Cochlear-implant spatial selectivity with monopolar, bipolar and tripolar stimulation

Abstract

Sharp spatial selectivity is critical to auditory performance, particularly in pitch related tasks. Most contemporary cochlear implants have employed monopolar stimulation that produces broad electric fields, which presumably contribute to poor pitch and pitch-related performance by implant users. Bipolar or tripolar stimulation can generate focused electric fields but requires higher current to reach threshold and, more interestingly, has not produced any apparent improvement in cochlear implant performance. The present study addressed this dilemma by measuring psychophysical and physiological spatial selectivity with both broad and focused stimulations in the same cohort of subjects. Different current levels were adjusted by systematically measuring loudness growth for each stimulus, each stimulation mode, and in each subject. Both psychophysical and physiological measures showed that, although focused stimulation produced significantly sharper spatial tuning than monopolar stimulation, it could shift the tuning position or even split the tuning tips. The altered tuning with focused stimulation is interpreted as a result of poor electrode-to-neuron interface in the cochlea, and is suggested to be mainly responsible for the lack of consistent improvement in implant performance. A linear model could satisfactorily quantify the psychophysical and physiological data and derive the tuning width. Significant correlation was found between the individual physiological and psychophysical tuning widths, and the correlation was improved by log-linearly transforming the physiological data to predict the psychophysical data. Because the physiological measure took only one-tenth of the time of the psychophysical measure, the present model is of high clinical significance in terms of predicting and improving cochlear implant performance.

Excitation Patterns of Standard and Steered Partial Tripolar Stimuli in Cochlear Implants

“…Several studies (Zhu et al 2012;Landsberger et al 2012;Saoji et al 2013) have recently measured the psychophysical spatial tuning curves and forward masking patterns for (partial or full) BP and TP stimuli. In Zhu et al (2012), the tip of spatial tuning and the peak of forward masking for full TP stimulation were shifted or split, possibly due to a dead region or poor neural survival around the main electrode. However, the PFM patterns of standard pTP stimulation with α = 0.5 in this study had a single peak on the main electrode EL8, suggesting that there was unlikely a dead region around EL8 for our subjects.…”

“…Previous studies mostly compared the excitation width of pBP or pTP stimulation with that of MP stimulation (e.g., Saoji et al 2013;Landsberger et al 2012). Only one study (Zhu et al 2012) directly compared the excitation width between full BP and TP stimuli. Similar to the present results, the width of spatial tuning curve was not significantly different between full BP and TP stimuli in Zhu et al (2012).…”

“…Only one study (Zhu et al 2012) directly compared the excitation width between full BP and TP stimuli. Similar to the present results, the width of spatial tuning curve was not significantly different between full BP and TP stimuli in Zhu et al (2012). It seems that the degree of current focusing may rely on the total amount of intracochlear return current, regardless of the distribution of return current between the flanking electrodes.…”

“…When the apical and/or basal flanking electrodes receive compensation or return current in the opposite phase to that of the middle main electrode, current spread is limited by the flanking electrodes, and excitation patterns are narrower than those of MP stimulation (e.g., Bierer 2007;Zhu et al 2012;Landsberger et al 2012;Saoji et al 2013). The use of a single or both flanking return electrodes is referred to as bipolar (BP) or tripolar (TP) stimulation, respectively.…”

“…The spatial profile of ECAP (e.g., Cohen et al 2003;Hughes and Abbas 2006) shows the spread of excitation for the probe by assessing the spatial and temporal interactions between the neurons activated by the masker and probe (due to the neural refractory effects). While ECAP patterns have been studied extensively for MP stimuli, those for focused TP stimuli have only been reported by Zhu et al (2012) to be irregular with multiple peaks. Within CI users, electrode pitch-ranking performance in MP mode has been found to be correlated with the separation of ECAP patterns between electrodes (Hughes 2008).…”