Abstract
In cochlear implants, loudness has been shown to grow more slowly with increasing pulse phase duration (PPD) than with pulse amplitude (PA), possibly due to “leaky” charge integration. This leakiness has been recently quantified in terms of “charge integration efficiency,” defined as the log difference between the PPD dynamic range and PA dynamic range (both expressed in charge units), relative to a common threshold anchor. Such leakiness may differ across electrodes and/or test ears, and may reflect underlying neural health. In this study, we examined the across-site variation of charge integration in recipients of Cochlear© devices. PPD and PA dynamic ranges were measured relative to two threshold anchors with either a 25- or 50-microsecond PPD. Strength-duration functions, previously shown to relate to survival of spiral ganglion cells and peripheral processes, were compared to charge integration efficiency on selected electrodes. Results showed no significant or systematic relationship between the across-site variation in charge integration efficiency and electrode position or threshold levels. Charge integration efficiency was poorer with the 50-μs threshold anchor, suggesting that greater leakiness was associated with larger PPD dynamic ranges. Poorer and more variable charge integration efficiency across electrodes was associated with longer duration of any hearing loss, consistent with the idea that poor integration is related to neural degeneration. More variable integration efficiency was also associated with poorer speech recognition performance across test ears. The slopes of the strength-duration functions at maximum acceptable loudness were significantly correlated with charge integration efficiency. However, the strength-duration slopes were not predictive of duration of any hearing loss or speech recognition performance in our participants. As such, charge integration efficiency may be a better candidate to measure leakiness in neural populations across the electrode array, as well as the general health of the auditory nerve in human cochlear implant recipients.
Highlights
In cochlear implants, loudness of the electrical pulses can be increased by increasing the pulse amplitude (PA) or pulse phase duration (PPD)
Loudness has been shown to grow more slowly with increasing pulse phase duration (PPD) than with pulse amplitude (PA), possibly due to “leaky” charge integration. This leakiness has been recently quantified in terms of “charge integration efficiency,” defined as the log difference between the PPD dynamic range and PA dynamic range, relative to a common threshold anchor
Charge integration efficiency was poorer with the 50-μs threshold anchor, suggesting that greater leakiness was associated with larger PPD dynamic ranges
Summary
Loudness of the electrical pulses can be increased by increasing the pulse amplitude (PA) or pulse phase duration (PPD). We predicted that greater across-site variance in CIE and poorer across-site mean in CIE (i.e., degeneration across the cochlea) would be associated with longer hearing deprivation and poorer speech recognition performance Another method to quantify the integration property of the auditory nerve is the strength-duration function (Pfingst et al 1991; Moon et al 1993; Zeng et al 1998), where the PA is adjusted to maintain threshold for a range of PPDs. Assuming an equal tradeoff between PA and PPD, the slope of the strength-duration function would be − 6 dB/doubling of PPD, reflecting perfect charge integration over time. We expected that these strength-duration slopes and CIE would be correlated across electrodes and test ears
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