Abstract
Previous psychophysical and modeling studies suggest that cathodic stimulation by a cochlear implant (CI) may preferentially activate the peripheral processes of the auditory nerve, whereas anodic stimulation may preferentially activate the central axons. Because neural degeneration typically starts with loss of the peripheral processes, lower thresholds for cathodic than for anodic stimulation may indicate good local neural survival. We measured thresholds for 99-pulse-per-second trains of triphasic (TP) pulses where the central high-amplitude phase was either anodic (TP-A) or cathodic (TP-C). Thresholds were obtained in monopolar mode from four or five electrodes and a total of eight ears from subjects implanted with the Advanced Bionics CI. When between-subject differences were removed, there was a modest but significant correlation between the polarity effect (TP-C threshold minus TP-A threshold) and the average of TP-C and TP-A thresholds, consistent with the hypothesis that a large polarity effect corresponds to good neural survival. When data were averaged across electrodes for each subject, relatively low thresholds for TP-C correlated with a high “upper limit” (the pulse rate up to which pitch continues to increase) from a previous study (Cosentino et al. J Assoc Otolaryngol 17:371–382). Overall, the results provide modest indirect support for the hypothesis that the polarity effect provides an estimate of local neural survival.
Highlights
Many cochlear implant (CI) listeners understand speech well, at least in quiet
It can be seen that the polarity effect (TTP-CTTP-A) varies across subjects and electrodes
Thresholds were lower for TP-C than for the central high-amplitude phase was either anodic (TP-A) pulses for 27 % of the electrode-subject combinations tested here
Summary
Many cochlear implant (CI) listeners understand speech well, at least in quiet. There remains substantial across-listener variability, with some struggling even in favorable listening conditions (Holden et al 2013). There has been considerable interest in identifying the reasons for poor speech perception, and in identifying the relationship between performance and the global and local pattern of neural survival in each individual patient. A potentially useful approach comes from singleelectrode psychophysical measures, which have revealed substantial across-listener and across-electrode variability in a number of tasks. These include signal detection in quiet and supra-threshold measures.
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More From: Journal of the Association for Research in Otolaryngology
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