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. 2010 Jan;127(1):415-26.
doi: 10.1121/1.3257591.

A relation between electrode discrimination and amplitude modulation detection by cochlear implant listeners

Monita Chatterjee  1 , Jian Yu
Affiliations

A relation between electrode discrimination and amplitude modulation detection by cochlear implant listeners

Monita Chatterjee et al. J Acoust Soc Am. 2010 Jan.

Abstract

The objective of this study was to examine the relation between measures of spectral and temporal resolutions in cochlear implant listeners at a particular electrode location. The hypothesis was that a common underlying factor, such as the health of local groups of neurons, might partially determine patients' sensitivity to both spectral and temporal cues at specific tonotopic locations. Participants were adult cochlear implant listeners. A significant correlation was found between electrode discrimination measured at soft levels (20% and 30% of the dynamic range) and modulation sensitivity at those levels, for stimulation in bipolar mode and a 100 Hz modulation rate. Correlations between the two measures were weaker under monopolar stimulation, or when the modulation rate was 10 Hz. At a higher stimulation level (40% of the dynamic range), no significant correlations between these measures were observed. It is hypothesized that the more restricted excitation pattern at lower levels and/or with a narrower stimulation mode allows the measurement of locally driven sensitivity to spectral and temporal cues, particularly under more challenging listening conditions. Thus, psychophysical measures obtained under conditions that evoke a narrower excitation pattern may serve as a useful indicator of the functional health of local neural populations.

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Figures

Figure 1
Figure 1
Threshold modulation index (in %) plotted as a function of stimulus level. Upper and lower panels show results obtained in bipolar and monopolar modes, respectively. Left- and right-hand panels show results obtained using the 100 Hz and the 10 Hz modulation rates, respectively. Within each panel, different symbols refer to different subjects. The thick solid line indicates the mean calculated across the seven subjects, in each case. Error bars show ±1 s.d. Note that the subject pools for bipolar and monopolar modes are overlapping, but not identical.
Figure 2
Figure 2
Mean modulation detection thresholds plotted as a function of stimulus level, for bipolar and monopolar stimulation modes (open and filled symbols, respectively), and 100 and 10 Hz modulation rates (circles and squares respectively). Results are shown for the eight subjects who were tested in both stimulation modes.
Figure 3
Figure 3
Percent correct in the electrode discrimination task, obtained using the bipolar stimulation mode. Each panel shows results obtained with a different subject. Within each panel, the parameter is stimulus level (in % DR). The abscissa shows the location of the comparison electrode (the reference was always at electrode 10).
Figure 4
Figure 4
Percent correct scores in the electrode discrimination task obtained using the monopolar stimulation mode. Otherwise, identical to Fig. 2.
Figure 5
Figure 5
Performance in the electrode discrimination task as a function of (absolute) distance from the reference electrode (in numbers of electrodes). The ordinate shows the average d′ taken across results obtained on equidistant electrodes on the basal and apical sides of the reference. Left-hand panels show results obtained using bipolar stimulation, and right-hand panels show results obtained using monopolar stimulation. Right-hand pairs of panels show results obtained in subjects S1, S2, S3, and S7; left-hand pairs show results obtained in subjects S8, S9, S10, and S11. Within each panel, the parameter is stimulus level.
Figure 6
Figure 6
EDI1 (left-hand panel) and EDI2 (right-hand panel) averaged across all eight subjects who provided data in both stimulation modes, plotted against the stimulus level. Dark and light bars represent results obtained in bipolar and monopolar modes, respectively. Error bars show ±1 s.d. Asterisks indicate significant differences between means (paired t-test).
Figure 7
Figure 7
The coefficient of variation for electrode discrimination measures (upper panel) and modulation detection thresholds (lower panel) plotted as a function of carrier level, for 20%, 30% and 40% DR levels.
Figure 8
Figure 8
Scatterplots of EDI2 obtained at 20% and 30% DRs (A and B, respectively) plotted against the MDT obtained at each level. Panel C shows the EDImean plotted against the MDTmean.
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References

    1. Bierer, J. A., and Middlebrooks, J. C. (2002). “Auditory cortical images of cochlear-implant stimuli: Dependence on electrode configuration,” J. Neurophysiol. 87, 478–492. - PubMed
    1. Cazals, Y., Pelizzone, M., Saudan, O., and Boex, C. (1994). “Low-pass filtering in amplitude modulation detection associated with vowel and consonant identification in subjects with cochlear implants,” J. Acoust. Soc. Am. 96, 2048–2054. 10.1121/1.410146 - DOI - PubMed
    1. Chatterjee, M., and Robert, M. E. (2001). “Noise enhances modulation sensitivity in cochlear implant listeners: Stochastic resonance in a prosthetic sensory system?,” J. Assoc. Res. Otolaryngol. 2, 159–171. 10.1007/s101620010079 - DOI - PMC - PubMed
    1. Chatterjee, M., and Oba, S. I. (2005). “Noise improves modulation detection by cochlear implant listeners at moderate carrier levels,” J. Acoust. Soc. Am. 118, 993–1002. 10.1121/1.1929258 - DOI - PubMed
    1. Chatterjee, M., Shannon, R. V., Galvin, J. J., and Fu, Q.-J. (2001). “Spread of excitation and its effect on auditory perception with cochlear implants,” in Physiological and Psychophysical Bases of Auditory Function, Proceedings of the 12th International Symposium on Hearing, edited by Houtsma A. J. M., Kohlrausch A., Prijs V. F., and Schoonhoven R. (Shaker, Maastricht: ).

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