrode with opposite polarity, thereby shifting the electrical field toward the apex and eliciting a lower pitch percept. The current study compared the effect sizes (in shifts of place of excitation) of multiple phantom configurations by matching the perceived pitch with phantom stimulation to that perceived with monopolar stimulation. Additionally, the effects of electrode location, type of electrode array, and stimulus level on the perceived pitch were investigated. DESIGN: Fifteen adult advanced bionics CI users participated in this study, which included four experiments to eventually measure the shifts in place of excitation with five different phantom configurations. The proportions of current delivered to the compensating electrode, expressed as σ, were 0.5, 0.6, 0.7, and 0.8 for the symmetrical biphasic pulses (SBC0.5, SBC0.6, SBC0.7, and SBC0.8) and 0.75 for the pseudomonophasic pulse shape (PSA0.75). A pitch discrimination experiment was first completed to determine which basal and apical electrode contacts should be used for the subsequent experiments. An extensive loudness balancing experiment followed where both the threshold level (T-level) and most comfortable level (M-level) were determined to enable testing at multiple levels of the dynamic range. A pitch matching experiment was then performed to estimate the shift in place of excitation at the chosen electrode contacts. These rough shifts were then used in the subsequent experiment, where the shifts in place of excitation were determined more accurately. RESULTS: Reliable data were obtained from 20 electrode contacts. The average shifts were 0.39, 0.53, 0.64, 0.76, and 0.53 electrode contacts toward the apex for SBC0.5, SBC0.6, SBC0.7, SBC0.8, and PSA0.75, respectively. When only the best configurations per electrode contact were included, the average shift in place of excitation was 0.92 electrode contacts (range: 0.25 to 2.0). While PSA0.75 leads to equal results as the SBC configurations in the apex, it did not result in a significant shift at the base. The shift in place of excitation was significantly larger at the apex and with lateral wall electrode contacts. The stimulus level did not affect the shift. CONCLUSIONS: Phantom stimulation results in significant shifts in place of excitation, especially at the apical part of the electrode array. The phantom configuration that leads to the largest shift in place of excitation differs between subjects. Therefore, the settings of the phantom electrode should be individualized so that the phantom stimulation is optimized for each CI user. The real added value to the sound quality needs to be established in a take-home trial.

Evidence-Based Inclusion Criteria for Cochlear Implantation in Patients With Postlingual Deafness.

OBJECTIVES: This study determined the relationship between preoperative phoneme and word scores and the gain in speech perception after cochlear implantation in a large cohort of patients. The authors aimed to define evidence-based selection criteria for cochlear implantations in adults with postlingual deafness. DESIGN: This retrospective study included 364 adults with postlingual deafness who received a cochlear implant between 2000 and 2013 at the Leiden University Medical Center. The gain in speech perception observed postimplantation was compared with preoperative-aided/binaural speech perception scores, measured at 65 dB SPL in quiet. Patients who showed preoperative phoneme scores on monosyllabic words above 50% were also tested for speech perception in the presence of speech-shaped background noise, at a +5 dB signal to noise ratio. RESULTS: Speech perception in quiet improved after implantation in all except 7 patients. Average scores in quiet continued to improve, up to approximately 1 year after implantation. When participants were divided into five groups, based on preoperative speech perception scores, all groups showed gains in speech perception, except for the group with aided preoperative phoneme scores above 80%. However, all patients who were tested preoperatively with the addition of background noise showed improvements in speech perception in noise after the cochlear implantation. CONCLUSIONS: Although the decision to implant should consider individual ear differences and other factors that might apply to a particular case, based on our data, all patients with preoperative scores of either 80% (phonemes correct) or 60% (words correct) and lower in an optimal-aided situation are potential candidates for a cochlear implant, provided that their preoperative speech perception score decreases below 50% (phonemes correct) or 20% (words correct), when background noise is added at a +5 dB signal to noise ratio.

Diversity in cochlear morphology and its influence on cochlear implant electrode position.

OBJECTIVES: To define a minimal set of descriptive parameters for cochlear morphology and study its influence on the cochlear implant electrode position in relation to surgical insertion distance. DESIGN: Cochlear morphology and electrode position were analyzed using multiplanar reconstructions of the pre- and postoperative CT scans in a population of 336 patients (including 26 bilaterally implanted ones) with a CII HiFocus1 or HiRes90K HiFocus1J implant. Variations in cochlear diameter and cochlear canal size were analyzed. The relationship between the outer and inner walls was investigated. Size differences based on sex, age, and ear side were investigated using linear mixed models. Two new methods, spiral fitting and principal component analysis, were proposed to describe cochlear shape, and the goodness of fit was investigated. The relationship between cochlear shape and electrode position, in terms of modiolus proximity and insertion depth, was analyzed using clustering, one-way analysis of variance (ANOVA) and simple linear regression analysis. RESULTS: Large variations in cochlear morphology were found, with cochlear canal sizes ranging from 0.98 to 2.96 mm and average cochlear diameters between 8.85 and 5.92 mm (with standard deviations of around 0.4 mm). The outer and inner walls were significantly correlated (p < 0.01), and a size difference of 4% in favor of males was found. Spiral fitting shows good alignment of the true measurements, with residuals having a mean of 0.01 mm and a standard deviation of 0.29 mm. Principal component analysis (PCA) showed that the use of one component, which describes size, is sufficient to explain 93.6% of the cochlear shape variance. A significant sex difference was also found with spiral fitting and PCA. Cochlear size was found to have a significant influence on modiolus proximity and insertion depth of the electrode (p < 0.01). Cochlear size explained around 13% of the variance in electrode position. When cochlear size was combined with surgical insertion, more than 81% of the variance in insertion depth can be explained. CONCLUSIONS: This study demonstrates a large variety in cochlear morphology, which significantly impacts electrode position in terms of modiolus proximity and insertion depth. The effect size is, however, relatively small compared with surgical insertion distance. PCA is shown to be an accurate reduction method for describing cochlear shape.

