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.

Figure 1

A schematic diagram of three stimulation modes. Monopolar (top), bipolar (middle), and tripolar (bottom) stimulations all have the same active electrode (gray bar; EL=electrode), but different return electrodes (hatched bars; EL−1 or EL+1 or EC=extra-cochlear electrode). The solid lines indicates the electric circuit; the dashed arrow represents the current path from active to return electrode, with (-I) indicating the cathodic phase.

Figure 2. Loudness growth function for single pulse and pulse trian

Individual loudness growth (rows) as a function of logarithmic current level (x-axis) using three stimulation modes (columns) and three stimulation durations (symbols). The data were fitted using a logarithmic function (see text for details).

Figure 3. STC (dB re:1uA)

Individual psychophysical spatial tuning curves plotting masker level as a function of masker electrode position. Rows represent individual subjects while columns represent stimulation modes. Symbols represent different probe levels. The dotted vertical line represents the probe electrode position (E8).

Figure 4. ave STC (dB re: 1uA)

Individual averaged spatial tuning curves (open circles) and their best fit using a linear function (solid lines). The range of the solid lines represents the electrode range used to produce the highest R² value (accounting for the most variance). The solid red symbol shows the tip position of the fitted tuning curve. The horizontal dashed line represents the Q_1dB width of the fitted tuning curve.

Figure 5. SMC (uV)

Individual physiological spatial masking curves plotting electrically-evoked compound action potential (ECAP) amplitude as a function of masker electrode position. Rows represent individual subjects while columns represent stimulation modes. Symbols represent different probe levels. The dotted vertical line represent the probe electrode position.

Figure 6. ave SMC (uV)

Individual averaged spatial masking curves (open circles) and their best fit using a linear function (solid lines). The range of the solid lines represents the electrode range used to produced the highest R² value. The solid red symbol shows the peak position of the fitted tuning curve. The horizontal dashed line represents the Q_1dB width of the fitted spatial masking curve.

Figure 7. Prediction of STC from SMC (μV)

Actual (open circles) and predicted (solid lines) psychophysical spatial tuning data by a linear-logarithmic transformation (Eq. 1 in the text) of the selected ECAP spatial masking data. The selected ECAP range was determined a subset of electrodes that best described the tip of the psychophysical tuning curve in Fig. 4.

Figure 8. Correlation

Correlations between psychophysical and physiological spatial selectivity measures (solid circles represent monoplor data, inverted triangles represent bipolar data, and solid squares represent tripolar data). The top panel shows correlation between spatial tuning curve width (from Fig. 4) versus spatial masking curve width (from Fig. 6). The bottom panel shows correlation with predicted spatial tuning curve width by nonlinearly transforming the ECAP masking data (from Fig. 7). The dashed diagonal line represents perfect prediction of the psychophysical tuning width from the physiolgical data.

Figure 9. Spreads of excitation with various neuron survival

Schematic excitation patterns of broad (left column) and focused (right column) electric stimulation (shaded area) and its interactions with nerve survival patterns (row 1=good survival; rows 2 and 3=poor survival). See text for details.