The effect of presentation level and stimulation rate on speech perception and modulation detection for cochlear implant users

Abstract

In order to improve speech understanding for cochlear implant users, it is important to maximize the transmission of temporal information. The combined effects of stimulation rate and presentation level on temporal information transfer and speech understanding remain unclear. The present study systematically varied presentation level (60, 50, and 40 dBA) and stimulation rate [500 and 2400 pulses per second per electrode (pps)] in order to observe how the effect of rate on speech understanding changes for different presentation levels. Speech recognition in quiet and noise, and acoustic amplitude modulation detection thresholds (AMDTs) were measured with acoustic stimuli presented to speech processors via direct audio input (DAI). With the 500 pps processor, results showed significantly better performance for consonant-vowel nucleus-consonant words in quiet, and a reduced effect of noise on sentence recognition. However, no rate or level effect was found for AMDTs, perhaps partly because of amplitude compression in the sound processor. AMDTs were found to be strongly correlated with the effect of noise on sentence perception at low levels. These results indicate that AMDTs, at least when measured with the CP910 Freedom speech processor via DAI, explain between-subject variance of speech understanding, but do not explain within-subject variance for different rates and levels.

FIG. 1.

Mean CNC phoneme scores compared to stimulation rate and presentation level for nine participants. The error bars represent ±1 standard error of the mean.

FIG. 2.

Mean Signal to noise ratio (SNR) required to achieve 70% of the sentences in quiet score, versus stimulation rate and presentation level for nine participants. The error bars represent ±1 standard error of the mean.

FIG. 3.

Mean acoustic modulation detection thresholds, versus stimulation rate and presentation level for nine participants. The error bars represent ±1 standard error of the mean.

FIG. 4.

Pearson correlation analyses between CNC phoneme scores and AMDTs for nine participants, at different stimulation rates and presentation levels.

FIG. 5.

Pearson correlation analyses between SNR_70% and AMDTs for nine participants, at different stimulation rates and presentation levels.

FIG. 6.

Speech processor outputs at channel 19 (625 Hz center frequency, upper panel) and channel 1 (7063 Hz center frequency, lower panel), for a 500 ms test signal of 10 Hz modulated noise, lowpass filtered at 1500 Hz, plus a constant 7063 Hz sine tone. The modulation depth of the noise was −6 dB re 100% modulation.

FIG. 7.

Electrical output level at electrode 19 (625 Hz center frequency) for different acoustic input levels of a 625 Hz sine tone.