Threshold levels of dual electrode stimulation in cochlear implants.

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.

Restoring speech perception with cochlear implants by spanning defective electrode contacts.

CONCLUSION: Even with six defective contacts, spanning can largely restore speech perception with the HiRes 120 speech processing strategy to the level supported by an intact electrode array. Moreover, the sound quality is not degraded. OBJECTIVES: Previous studies have demonstrated reduced speech perception scores (SPS) with defective contacts in HiRes 120. This study investigated whether replacing defective contacts by spanning, i.e. current steering on non-adjacent contacts, is able to restore speech recognition to the level supported by an intact electrode array. METHODS: Ten adult cochlear implant recipients (HiRes90K, HiFocus1J) with experience with HiRes 120 participated in this study. Three different defective electrode arrays were simulated (six separate defective contacts, three pairs or two triplets). The participants received three take-home strategies and were asked to evaluate the sound quality in five predefined listening conditions. After 3 weeks, SPS were evaluated with monosyllabic words in quiet and in speech-shaped background noise. RESULTS: The participants rated the sound quality equal for all take-home strategies. SPS with background noise were equal for all conditions tested. However, SPS in quiet (85% phonemes correct on average with the full array) decreased significantly with increasing spanning distance, with a 3% decrease for each spanned contact.

Spread of excitation and channel interaction in single- and dual-electrode cochlear implant stimulation.

OBJECTIVES: To determine how simultaneous dual-electrode stimulation (DES) can be optimized for the individual patient to deliver better sound quality and speech recognition. DES was compared with single-electrode stimulation (SES) with respect to the site of stimulation (X) in the cochlea, the spread of excitation (SOE), and channel interaction. Second, it was investigated whether the number of intermediate pitches created with DES can be predicted from SOE, channel interaction measures, current distribution in the cochlea, or distance of the electrode to the medial wall. DESIGN: Twelve users of the HiRes90K cochlear implant with HiFocus1J electrode were randomly selected to participate in this study. Electrode contacts were selected based on their location in the cochlea as determined by multislice computed tomography, viz. 120 degrees (basal), 240 degrees (middle), and 360 degrees (apical) from the round window. The number of intermediate pitches with simultaneous DES was assessed with a three-alternative forced choice pitch discrimination experiment. The channel interactions between two single-electrode contacts and two DES pairs were determined with a threshold detection experiment (three-alternative forced choice). The eCAP-based SOE method with fixed probe and variable masker was used to determine the location of the neurons responding to a single-electrode contact or dual-electrode contact stimulus. Furthermore, the intracochlear electrical fields were determined with the Electrical Field Imaging tool kit. RESULTS: DES was not different from SES in terms of channel interaction and SOE. The X of DES was 0.54 electrode contacts more basal compared with SES stimulation, which was not different from the predicted shift of 0.5. SOE and current distribution were significantly different for the three locations in the cochlea but showed no correlation with the number of perceivable pitches. A correlation was found between channel interaction and the number of intermediate pitches along the array within a patient, not between patients. CONCLUSION: SES and DES are equivalent with regard to SOE and channel interaction. The excitation site of DES has the predicted displacement compared with the excitation region induced by the neighboring single-electrode contact. Unfortunately, no predictor for the number of intermediate pitches was found.

Influence of widening electrode separation on current steering performance.

OBJECTIVES: Current steering between adjacent electrodes makes it possible to create more spectral channels than the number of electrodes in an electrode array. With current steering on nonadjacent electrodes, called "spanning," it could be possible to bridge a defective electrode contact or potentially reduce the number of electrode contacts for the same level of access to the auditory nerve. This study investigates the effectiveness of spanning in terms of the number of intermediate pitches, loudness effects, and linearity of the current weighting coefficient (α) with respect to the perceived pitch. DESIGN: Twelve postlingually deafened users of the HiRes90K cochlear implant with HiFocus1j electrode were randomly selected to participate in this study. Electrode contacts were selected at two locations in the cochlea, as determined on multislice computed tomography: 180° (basal) and 360° (apical) from the round window. For both cochlear locations, three psychophysical experiments were performed using simultaneous stimulation of electrode contacts. An adaptive staircase-based procedure was used. The number of intermediate pitches was assessed with a three-alternative forced choice (3AFC) pitch discrimination task, and the extent of current adjustment required when varying the current weighting coefficient (α) was determined with loudness balancing (2AFC). Finally, the pitch of a spanned channel was matched with the pitch of an intermediate physical electrode in a 2AFC procedure to assess the place of the spanned channel on the electrode array. RESULTS: Spanning required significantly more current compensation to maintain equal loudness than current steering between adjacent electrode contacts. A significant decrease of discriminable intermediate pitches occurred with spanning in comparison with current steering between adjacent electrode contacts. No significant difference was found between the pitch-matched current steering coefficient and the theoretical coefficient corresponding a priori with the intermediate physical electrode. No significant difference was found between the data from the apical and the basal sections of the electrode array. CONCLUSIONS: Spanning over wider electrode distance is feasible. With increasing electrode spanning distance, more current compensation is needed to maintain equal loudness, and a gradual deterioration in the just noticeable difference for pitch is observed. However, the pitch progression is linear. For a spanned signal with equal proportions of current delivered to both electrodes, pitch is equivalent to that produced by an intermediate physical electrode.