Amplitude compression in cochlear implants artificially restricts the perception of temporal asymmetry

This paper presents a study in which five cochlear implantees were asked to discriminate the timbre of stimuli with temporally asymmetric envelopes. S timuli were damped and ramped sinusoids presented acoustically. They were t ransformed by the speech processor of the implant and were presented throug h one electrode. All cochlear implantees could discriminate the damped and ramped sinusoids when the half-life was 4 ms, the carrier frequency was 400 Hz, and the period of the envelope was 50 ms. In a second experiment, timb re discrimination performance was measured as a function of half-life for t wo cochlear implantees. Both showed that timbre discrimination was possible over the range 1-24 ms. In normal-hearing listeners, the range is 1-64 ms and in cochlear implantees, stimulated directly without the speech processo r, the range is 1-300 ms. At long half-lives, the decrease in discriminatio n performance observed with the speech processor appears to be due to the a mplitude compression applied by the device. The present results suggest tha t it may be important to ensure that cochlear implants do not restrict temp oral asymmetry unduly when applying compression to control level.