Current focusing sharpens local peaks of excitation in cochlear implant stimulation

Abstract

Cochlear implant (CI) users' spectral resolution is limited by the number of implanted electrodes, interactions between the electrodes, and the underlying neural population. Current steering has been proposed to increase the number of spectral channels beyond the number of physical electrodes, however, electric field interactions may limit CI users' access to current-steered virtual channels (VCs). Current focusing (e.g tripolar stimulation) has been proposed to reduce current spread and thereby reduce interactions. In this study, current steering and current focusing were combined in a four-electrode stimulation pattern, i.e quadrupolar virtual channels (QPVCs). The spread of excitation was measured and compared between QPVC and Monopolar VC (MPVC) stimuli using a forward masking task. Results showed a sharper peak in the excitation pattern and reduced spread of masking for QPVC stimuli. Results from the forward masking study were compared with a previous study measuring VC discrimination ability and showed a weak relationship between spread of excitation and VC discriminability. The results suggest that CI signal processing strategies that utilize both current steering and current focusing might increase CI users' functional spectral resolution by transmitting more channels and reducing channel interactions.

Figure 1

Illustration of different stimulation modes. The oval below each rectangular electrode array represents the extracochlear electrode. Note that this is the second phase of a cathodic-first bi-phasic pulse. “i” represents the current amplitude, α represents the fraction of current delivered to the basal electrode, and σ represents the fraction of current returned on the flanking electrodes within the array.

Figure 2

Loudness balanced levels for MPVC and QPVC stimuli (in dB) for individual subjects and electrode pairs (with α = 0.5 in all conditions) as measured in the forward masking task. Black bar indicates MPVC amplitude, gray bar indicates amplitude increase required for loudness balanced QPVC stimulus.

Figure 3

Normalized forward masking patterns with MPVC (filled symbols) and QPVC (open symbols) maskers, for individual subjects. Data is shown for the apical (left), middle (middle), and basal (right) electrode pairs; the specific VC component electrodes are listed in Table 1. The lower x-axis shows the α-value for the QPVC probe and the upper x-axis shows the distance (in mm) of the probe from the masker. The masker α value was always 0.5. The y-axis shows the normalized threshold (in μA), relative to the peak masking. Data were not collected at all locations for all subjects.

Figure 4

Top: Difference in the areas under the normalized masked threshold curves (MPVC area – QPVC area) shown in Figure 3. Subjects and stimulation sites are ordered according to the magnitude of difference. Bottom: Difference in the average percent masking at the endpoints of the normalized masked threshold curves (MPVC percent masking – QPVC percent masking) shown in Figure 3. Subjects and stimulation sites are shown in the same order as in the top panel.

Figure 5

A summary of VC discrimination results for all electrode pairs measured. Bars show the cumulative d′ score for each electrode pair in a stimulation mode. A larger cumulative d′ indicates greater perceptual resolution. Data to the left of the dashed line indicates data described in a previous paper (Landsberger & Srinivasan, 2009) and data to the right was collected specifically for this study.

Figure 6

Panel A: MPVC cumulative d′ as a function of the area under the normalized MPVC masking curve. Panel B: QPVC cumulative d′ as a function of the area under the normalized QPVC masking curve. Panel C: MPVC cumulative d′ as a function of the average percent masking at the endpoints of the MPVC masking curve. Panel D: QPVC cumulative d′ as a function of the average percent masking at the endpoints of the QPVC masking curve. The correlation coefficient r and the p-value calculated from the Spearman’s rank correlation are shown.

Figure 7

Top: difference in cumulative d′ (see Figure 5) between stimulation modes as a function of the difference in the area under the normalized forward masking functions (see Figure 3). Bottom: difference in cumulative d′ between stimulation modes as a function of the difference in percent masking response at the endpoints of the normalized forward masking functions. The dashed lines show no difference between stimulation modes. The correlation coefficient r and the p-value calculated from the Spearman’s rank correlation are shown.