Combining Place and Rate of Stimulation Improves Frequency Discrimination in Cochlear Implant Users

Abstract

In the auditory system, frequency is represented as tonotopic and temporal response properties of the auditory nerve. While these response properties are inextricably linked in normal hearing, cochlear implants can separately excite tonotopic location and temporal synchrony using different electrodes and stimulation rates, respectively. This separation allows for the investigation of the contributions of tonotopic and temporal cues for frequency discrimination. The present study examines frequency discrimination in adult cochlear implant users as conveyed by electrode position and stimulation rate, separately and combined. The working hypothesis is that frequency discrimination is better provided by place and rate cues combined compared to either cue alone. This hypothesis was tested in two experiments. In the first experiment, frequency discrimination needed for melodic contour identification was measured for frequencies near 100, 200, and 400 Hz using frequency allocation modeled after clinical processors. In the second experiment, frequency discrimination for pitch ranking was measured for frequencies between 100 and 1600 Hz using an experimental frequency allocation designed to provide better access to place cues. The results of both experiments indicate that frequency discrimination is better with place and rate cues combined than with either cue alone. These results clarify how signal processing for cochlear implants could better encode frequency into place and rate of electrical stimulation. Further, the results provide insight into the contributions of place and rate cues for pitch.

Keywords: Auditory neuroscience; Cochlear implant; Pitch; Psychophysics.

Fig. 1.

Example stimuli for experimental conditions of place, rate, and combined place-rate. The melodic contour is “rising”, but the frequency allocation table used for the melodic contour identification task, modeled after the frequency allocation for typical Cochlear Corporation devices, has a cutoff of 200 Hz limiting the place information at the lower frequencies and making it look more like the “flat-rising” contour for place-of-excitation cues. The first panel shows the condition where place of stimulation is varied, and rate is held constant at the center-note frequency. The second panel shows the condition where rate of stimulation is varied, and place of stimulation is held constant at the center-note frequency. The third panel shows the combined place-rate condition with both place and rate covaried for all notes.

Fig. 2.

Internote frequency spacing thresholds for melodic contour identification as a function of center-note frequency. Symbols indicate thresholds averaged across subjects with shaded error bars showing standard errors of the means.

Fig. 3.

Individual internote frequency spacing thresholds for melodic contour as a function of center-note frequency. Symbols indicate internote frequency spacing thresholds averaged across repetitions.

Fig. 4.

Example stimuli for experimental conditions of place, rate, and combined place-rate. The first panel shows the condition where place of stimulation is varied from 100 to 200 Hz and rate is held constant at the base frequency of 100 Hz for both the standard and target stimuli. The second panel shows the condition where rate of stimulation is varied from 100 to 200 Hz and place of stimulation is held constant at the base frequency of 100 Hz for both the standard and target stimuli. The third panel shows the combined place-rate condition with both place and rate covaried from 100 Hz for the standard stimulus to 200 Hz for the target stimuli.

Fig. 5.

Frequency discrimination with multi-electrode stimuli averaged across participants for the factors of stimulation cue and frequency with shaded error bars showing standard errors of the means.

Fig. 6.

Individual frequency discrimination as a function of frequency. Symbols show discrimination thresholds averaged across repetitions for each cue type.

Fig. 7.

Single and multi-electrode rate discrimination as a function of frequency. Symbols indicate discrimination thresholds averaged across subjects with shaded error bars indicating standard errors of the means.

Fig. 8.

Correlations between forward-masked threshold slopes normalized by the subtraction of the average with frequency discrimination thresholds for subjects 1 through 5 for the stimulation cues of place, rate, place-rate, and single-electrode rate.

Fig. 9.

Correlations between rate discrimination, as measured by melodic contour and simple frequency discrimination, and the metrics of duration of deafness before implantation and cochlear implant experience.