Hearing-in-noise benefits after bilateral simultaneous cochlear implantation continue to improve 4 years after implantation

Abstract

Objective: The purpose of this 4-year longitudinal study was to assess the stability of the binaural benefits of head shadow, summation, and squelch for bilateral cochlear implant recipients and to quantify these benefits for the understanding of speech in noise.

Design: This is a prospective study of 9 patients who received simultaneous bilateral insertion of MED-EL COMBI +40 cochlear implants in a single-stage operation at the University of North Carolina, Chapel Hill, NC. Each patient had postlingual deafness of short duration before insertion of the device. Each year, the patients were tested for word recognition using consonant-nucleus-consonant words in quiet and speech perception in noise using City University of New York sentences. These tests were administered using direct audio input to the implants. Head-related transfer functions were used to simulate speech in noise testing in a spatial environment. Speech was always presented at midline (0 degrees), and the noise masker was presented at either side or midline (-90, 0, +90 degrees).

Results: The binaural benefits of head shadow and summation effects developed early in the postoperative period and remained stable throughout the follow-up period. Squelch developed more slowly and was first demonstrated at 12 months after implantation but continued to increase beyond the first year of follow-up.

Conclusion: Benefits of head shadow and summation emerge early and remain stable. However, squelch has the most protracted period of development, with increasing benefit after a year or more of implant experience. These data support the idea that binaural integration continues several years after insertion of bilateral cochlear implant devices.

FIG. 1

A diagram of the testing paradigm for summation and squelch. A, The stimulus configuration for assessing summation. In this image, the signal is presented with noise to the listener as if it were directly in front of her. The benefit is achieved through stimulation of both ears by the same target signal. B, The stimulus configuration used for estimating squelch. In this image, the signal is presented from the front, and the left ear is more heavily masked than the right. Squelch can be described as the benefit obtained by the addition of information from the masked side. Illustrations for Figure 1 were provided by Laura M. Buss.

FIG. 2

A comparison of percent correct for CNC words in quiet over time. Bar shading reflects listening condition: worse side alone (black), better side alone (white), or bilateral (hatched bars). Error bars show 1 standard error of the mean.

FIG. 3

The derived measures of binaural benefit--summation, head shadow, and squelch--are plotted by patient number and shown separately for each of the follow-up intervals. In the case of head shadow and squelch, estimates are shown separately for the left and right side (left- and right-pointing triangles, respectively).