The normal-hearing ear processes complex sounds, such as speech, by stimulating different groups of neurons distributed along the length of the basilar membrane with different frequency components. Cochlear implants (CIs) attempt to mimic this spatial separation of frequency by allocating certain spectral components within the acoustic signal to different intracochlear electrodes. A limitation of this processing is that any two electrodes may stimulate overlapping groups of neurons. We can use forward masking of electrically evoked compound action potentials (ECAPs) to measure the degree of overlap in individual CI recipients, including those who can't participate in behavioral or speech perception testing. Unfortunately, these peripheral measures of spatial resolution have not been found to correlate with speech perception, which limits our ability to use these measures to evaluate and optimize the benefit a recipient receives from a CI. We will evaluate two hypothesizes relative to these limitations: (1) The integrity of the central auditory system is not reflected in ECAP forwar masking but is involved in speech perception, and (2) Spatial resolution, as measured by a limited set of electrode pairs, may not appropriately characterize how a complex signal, such as speech, stimulates multiple intracochlear electrodes. To address these issues, in addition to extensive peripheral measures of spatial resolution (ECAP forward masking), we propose using two electrophysiological measures of processing at the auditory cortex: a measure of spatial resolution (discrimination between two electrodes) and a measure of spectral resolution (discrimination between two spectrally complex acoustic signals). Three cumulative experiments have been proposed. Experiment 1 examines the relationship between peripheral/central measures of spatial resolution (i.e. electrode selectivity) within individual CI recipients. For Experiments 2 and 3, three novel CI programs will be used to effectively vary the spatial resolution within individual CI recipients. Experiment 2 examines the relationship between the processing of simple and complex stimulation. Experiment 3 examines the relationship between the three electrophysiological measures and vowel perception. Simple and multiple regression analyses will be performed using mixed models as needed to account for repeated measures. The results should more accurately define the relationship among these three electrophysiological measures and their ability to predict the way individual CI recipients perceive vowels.
The youngest cochlear implant recipients can't participate in extensive behavioral or speech perception testing, leaving clinicians with limited means for evaluating whether the child is receiving optimal benefit from the implant. This is a critical problem, as further development and maturation of a child's auditory system is dependent upon appropriate input from the implant. Here we propose to obtain an extensive set of electrophysiological measures, which can be obtained in children, to investigate whether any, combined or in isolation, can be used to predict speech perception.