Qualities of Single Electrode Stimulation as a Function of Rate and Place of Stimulation with a Cochlear Implant

Abstract

Objectives Although it has been previously shown that changes in temporal coding produce changes in pitch in all cochlear regions, research has suggested that temporal coding might be best encoded in relatively apical locations. We hypothesized that although temporal coding may provide useable information at any cochlear location, low rates of stimulation might provide better sound quality in apical regions that are more likely to encode temporal information in the normal ear. In the present study, sound qualities of single electrode pulse trains were scaled to provide insight into the combined effects of cochlear location and stimulation rate on sound quality. Design Ten long term users of MED-EL cochlear implants with 31 mm electrode arrays (Standard or FLEXSOFT) were asked to scale the sound quality of single electrode pulse trains in terms of how “Clean”, “Noisy”, “High”, and “Annoying” they sounded. Pulse trains were presented on most electrodes between 1 and 12 representing the entire range of the long electrode array at stimulation rates of 100, 150, 200, 400, or 1500 pulses per second. Results While high rates of stimulation are scaled as having a “Clean” sound quality across the entire array, only the most apical electrodes (typically 1 through 3) were considered “Clean” at low rates. Low rates on electrodes 6 through 12 were not rated as “Clean” while the low rate quality of electrodes 4 and 5 were typically in between. Scaling of “Noisy” responses provided an approximately inverse pattern as “Clean” responses. “High” responses show the trade-off between rate and place of stimulation on pitch. Because “High” responses did not correlate with “Clean” responses, subjects were not rating sound quality based on pitch. Conclusions If explicit temporal coding is to be provided in a cochlear implant, it is likely to sound better when provided apically. Additionally, the finding that low rates sound clean only at apical places of stimulation is consistent with previous findings that a change in rate of stimulation corresponds to an equivalent change in perceived pitch at apical locations. Collectively, the data strongly suggests that temporal coding with a cochlear implant is optimally provided by electrodes placed well into the second cochlear turn.

Neural Processing of Acoustic and Electric Interaural Time Differences in Normal-Hearing Gerbils

“…This proportion is similar to the 93% reported by Siveke et al (2006), who used a weighted mean-square-error measure (Kuwada et al, 1987). The CP is defined as the y-intercept of the regression line at 0 Hz and describes the mean interaural phase at which the characteristic delay (CD) occurs; that is, where the ITD functions for different frequencies display constant relative amplitude (Yin and Kuwada, 1983b;Kuwada et al, 1987). CP values at or near 0 cycles (between 0 and Ϯ0.18 cycles) indicate CDs at or close to the peaks of the ITD functions (peak-type neurons).…”

“…Peak-type neurons were most common (56/126, 44%), followed by intermediate-type neurons (36/129, 29%) and trough-type neurons (34/126, 27%). The dominance of peak-type neurons is consistent with previous studies of pure tone ITD processing in DNLL and IC (Kuwada et al, 2006;Siveke et al, 2006).…”

“…The basic features of neural ITD processing (shape distributions of ITD functions, ITD tuning metrics, and neural ITD discrimination thresholds; Table 1) were similar in DNLL and IC independent of the stimulus mode or the spectrotemporal properties of the acoustic stimuli. These results are consistent with previous studies on acoustic ITD processing in unanesthetized rabbits except for a sharpening of ITD tuning in the IC observed by Kuwada et al (2006). The similarity in ITD processing between the two structures suggests that projections to DNLL and IC from primary ITD-processing structures (MSO and LSO) (Brunso-Bechtold et al, 1981;Glendenning and Masterton, 1983) and projections from DNLL to IC are similarly engaged by the two stimulus modes.…”

“…When advanced ventrally, the electrode typically encountered a silent zone (width ϳ500 m) between the ventral part of the IC and the dorsal part of the DNLL, where no neurons could be isolated. The following entry into the DNLL was marked by an abrupt increase in spontaneous activity and neuronal discharge rates (Kuwada et al, 2006).…”

“…Acoustic studies have shown that ITD response properties of neurons in the dorsal nucleus of the lateral lemniscus (DNLL) and the IC are similar (Kuwada et al, 2006). Electric and acoustic stimulation might engage different binaural excitatory and inhibitory brain circuits that may affect ITD processing of electric stimuli in DNLL and IC differently.…”