Effects of stimulation rate, mode and level on modulation detection by cochlear implant users

Abstract

In cochlear implant (CI) patients, temporal processing is often poorest at low listening levels, making perception difficult for low-amplitude temporal cues that are important for consonant recognition and/or speech perception in noise. It remains unclear how speech processor parameters such as stimulation rate and stimulation mode may affect temporal processing, especially at low listening levels. The present study investigated the effects of these parameters on modulation detection by six CI users. Modulation detection thresholds (MDTs) were measured as functions of stimulation rate, mode, and level. Results show that for all stimulation rate and mode conditions, modulation sensitivity was poorest at quiet listening levels, consistent with results from previous studies. MDTs were better with the lower stimulation rate, especially for quiet-to-medium listening levels. Stimulation mode had no significant effect on MDTs. These results suggest that, although high stimulation rates may better encode temporal information and widen the electrode dynamic range, CI patients may not be able to access these enhanced temporal cues, especially at the lower portions of the dynamic range. Lower stimulation rates may provide better recognition of weak acoustic envelope information.

Figures

Fig. 1

Modulation detection thresholds (MDTs) for individual CI subjects. Each panel shows individual subject results. The x-axis shows the reference loudness level, in percent DR of the reference electrode. The y-axis shows the MDTs in log scale; note that the scale is optimized for each subject's range of MDTs. The filled circles represent MDTs with the 2000 pps carrier and the open circles represent MDTs with the 250 pps carrier; the error bars represent 1 standard deviation. The solid and dashed lines represent sigmoid fits to the MDT data for the 2000 and 250 pps carriers, respectively.

Fig. 2

Shift in MDTs between stimulation rates, for BP + 3 stimulation. For each subject, at each loudness-balanced reference level, the MDT with the 2000 pps carrier was subtracted from the MDT with the 250 pps carrier. The x-axis shows the reference loudness level, in percent dynamic range of the reference electrode. The y-axis shows the difference in MDTs between the carrier rates (in dB). The reference line at 0 dB represents no difference in MDTs between the carrier rates. The thick solid line shows the mean performance shift, across subjects. Individual subject data are represented by the different symbols. The filled symbols represent a statistically significant difference in MDTs between the carrier rates (paired t test: p < 0.05); the open symbols represent differences in MDTs that were not statistically significant between the carrier rates (p > 0.05).

Fig. 3

Shift in MDTs between stimulation modes, for 250 pps stimulation rate. For subjects S1 and S2, the MDT with the BP + 3 configuration was subtracted from the MDT with the monpolar configuration. For subjects S3 and S4, the MDT with the BP + 3 configuration was subtracted from the MDT with the BP + 13 configuration. The x-axis shows the reference loudness level, in percent dynamic range of the reference electrode. The y-axis shows the difference in MDTs between the electrode configurations (in dB). The reference line at 0 dB represents no difference in MDTs between the carrier rates. The thick solid line shows the mean performance shift, across subjects. Individual subject data are represented by the different symbols. The filled symbols represent a statistically significant difference in MDTs between the carrier rates (t test: p< 0.05); the open symbols represent differences in MDTs that were not statistically significant between the carrier rates (p > 0.05).

Fig. 4

Mean MDTs (across entire DR). The x-axis shows individual subjects. The y-axis shows the mean MDT in dB; for each subject and condition, MDTs were averaged across all reference levels. The bars represent different electrode configuration and carrier rate conditions.

Fig. 5

Mean MDTs (across the range of reference levels) as a function of dynamic range (measured between the minimum and maximum reference levels). Individual subject data are represented by the different symbols; the different stimulation mode/rate conditions are represented by the different fill shades. The solid line represents the linear regression for all data.