Binaural unmasking with temporal envelope and fine structure in listeners with cochlear implants

Abstract

For normal-hearing (NH) listeners, interaural information in both temporal envelope and temporal fine structure contribute to binaural unmasking of target signals in background noise; however, in many conditions low-frequency interaural information in temporal fine structure produces greater binaural unmasking. For bilateral cochlear-implant (CI) listeners, interaural information in temporal envelope contributes to binaural unmasking; however, the effect of encoding temporal fine structure information in electrical pulse timing (PT) is not fully understood. In this study, diotic and dichotic signal detection thresholds were measured in CI listeners using bilaterally synchronized single-electrode stimulation for conditions in which the temporal envelope was presented without temporal fine structure encoded (constant-rate pulses) or with temporal fine structure encoded (pulses timed to peaks of the temporal fine structure). CI listeners showed greater binaural unmasking at 125 pps with temporal fine structure encoded than without. There was no significant effect of encoding temporal fine structure at 250 pps. A similar pattern of performance was shown by NH listeners presented with acoustic pulse trains designed to simulate CI stimulation. The results suggest a trade-off across low rates between interaural information obtained from temporal envelope and that obtained from temporal fine structure encoded in PT.

FIG. 1.

Representations of an NoSπ electrical stimulus at 125 pps in the PA (left) and PA + PT (right) conditions at 0-dB SNR. Left and right channels are shown in gray and black, respectively. The temporal envelope of each channel is traced with a line for clarity. The units of the y axis are clinical current units (CU). The stimulus was compressed between 134 and 190 CU (arbitrarily chosen) for the figure. Only the middle 150 ms of the stimulus is shown.

FIG. 2.

NoSo (gray) and NoSπ (black) signal detection thresholds (dB SNR) in the PA and PA + PT conditions averaged across CI participants. Left, middle, and right panels show thresholds for 125 pps, 250 pps, and 1000 pps, respectively. Data were not collected for the 1000-pps PA + PT condition. Error bars show ±1 standard error of the mean.

FIG. 3.

The waveform (left) and spectrum (right) of an acoustic stimulus with a 125-pps pulse rate in the PA condition. Only the left channel is shown. The waveform shows the middle 150 ms of the stimulus.

FIG. 4.

NoSo (gray) and NoSπ (black) signal detection thresholds (dB SNR) in the PA, PA + PT, and PT conditions with the 50-Hz bandwidth stimuli, averaged across NH participants. Left, middle, and right panels show thresholds for 125 pps, 250 pps, and 500 pps, respectively. NoSo PA + PT and NoSo PT thresholds are the same data. Error bars show ±1 standard error of the mean. Unfilled black circles show model predictions of NoSπ thresholds (see the Appendix).

FIG. 5.

NoSo (gray) and NoSπ (black) signal detection thresholds (dB SNR) in the PA, PA + PT, and PT conditions at 500 pps averaged across NH participants. Left and right panels show thresholds for the 50-Hz and 125-Hz noise-bandwidth (BW) conditions, respectively. NoSo PA + PT and NoSo PT thresholds are the same data. The 50-Hz BW data are the same as the 500-pps data in Fig. 4. Error bars show ±1 standard error of the mean.

FIG. 6.

Representations of an acoustic PA + PT NoSπ stimulus at 125 pps (left), 250 pps (middle), and 500 pps (right) after physiologically inspired preprocessing. Stimuli were at −4-dB SNR. Left and right channels are shown in gray and black, respectively. Preprocessing consisted of fourth-order gammatone filtering centered at 9.2 kHz, temporal envelope compression with a power of 0.46, half-wave rectification, and low-pass filtering with a fourth-order Butterworth filter with a 425-Hz cutoff frequency, and a second low-pass filtering with a second-order Butterworth filter with a 100-Hz cutoff frequency.

FIG. 7.

Metrics of interaural “correlation” of the left and right channels of the NoSπ stimuli as a function of SNR (dB) in the PA (left column), PA + PT (middle column), and PT (right column) conditions. Stimuli were that of the NH listeners in the 50-Hz bandwith condition (experiment 2). Rows from top to bottom show the normalized envelope correlation, the normalized envelope covariance 30%, and the normalized envelope covariance, respectively. The low-pass cutoff frequency of the second-order filter was 100 Hz for all metrics in the figure. The pulse rate of the stimuli is shown with the darkness of shading. Error bars show ±1 standard deviation from the mean. Dotted lines show the metric value at which predicted thresholds (corresponding SNRs) explained the highest amount of variance in the average observed NH thresholds.

FIG. 8.

The variance of the average NH NoSπ thresholds explained (%) by three metrics as a function of the low-pass second-order cutoff frequency (Hz) used in calculating the metrics. The three metrics shown are the normalized envelope correlation (Corr.), the normalized envelope covariance 30% (Cov. 30%), and the normalized envelope covariance (Cov.). Each metric is shown with a different symbol and a connecting line.