Comparing spatial tuning curves, spectral ripple resolution, and speech perception in cochlear implant users
Spatial tuning curve for subject D08, measured in a previous study (Nelson et al., 2008). Depicted by the black squares in this figure is the current level needed to just mask a low-level probe presented to electrode 8, as a function of masker electrode number. Stimulus amplitude (µA) is shown on the ordinate, with research electrode number (rEL) displayed on the abscissa. (The rEL numbering system normalizes the different numbering systems used by different implant devices; number 1 is assigned to the most apical electrode, with consecutive numbering proceeding to the basal end of the array.) Error bars indicate standard deviations. Gray squares represent current levels at maximum acceptable loudness (MAL) for each electrode; gray diamonds indicate current levels at threshold (THS) for each electrode. The open circle at the tip of the curve indicates the probe electrode, with probe level shown by the symbol’s vertical position and sensation level indicated by the height of the vertical line beneath it. Tuning curve bandwidth (BW) is defined as the width of the STC at 1 dB above the tip of the STC (Q1 dB Level).
Ripple discrimination thresholds as a function of transformed STC bandwidth (from Nelson et al., 2008). (a) Broadband ripple discrimination thresholds, in ripples per octave (rpo) with a linear regression fit to all 15 data points; (b) the regression line for the least squares fit to data from 13 subjects, excluding subjects C05 and D10.
Plots of windowed (left) and non-windowed (right) stimulus spectra. Stimuli are broadband (350–5600 Hz) noise with sinusoidal spectral ripples; spectral modulation frequency is 1 ripple per octave. The windowed stimulus includes Hanning (raised-cosine) ramps applied to the spectral edges. The non-windowed stimulus has steep spectral edges.
Ripple discrimination thresholds for shallow-sloped (windowed) stimuli, with half-octave Hanning ramps on each side, as a function of thresholds for steep-sloped (non-windowed) stimuli. The diagonal line represents perfect correspondence between the two types of stimuli (slope = 1).
Octave-band ripple discrimination (from Experiment 2) as a function of transformed STC bandwidth. Data from all subjects are included in the solid regression line; subjects C05 and D10 are identified with different symbol shapes. Data from 13 subjects, excluding C05 and D10, are included in the dotted regression line.
Ripple discrimination thresholds as a function of low-frequency cutoff of octave-band rippled noise stimuli, for 15 individual subjects, separated by device type. (a) Performance of Clarion I subjects; (b) Clarion II subjects; and (c) Nucleus 22 subjects.
(a) Sentence recognition in quiet (rau scores) as a function of broadband ripple discrimination threshold. (b) Vowel recognition in quiet (rau scores) as a function of broadband ripple discrimination threshold. (c) SNR for 50% correct sentence recognition, interpolated/extrapolated from performance-intensity (P-I) functions, as a function of broadband ripple discrimination threshold. Data from 12 subjects are included. (d) SNR for 50% correct vowel recognition, which was interpolated/extrapolated from P-I functions for 14 subjects, as a function of broadband ripple discrimination threshold. Regression lines are all logarithmic fits, with the exception of (c), which shows a linear fit.
(a) Sentence recognition in quiet (rau scores) as a function of 1/BW. (b) Vowel recognition in quiet (rau scores) as a function of 1/BW. Regression lines are log fits. (c) SNR for 50% correct sentence recognition, interpolated/extrapolated from P-I functions, as a function of 1/BW. The regression line is a linear fit. Data from 12 subjects are included. (d) SNR for 50% correct vowel recognition, which was interpolated/extrapolated from P-I functions for 14 subjects, as a function of 1/BW. The regression line is a linear fit.
Output patterns based on characteristics of one C-I subject and one C-II subject. Electrode number is shown on the abscissa, with current output (in arbitrary units) displayed on the ordinate. The top row of panels shows four stimulation patterns for an 8-channel CIS processor, and the bottom row simulates a 16-channel processor. From left to right, the plots illustrate the output for rippled noise stimuli of 0.5, 1, 2, and 4 rpo.
Summary of CI subject characteristics. The subject identifiers C, D, and N denote Clarion I, Clarion II, and Nucleus users, respectively.
Individual STC data (from Nelson et al., 2008).
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