Objective: To test localization of sound sources in horizontal and vertical dimensions in cochlear-implant (CI) listeners using clinical bilateral CI systems.
Design: Five bilateral CI subjects listened via their clinical speech processors to noises filtered with subject-specific, behind-the-ear microphones and head-related transfer functions. Subjects were immersed in a visual virtual environment presented via a head-mounted display. Subjects used a manual pointer to respond to the perceived sound location and received visual response feedback via the head-mounted display during the tests. The target positions were randomly distributed in two-dimensional space over an azimuth range of 0° to 360° and over an elevation range of -30° to +80°. In experiment 1, the signal level was roved in the range of ±2.5 dB from trial to trial. In experiment 2, the signal level was roved in the range of ±5 dB.
Results: CI subjects were generally worse at sound localization than normal-hearing listeners tested in a previous study, in both the horizontal and vertical dimensions. In the horizontal plane, subjects could determine the correct side and locate the target within the side at better than chance performance. In the vertical plane, with a smaller level-roving range, subjects could determine the correct hemifield at better than chance performance but could not locate the target within the correct hemifield. The target angle and response angle were correlated as expected. The response angle and signal level range were also correlated, raising concerns that subjects were using only level cues for the task. With a larger level-roving range, the number of front-back confusions increased. The correlation between the target and response angles decreased, whereas the correlation between the level and response angle did not change, which is an indication that the subjects were relying heavily on level cues.
Conclusions: For the horizontal plane, the results are in agreement with previous CI studies performed in the horizontal plane with a comparable range of targets. For the vertical plane, CI listeners could discriminate front from back at better than chance performance; however, there are strong indications that the broadband level, not the spectral profile, was used as the primary localization cue. This study indicates the necessity of new CI processing strategies that encode spectral localization cues.