Effect of channel separation and interaural mismatch on fusion and lateralization in normal-hearing and cochlear-implant listeners

Abstract

Bilateral cochlear implantation has provided access to some of the benefits of binaural hearing enjoyed by normal-hearing (NH) listeners. However, a gap in performance still exists between the two populations. Single-channel stimulation studies have shown that interaural place-of-stimulation mismatch (IPM) due to differences in implantation depth leads to decreased binaural fusion and lateralization of interaural time and level differences (ITDs and ILDs, respectively). While single-channel studies are informative, multi-channel stimulation is needed for good speech understanding with cochlear implants (CIs). Some multi-channel studies have shown that channel interaction due to current spread can affect ITD sensitivity. In this work, we studied the effect of IPM and channel spacing, along with their potential interaction, on binaural fusion and ITD/ILD lateralization. Experiments were conducted in adult NH listeners and CI listeners with a history of acoustic hearing. Results showed that IPM reduced the range of lateralization for ITDs but not ILDs. CI listeners were more likely to report a fused percept in the presence of IPM with multi-channel stimulation than NH listeners. However, no effect of channel spacing was found. These results suggest that IPM should be accounted for in clinical mapping practices in order to maximize bilateral CI benefits.

FIG. 1.

(Color online) Examples of how IPM was simulated for the two channel-spacing configurations tested. Each row shows a schematic of an unrolled cochlea with a grey 22-electrode array with different insertion depths across the ears. The yellow markers indicate the place along the cochlea being stimulated. In the close-spacing configuration (A), the place of stimulation is held consistently at the same distance apart (1.5 mm in NH and ∼2 electrode spacing in CI listeners), and the place of stimulation in the contralateral ear is varied depending on the desired amount of simulated IPM. In the far-spacing configuration (B), the places of stimulation in each ear are further apart (4.5 mm in NH and ∼6 electrode spacing in CI listeners).

FIG. 2.

(Color online) Example of a fit to the lateralization data with ILD cues of one of the NH listeners. L, C, and R stand for the left, center, and right of the GUI. The size of each circle represents the number of responses in a particular location on the GUI. The solid line shows the fit to the responses.

FIG. 3.

(Color online) Spectrum of the stimulus used in the NH experiments for each IPM and spacing configuration. The red line shows the spectrum of the stimulus in the ear that was held constant and the blue line shows the spectrum of the stimulus in the ear that was changed.

FIG. 4.

(Color online) Proportion of trials where only one auditory object was reported as a function of IPM in NH listeners. (A) shows individual data and (B) shows group averaged data for the two spacing-channel configurations tested.

FIG. 5.

(Color online) Perceived lateral position of the dominant auditory object for NH listeners. (A) shows the data for the close-spacing configuration where the upper panels show the data for listeners who always heard one fused auditory object and the lower panels show the data for listeners who reported hearing more than one auditory object. (B) shows the data for the far-spacing configuration arranged in a similar manner as that of (A).

FIG. 6.

(Color online) Results from the three analysis metrics calculated from the NH lateralization data obtained when a non-zero ITD was applied to the stimulus. (A) shows the proportion of trials where one auditory object was reported, (B) shows the ULR, and (C) shows the estimated threshold. The top, middle, and bottom rows show the individual data for the close-spacing configuration, individual data for the far-spacing configuration, and group data, respectively.

FIG. 7.

(Color online) Results from the three analysis metrics calculated from the NH lateralization data obtained when a non-zero ILD was applied to the stimulus. The panels are arranged in the same way as in Fig. 6.

FIG. 8.

(Color online) Proportion of trials where only one auditory object was reported as a function of IPM in CI listeners. The panels are arranged in the same way as in Fig. 4.

FIG. 9.

(Color online) Perceived lateral position of the dominant auditory object for CI listeners. The panels are arranged in the same way as in Fig. 5.

FIG. 10.

(Color online) Results from the three analysis metrics calculated from the CI lateralization data obtained when a non-zero ITD was applied to the stimulus. The panels are arranged in the same way as in Fig. 6.

FIG. 11.

(Color online) Results from the three analysis metrics calculated from the CI lateralization data obtained when a non-zero ILD was applied to the stimulus. The panels are arranged in the same way as in Fig. 6.