Binaural unmasking of multi-channel stimuli in bilateral cochlear implant users

Abstract

Previous work suggests that bilateral cochlear implant users are sensitive to interaural cues if experimental speech processors are used to preserve accurate interaural information in the electrical stimulation pattern. Binaural unmasking occurs in adults and children when an interaural delay is applied to the envelope of a high-rate pulse train. Nevertheless, for speech perception, binaural unmasking benefits have not been demonstrated consistently, even with coordinated stimulation at both ears. The present study aimed at bridging the gap between basic psychophysical performance on binaural signal detection tasks on the one hand and binaural perception of speech in noise on the other hand. Therefore, binaural signal detection was expanded to multi-channel stimulation and biologically relevant interaural delays. A harmonic complex, consisting of three sinusoids (125, 250, and 375 Hz), was added to three 125-Hz-wide noise bands centered on the sinusoids. When an interaural delay of 700 μs was introduced, an average BMLD of 3 dB was established. Outcomes are promising in view of real-life benefits. Future research should investigate the generalization of the observed benefits for signal detection to speech perception in everyday listening situations and determine the importance of coordination of bilateral speech processors and accentuation of envelope cues.

FIG. 1

Block diagram of the processing scheme for the BMLD stimuli, showing the conversion of digital signal (S) and noise waveforms into electrical pulses.

FIG. 2

Time-domain representation of the stimulus presented to the left (L) and right (R) electrodes in the N₀S_τ condition at different SNRs (columns), after signal processing, including compression. The dashed line indicates a delay of 4 ms between the left and right ear. The pulse amplitude is expressed as a proportion of the dynamic range (DR).

FIG. 3

Electrical stimulation patterns applied to the left (L) and right (R) ear at A electrode 14, B electrode 13, and C electrode 12, for the N₀S_τ stimulus at different SNRs (columns). The dashed line indicates a delay of 2 ms between the left and right ear. The pulse amplitude is expressed as a proportion of the dynamic range (DR).

FIG. 4

Signal detection thresholds (in decibels SNR) for a single-channel stimulus in diotic conditions (N₀S₀—yellow) and conditions with a 4-ms (N₀S_τ 4 ms—purple), 2-ms (N₀S_τ 2 ms—blue), and 0.7-ms (N₀S_τ 0.7 ms—green) delay. Error bars depict 90% confidence intervals. Non-overlapping error bars indicate significant differences (also indicated with stars).

FIG. 5

Signal detection thresholds (in decibels SNR) for a multi-channel stimulus in diotic conditions (N₀S₀—yellow) and conditions with a 2-ms (N₀S_τ 2 ms—blue) and 0.7-ms (N₀S_τ 0.7 ms—green) delay. Error bars depict 90% confidence intervals. Non-overlapping error bars indicate significant differences (also indicated with stars).

FIG. 6

Box plots of single- (left side) and multi-channel (right side) BMLDs for a 2-ms (blue) and 0.7-ms delay (green). Boxes represent the median (thick horizontal line), lower and upper quartiles (ends of boxes), minimum and maximum values (ends of whiskers), outliers (values between 1.5 and 3 times the interquartile range below the first quartile or above the third quartile—circles).

FIG. 7

Signal detection thresholds (in decibels SNR) in diotic (N₀S₀—white) and dichotic (N₀S_τ—gray) conditions for A different sine wave frequencies (125 Hz—plain bars versus 375 Hz—hatched bars), B different noise bandwidths (50 Hz—plain bars versus 375 Hz—hatched bars), C different number of channels (one—plain bars versus three—hatched bars), and D uniform stimulus (plain bars) versus harmonic complex (hatched bars) on three channels. The size of the interaural delay in the N₀S_τ condition was dependent on the stimulus type and is indicated in the legend. Error bars depict 90% confidence intervals. Non-overlapping error bars indicate significant differences (also indicated with stars).