Frequency importance functions in simulated bimodal cochlear-implant users with spectral holes

Abstract

Frequency importance functions (FIFs) for simulated bimodal hearing were derived using sentence perception scores measured in quiet and noise. Acoustic hearing was simulated using low-pass filtering. Electric hearing was simulated using a six-channel vocoder with three input frequency ranges, resulting in overlap, meet, and gap maps, relative to the acoustic cutoff frequency. Spectral holes present in the speech spectra were created within electric stimulation by setting amplitude(s) of channels to zero. FIFs were significantly different between frequency maps. In quiet, the three FIFs were similar with gradually increasing weights with channels 5 and 6 compared to the first three channels. However, the most and least weighted channels slightly varied depending on the maps. In noise, the patterns of the three FIFs were similar to those in quiet, with steeper increasing weights with channels 5 and 6 compared to the first four channels. Thus, channels 5 and 6 contributed to speech perception the most, while channels 1 and 2 contributed the least, regardless of frequency maps. Results suggest that the contribution of cochlear implant frequency bands for bimodal speech perception depends on the degree of frequency overlap between acoustic and electric stimulation and if noise is absent or present.

FIG. 1.

(Color online) Mean percent correct scores with standard errors in quiet (top panel) and at 6 dB SNR (bottom panel) as a function of listening conditions for each map. Each number on the x-axis indicates which frequency band(s) were off. Scores with acoustic alone are indicated by horizontal dotted lines. Baseline indicates the bimodal listening condition with no spectral holes present.

FIG. 2.

(Color online) Comparisons of the FIFs with standard errors across frequency maps in quiet and at 6 dB SNR. In the quiet condition, the top asterisks indicate a significant difference between the highest and lowest FIF values at each channel, while the bottom asterisks indicate a significant difference between the second highest and lowest FIF values at each channel. For example, at channel 1 in the quiet condition, *** indicates a significant difference between overlap and gap FIF values, while * indicates a significant difference between overlap and meet FIF values. In the noise condition, asterisks indicate a significant difference between the highest and lowest FIF values. *** indicates p < 0. 001, ** p < 0. 01, and * p < 0. 05.