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... Show moreObjectives: 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 sigma, 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 (PSA(0.75)). 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 PSA(0.75), 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 PSA(0.75)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. Show less
With commonly used monopolar or __single electrode stimulation__ (SES) in cochlear implants the perceived pitch depends on the place in the cochlea of the stimulated contact. When two contacts are... Show moreWith commonly used monopolar or __single electrode stimulation__ (SES) in cochlear implants the perceived pitch depends on the place in the cochlea of the stimulated contact. When two contacts are stimulated simultaneously, __dual electrode stimulation__(DES), intermediate pitches can be elicited. The place and precise pitch can be adjusted by varying the current ratio between these two contacts. In this thesis the mechanism of DES is investigated psychophysically, electrophysiologically and in a computational model of the cochlea. It was concluded that DES and SES are indistinguishable in terms of spread of excitation and sequential channel interaction, while with DES the pitch depends linearly on the current ratio. On adjacent contacts, DES turned out to be effective for the entire dynamic range without the need for any current correction to equalize loudness between pitches. DES is also feasible on non-adjacent contacts (__spanning__) up till 4.4 mm, but with increasing distance between the contacts, such a current correction becomes mandatory, while also the number of discriminable pitches decreases. Finally, spanning was implemented in a speech coding strategy and tested in a take-home trial, which demonstrated that even with two groups of three adjacent defective contacts, speech perception and sound quality were retained. Show less
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... Show moreOBJECTIVES:: 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. Show less
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... Show moreObjectives: 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 (alpha) 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 degrees (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 (alpha) 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. (Ear & Hearing 2011;32;221-229) Show less
