Consonant recognition as a function of the number of stimulation channels in the Hybrid short-electrode cochlear implant.

Consonant recognition was measured as a function of the number of stimulation channels for Hybrid short-electrode cochlear implant (CI) users, long-electrode CI users, and normal-hearing (NH) listeners in quiet and background noise. Short-electrode CI subjects were tested with 1-6 channels allocated to a frequency range of 1063-7938 Hz. Long-electrode CI subjects were tested with 1-6, 8, or 22 channels allocated to 188-7938 Hz, or 1-6 or 15 channels from the basal 15 electrodes allocated to 1063-7938 Hz. NH listeners were tested with simulations of each CI group/condition. Despite differences in intracochlear electrode spacing for equivalent channel conditions, all CI subject groups performed similarly at each channel condition and improved up to at least four channels in quiet and noise. All CI subject groups underperformed relative to NH subjects. These preliminary findings suggest that the limited channel benefit seen for CI users may not be due solely to increases in channel interactions as a function of electrode density. Other factors such as pre-operative patient history, location of stimulation in the base versus apex, or a limit on the number of electric channels that can be processed cognitively, may also interact with the effects of electrode contact spacing along the cochlea.

Changes in pitch with a cochlear implant over time.

In the normal auditory system, the perceived pitch of a tone is closely linked to the cochlear place of vibration. It has generally been assumed that high-rate electrical stimulation by a cochlear implant electrode also evokes a pitch sensation corresponding to the electrode's cochlear place ("place" code) and stimulation rate ("temporal" code). However, other factors may affect electric pitch sensation, such as a substantial loss of nearby nerve fibers or even higher-level perceptual changes due to experience. The goals of this study were to measure electric pitch sensations in hybrid (short-electrode) cochlear implant patients and to examine which factors might contribute to the perceived pitch. To look at effects of experience, electric pitch sensations were compared with acoustic tone references presented to the non-implanted ear at various stages of implant use, ranging from hookup to 5 years. Here, we show that electric pitch perception often shifts in frequency, sometimes by as much as two octaves, during the first few years of implant use. Additional pitch measurements in more recently implanted patients at shorter time intervals up to 1 year of implant use suggest two likely contributions to these observed pitch shifts: intersession variability (up to one octave) and slow, systematic changes over time. We also found that the early pitch sensations for a constant electrode location can vary greatly across subjects and that these variations are strongly correlated with speech reception performance. Specifically, patients with an early low-pitch sensation tend to perform poorly with the implant compared to those with an early high-pitch sensation, which may be linked to less nerve survival in the basal end of the cochlea in the low-pitch patients. In contrast, late pitch sensations show no correlation with speech perception. These results together suggest that early pitch sensations may more closely reflect peripheral innervation patterns, while later pitch sensations may reflect higher-level, experience-dependent changes. These pitch shifts over time not only raise questions for strict place-based theories of pitch perception, but also imply that experience may have a greater influence on cochlear implant perception than previously thought.