Spatial Selectivity in Cochlear Implants: Effects of Asymmetric Waveforms and Development of a Single-Point Measure

Abstract

Three experiments studied the extent to which cochlear implant users' spatial selectivity can be manipulated using asymmetric waveforms and tested an efficient method for comparing spatial selectivity produced by different stimuli. Experiment 1 measured forward-masked psychophysical tuning curves (PTCs) for a partial tripolar (pTP) probe. Maskers were presented on bipolar pairs separated by one unused electrode; waveforms were either symmetric biphasic ("SYM") or pseudomonophasic with the short high-amplitude phase being either anodic ("PSA") or cathodic ("PSC") on the more apical electrode. For the SYM masker, several subjects showed PTCs consistent with a bimodal excitation pattern, with discrete excitation peaks on each electrode of the bipolar masker pair. Most subjects showed significant differences between the PSA and PSC maskers consistent with greater masking by the electrode where the high-amplitude phase was anodic, but the pattern differed markedly across subjects. Experiment 2 measured masked excitation patterns for a pTP probe and either a monopolar symmetric biphasic masker ("MP_SYM") or pTP pseudomonophasic maskers where the short high-amplitude phase was either anodic ("TP_PSA") or cathodic ("TP_PSC") on the masker's central electrode. Four of the five subjects showed significant differences between the masker types, but again the pattern varied markedly across subjects. Because the levels of the maskers were chosen to produce the same masking of a probe on the same channel as the masker, it was correctly predicted that maskers that produce broader masking patterns would sound louder. Experiment 3 exploited this finding by using a single-point measure of spread of excitation to reveal significantly better spatial selectivity for TP_PSA compared to TP_PSC maskers.

Keywords: cochlear implants; spatial selectivity.

FIG. 1

Schematic representation of bipolar (BP), tripolar (TP), quadrupolar virtual channel (QPVC), and all-polar (AP) modes of stimulation. The direction and length of each arrow indicates the polarity and amplitude of stimulation. Only eight electrodes are shown, for clarity.

FIG. 2

a How the bimodal excitation patterns that arise from BP stimulation with symmetric waveforms may be reduced by the use of pseudomonophasic waveforms. The waveforms are shown to the left of each stimulating electrode, and the schematic excitation patterns are shown to the right of the electrodes. b, c How the relative amplitudes of the central and side lobes of excitation, produced by tripolar stimulation, may be affected by the polarity of stimulation. d The situation in experiment 1 where the symmetric bipolar masker (green) is presented on electrodes (-1,1), thereby straddling the partial-tripolar probe (purple) on electrode 0.

FIG. 3

Forward-masked PTCs from experiment 1. The first seven panels show the results for individual subjects. Mean data are shown in the eighth (bottom right) panel for all subjects (filled symbols) or for all except subject AB1 (open symbols, shifted downwards by 10 dB for clarity). PTCs for the PSA, PSC, and SYM maskers are shown by the upward triangles, downward triangles, and circles, respectively. Points where the MLTs for the PSA and PSC maskers differed significantly at the 0.05 and 0.01 levels are shown by single and double symbols, respectively; asterisks indicate instances where the difference is in the predicted direction (PSA > PSC for apical maskers, PSA < PSC for basal maskers; see text) whereas crosses show differences in the opposite direction. The ordinate shows the MLT for the PTC measures. The values on the ordinate also show the detection threshold for each probe in quiet, shown by the open squares joined by dashed lines, which are shifted upwards by an arbitrary amount for each subject. These amounts are, in ascending order of subject number, −9, 0, −8, −9, −12, −3, and 0 dB.

FIG. 4

Masked excitation patterns for each subject of experiment 2, for TP_PSA (upward triangles), TP_PSC (downward triangles), and MP_SYM (circles) maskers. Thresholds in quiet are shown by open squares connected by dashed lines. Error bars are plus and minus one standard deviation, in dB.

FIG. 5

Scatter plot showing the correlation between the masker level relative to its MCL (a measure of loudness) and the amount of off-site masking (a measure of the spread of excitation). Both measures are normalised so that the mean for each listener, across masker types, is zero.

FIG. 6

Masked thresholds for each masker type and subject/electrode combination of experiment 3. Each cluster of three bars shows the data for one subject/electrode combination, with conditions TP-PSA, TP_PSC, and MP_SYM ordered from left to right and shown in red (cross-hatch), blue (downward stripes), and green (upward stripes), respectively. Cases where the TP_PSC stimulus produced significantly more masking than either the TP_PSC or MP_SYM masker are shown by single (p < 0.05) or double (p < 0.01) asterisks, respectively. The one case where it produced significantly less masking than the MP_SYM stimulus (subject AB2, e10, p < 0.05) is shown by a cross.

FIG. 7

Decay of forward masking for TP_PSA and TP_PSC maskers for the two subjects tested on this measure in experiment 3.