Fan-Gang Zeng
Robert V. Shannon
House Ear Inst., 2100 W. 3rd St., Los Angeles, CA 90057
In cochlear implants loudness is a power function of electrical amplitude at low frequencies (<300 Hz) and an exponential function for higher frequencies [Zeng and Shannon, Science 264, 564--566 (1994)]. In addition, the just-noticeable-difference (jnd) in intensity is inversely proportional to the slope of the loudness function. Implant speech processors generally use low-frequency modulation of a high-frequency carrier. Because the loudness functions are different for the modulator and carrier frequencies, this raises the question: Do the loudness and jnd of a modulated stimulus follow the modulator or the carrier? Intensity discrimination was measured for a 100-Hz sinusoidally modulated 1000-Hz sinusoid or pulse train in implant listeners. The jnd was measured either as an increment in the carrier level for a fixed modulation depth or as an increment in the modulation depth for a fixed carrier level. The results showed that the jnd function of the modulated stimuli is similar to the jnd function of the high-frequency carrier. At high sensation levels, implant listeners can discriminate extremely small changes in modulation depth (1%--2%). This implies that modulated stimuli can produce more jnd steps across the dynamic range than either the modulator or the carrier alone.