Binaural interactions in the auditory midbrain with bilateral electric stimulation of the cochlea

Abstract

Bilateral cochlear implantation seeks to restore the advantages of binaural hearing to the profoundly deaf by giving them access to binaural cues normally important for accurate sound localization and speech reception in noise. This thesis characterizes binaural interactions in auditory neurons using a cat model of bilateral cochlear implants. Single neuron responses in the inferior colliculus (IC), the main nucleus of the auditory midbrain, were studied using electric stimulation of bilaterally implanted intracochlear electrode arrays. Neural tuning to interaural timing difference (ITD) was emphasized since it is an important binaural cue and is well represented in IC neural responses. Stimulation parameters were explored in an effort to find stimuli that might result in the best ITD sensitivity for clinical use. The majority of IC neurons were found to be sensitive to ITD with low-rate constant-amplitude pulse trains. Electric ITD tuning was often as sharp as that with acoustic stimulation in normal-hearing animals, but many neurons had dynamic ranges of ITD sensitivity limited to a few decibels. Consistent with behavioral results in bilaterally implanted humans, neural ITD discrimination thresholds degraded with increasing pulse rates above 100 pulses per second (pps).

(cont.) Since cochlear implants typically encode sounds by amplitude modulation (AM) of pulse-train carriers, ITD tuning of IC neurons was also studied using AM pulse trains. Many IC neurons were sensitive to ITD in both the amplitude envelope and temporal fine structure of the AM stimulus. Sensitivity to envelope ITD generally improved with increasing modulation frequency up to 160 Hz. However, the best sensitivity was to fine structure ITD of a moderate-rate (1000 pps) AM pulse train. These results show that bilateral electric stimulation can produce normal ITD tuning in IC neurons and suggest that the interaural timing of current pulses should be accurately controlled if one hopes to design a bilateral cochlear implant processing strategy that provides salient ITD cues. In additional experiments, we found that evoked potentials may be clinically useful for assigning frequency-channel mappings in bilateral implant recipients, such as pediatric patients, for which existing psychophysical methods of matching interaural electrodes are unavailable.