TIMING MATTERS! – THE BENEFIT OF TEMPORAL FINE STRUCTURE FOR BINAURAL HEARING WITH COCHLEAR IMPLANTS

The ability to perceive interaural time differences (ITDs) is very important for sound localization. Compared to normal hearing subjects, bilateral cochlear implanted (biCI) patients show largely reduced ITD sensitivity. This could be due to the lack of bilateral synchronization of the stimulation pulses by the two CIs in most clinical sound processors. An exception are sound processors using fine structure (FS) coding strategies. They transmit fine structure information of low-frequency sounds via apical stimulation electrodes, while more common clinical strategies do not encode fine structure.
We investigated ITD sensitivity in biCI children and biCI adults using low-frequency tone bursts with two different onset/offset ramp durations (4 and 100 ms), and two different coding strategies (FS and non-FS). Audio stimuli were fed directly into the sound processors, and ITD sensitivity was assessed using a lateralization task.
For tone bursts with short ramp, all subjects showed significantly better ITD sensitivity with FS than with non-FS, with some subjects even showing no sensitivity under non-FS. For tone bursts with long ramp, ITD sensitivity was reduced overall. Approximately 50% of the children and nearly all adults showed ITD sensitivity with FS, while sensitivity was lost in all subjects with non-FS for tone bursts with long ramp.
In contrast to non-FS, the FS coding strategy enabled significantly better ITD perception in biCI users. All patients benefited from additional onset/offset ITD information. This underscores the importance of coding natural temporal sound parameters in the stimulation patterns of cochlear implants to improve binaural hearing in biCI users.