Across- and Within-Channel Envelope Interactions in Cochlear Implant Listeners

Abstract

The effects of modulated maskers on detection thresholds of a 50-Hz sinusoidal amplitude modulation (SAM) in a signal carrier were measured in nine cochlear implant (CI) listeners as a function of masker envelope type and for different masker-signal electrode separations. Both signal and masker were 200-ms-long pulse trains, presented concurrently in an interleaved stimulation mode. Masker envelopes were SAM at 20, 50, (0-and p-phase re: the signal modulator), and 125 Hz, as well as noise amplitude modulated (NAM), all with a fixed 20% modulation depth. Comparisons were made against steady-state maskers that had an amplitude equal to the mean amplitude of the modulated maskers or to their peak amplitude (SS peak ). Modulation thresholds were larger in the presence of the dynamic maskers versus the SS peak maskers; however, there was significant intersubject variability in the pattern of results. Effects of relative phase between masker and signal were not consistent across subjects. Envelope masking (the dB difference in modulation detection thresholds between modulated and SS peak maskers) was generally larger for the lower-modulation-frequency maskers than the 125-Hz masker. The spatial distribution of masked modulation detection thresholds was found to be considerably different from spatial forwardmasking patterns obtained in the same subjects. Finally, modulation thresholds measured for a very wide separation between the masker and signal showed significant envelope masking. These results suggest that, as has been shown in acoustic stimulation, central, across-channel temporal processing mechanisms also occur in electrical stimulation.

Effects of Stimulation Rate, Mode and Level on Modulation Detection by Cochlear Implant Users

“…The effect of stimulation mode on MDTs was evaluated in the present experiment to test whether representation of the envelope would be enhanced by a larger spatial spread of excitation or by the change/shift in DR. Chatterjee (2003) showed that channel interaction increased when synchronized envelopes were presented to tonotopically remote electrodes. Modulation detection interference also increases when modulated maskers are presented to tonotopically remote electrode locations (Chatterjee and Oba 2005). In some way, these data imply that amplitude envelope cues might be more salient when presented to a wide region of the cochlea (e.g., BP + 13 and monopolar stimulation modes used in the present study) rather than a restricted region (e.g., BP + 3 in the present study).…”

Sensitivity of Bimodal Listeners to Interaural Time Differences with Modulated Single- and Multiple-Channel Stimuli

“…When more than 1 channel is stimulated simultaneously, one could expect negative effects of channel interaction due to spread of excitation and/or -if the channels are perceptually distinguishable -effects similar to mod- ulation detection interference, which is also found in CI listeners [Chatterjee and Oba, 2004;Richardson et al, 1998]. The results of the current study do not seem to reflect this finding.…”

Detection and rate discrimination of amplitude modulation in electrical hearing

“…Thus, when pulse phase duration is modulated, the same AM depth would result in even smaller loudness increases than those observed in the present study and by McKay and Henshall (2010). Chatterjee (2003) and Chatterjee and Oba (2004) reported on modulation detection interference (MDI) effects in electrical stimulation. The interference observed in those studies caused AM depths to increase by large amounts in order to reach AM detection threshold in many CI listeners.…”

“…If levelroving had been used in those studies, the MDI effects would have likely been magnified. Thus, the MDI reported by Chatterjee (2003) and Chatterjee and Oba (2004) was possibly underestimated relative to the true size of the effect.…”