Loudness of simple and complex stimuli in electric hearing

In a earlier study we showed that loudness function depends on stimulus frequency for simple sinusoidal and pulsatile stimuli in cochlear implants. Loudness is an exponential function of stimulus amplitude for high frequencies (> 300 Hz) and a power function for low frequencies (< 300 Hz). Two experiments were conducted to extend our previous work in eight Ineraid cochlear implant subjects. First, loudness functions were measured by means of a magnitude estimation technique for simple sinusoidal stimuli. Stimuli were 100- and 1,000-Hz sinusoids. Stimuli had a duration of 300 milliseconds and were presented at amplitudes representing 10%, 30%, 50%, 70%, and 90% of the dynamic range. Loudness estimates were best fit by an exponential function for the 1,000-Hz sinusoid and by a power function for the 100-Hz sinusoid. The estimation result is consistent with previous results that were obtained with a loudness balance technique. In a second experiment, loudness functions were studied for modulated stimuli, in which a 1,000-Hz sinusoid was modulated by a 100-Hz sinusoid at either 50% or 100% modulation depth. A linear loudness-balance function was obtained between the modulated stimuli and a 1,000-Hz sinusoidal standard, indicating that the modulated stimuli have the same exponential loudness function as the 1,000-Hz carrier despite the 100-Hz modulator. This result has important implications for speech processor design, because many devices use low-frequency speech envelopes to modulate a high-frequency carrier. This result also provides a psychophysical basis for the logarithmic amplitude transformation in commonly used speech processors to compress the wide acoustic dynamic range into a narrow electric dynamic range while preserving normal loudness function.