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Spiral Ganglion Cell Survival And Speech Perception In Ci Recipients

Cochlear implant recipients demonstrate an extremely wide range of auditory performance post-surgery. The benefit received by some individuals is little more than simple awareness of environmental sounds, while others enjoy significant degrees of open-set speech perception and even can converse freely on the telephone. And, unfortunately, there are a limited number of recipients who are unable to hear at all with their cochlear implants. One factor that could have a fundamental influence on outcomes for cochlear implant recipients is spiral ganglion cell survival. Since spiral ganglion cells are necessary for perception of cochlear stimulation, it seems intuitive that persons with very poor peripheral neural survival (or very poor neural function) would also have poorer speech perception than those with excellent neural survival. Studies aimed at quantifying spiral ganglion cell survival have found that even among individuals with similar hearing loss etiologies, spiral ganglion cell survival is widely variable and cannot be reliably predicted based on medical history. Electrophysiologic studies of spiral ganglion cell survival in cases of severe to profound hearing loss also have yielded mixed findings. Some investigators have shown significant correlations between spiral ganglion survival and measures such as amplitude growth function slope and maximum peak-to-peak amplitudes of various electrophysiologic measures (e.g., Smith and Simmons1), while others have failed to show evidence of such relationships (e.g., Stypulkowski et al.2). One electrophysiologic measure that has shown promise for predicting spiral ganglion cell survival is the electrically evoked compound action potential (ECAP). For a detailed description of the ECAP, see Abbas et al.3 and Brown et al.4 Hall5 and numerous others have demonstrated that this response correlates directly with neural survival, offering compelling evidence that the amplitude of the ECAP is directly proportional to the number of surviving spiral ganglion cells contributing to the response. Our research used the ECAP to examine possible differences in spiral ganglion cell survival between two groups of post-linguistically deafened adult cochlear implant recipients. To maximize the likelihood of finding a relationship between speech perception and the ECAP, we selected only subjects who fell at an extreme of speech perception performance (the top and bottom 10th percentiles), based on their CUNY sentence scores in quiet (high performers ≥98% correct; poor performers ≤40% correct). If we assume for a moment that the differences in speech perception ability in these two groups indeed are caused by differences in neural survival, we can make several predictions about the ECAP: We would expect the high performers to have larger maximum peakto-peak amplitudes, possibly shorter ECAP latencies, larger response dynamic ranges and lower response thresholds, more steeply sloping amplitude growth functions, and faster neural recovery speed. We evaluated all of these parameters in this investigation. Stated generally, the results of the current research showed no meaningful differences in the ECAP between two groups of adult cochlear implant recipients. Some might consider this a surprising finding, given the potential relationship between speech understanding and peripheral neural survival and the literature showing that various parameters of the ECAP are directly related to neural survival. Our results may provide evidence that there really is no difference in spiral ganglion cell survival between these two groups, in which case speech perception ability is fairly independent of number of surviving spiral ganglion cells. A more moderate interpretation of our findings is that perhaps a certain minimum number of spiral ganglion cells is required for speech perception (generally considered to be around 10,0006), and above that number, additional cells do not incrementally improve understanding ability. Speech understanding, as a highlevel cortical process, may be somewhat independent of peripheral neural survival, given a requisite degree of intactness.