Effects of carrier pulse rate and stimulation site on modulation detection by subjects with cochlear implants

Abstract

Most modern cochlear-implant speech processors convey speech-envelope information using amplitude-modulated pulse trains. The use of higher-rate carrier pulse trains allows more envelope detail in the signal. However, neural response properties could limit the efficacy of high-rate carriers. This study examined effects of carrier rate and stimulation site, on psychophysical modulation detection thresholds (MDTs). Both of these variables could affect the neural representation of the carrier and thus affect perception of the modulation. Twelve human subjects with cochlear implants were tested. Phase duration of symmetric biphasic pulses was modulated sinusoidally at 40 Hz. MDTs were determined for monopolar stimulation at two carrier rates [250 and 4000 pulses/s (pps)], three stimulation sites (basal, middle, and apical), and five stimulus levels (10%, 30%, 50%, 70%, and 90% of the dynamic range). MDTs were lower for 250 pps carriers than for 4000 pps carriers in 71% of the 180 cases studied. Effects of carrier rate were greatest at the apical stimulation site and effects of stimulation site on MDTs depended on carrier rate. The data suggest a distinct disadvantage to using carrier pulse rates as high as 4000 pps. Stimulation site should be considered in evaluating modulation detection ability.

Adjustments of the amplitude mapping function: Sensitivity of cochlear implant users and effects on subjective preference and speech recognition

“…Gallégo & Micheyl, 1998;Galvin & Fu, 2009;Kreft et al, 2004;Nelson et al, 1996). Similarly, detection thresholds for amplitude modulation of pulse trains tend to improve with the level across DR (Galvin & Fu, 2005;Pfingst et al, 2007Pfingst et al, , 2008. In the present study, the average non-zero stimulation level was higher for the sentences in noise (approximately 75%DR) than for the sentences in quiet (approximately 58%DR) when the default AMF programmed in a CP810 sound processor was used for processing.…”

Psychophysical assessment of stimulation sites in auditory prosthesis electrode arrays

“…Some of these data were presented at the Fourth Joint Meeting of the Acoustical Society of America and the Acoustical Society of Japan (Pfingst et al, 2006). Procedures were similar to those for a previously published experiment (Pfingst et al, 2007b). Effects of stimulation level on modulation detection thresholds (MDTs) (from Pfingst et al, 2007b).…”

“…Procedures were similar to those for a previously published experiment (Pfingst et al, 2007b). Effects of stimulation level on modulation detection thresholds (MDTs) (from Pfingst et al, 2007b). Each panel shows MDT-versus-level functions for a single subject for three stimulation sites (basal, middle and apical) at two carrier rates (250 pps and 4000 pps).…”

“…For example, modulation detection is generally better for low pulse rate carriers than for very high pulse rate carriers (Galvin and Fu, 2005;Pfingst et al, 2007b). Effects of carrier rate are somewhat variable across subjects and across stimulation sites, but further research is needed to determine if there are optimal rates for individual stimulation sites.…”

“…All temporal processing skills that have been tested also benefit. These include pulse rate discrimination (Pfingst et al, 1994), frequency discrimination (Pfingst and Rai, 1990), modulation frequency discrimination (Morris and Pfingst, 2000), modulation detection (Shannon 1992(Shannon , 1993Galvin and Fu, 2005;Pfingst et al, 2007b), gap detection (Shannon, 1989;Chatterjee et al, 1998;Hanekom and Shannon, 1998), and sensitivity to binaural timing (van Hoesel, 2007). Examples of MDT versus level functions are shown in Figure 3.…”