Pulse-rate discrimination by cochlear-implant and normal-hearing listeners with and without binaural cues

Abstract

Experiment 1 measured rate discrimination of electric pulse trains by bilateral cochlear implant (CI) users, for standard rates of 100, 200, and 300 pps. In the diotic condition the pulses were presented simultaneously to the two ears. Consistent with previous results with unilateral stimulation, performance deteriorated at higher standard rates. In the signal interval of each trial in the dichotic condition, the standard rate was presented to the left ear and the (higher) signal rate was presented to the right ear; the non-signal intervals were the same as in the diotic condition. Performance in the dichotic condition was better for some listeners than in the diotic condition for standard rates of 100 and 200 pps, but not at 300 pps. It is concluded that the deterioration in rate discrimination observed for CI users at high rates cannot be alleviated by the introduction of a binaural cue, and is unlikely to be limited solely by central pitch processes. Experiment 2 performed an analogous experiment in which 300-pps acoustic pulse trains were bandpass filtered (3900-5400 Hz) and presented in a noise background to normal-hearing listeners. Unlike the results of experiment 1, performance was superior in the dichotic than in the diotic condition.

Figure 1

Part a) shows a schematic of the pulses in each ear for a signal in the dichotic condition with a baseline rate of 100 pps and Δr=10 pps. For clarity, only the first 200 ms of the stimulus is shown. It can be seen that the asynchrony between the nth pulse in each ear increases up to 100 ms (1/Δr sec), at which point the nth pulse in one ear is synchronous with the n+1th in the other. The wide gray line in part b) illustrates this fact: for a Δr of x pps, the ITD between the pulses in the two ears rotates through a whole period once every 1/x sec, with a sawtooth function. The dotted black line shows the asynchrony between each pulse in one ear and the nearest pulse in the other

Figure 2

Psychometric functions describing percent correct as a function of Δr for the four CI users who took part in experiment 1. Each row shows the data for one listener, and each column shows the data for one basleine pulse rate. Performance in the diotic and dichotic conditions is shown by circles and triangles, respectively. Error bars show +/− one standard error, estimated from the normal approximation to the binomial distribution. The solid symbols in the top-right panel show performance at a rate of 400 pps for listener CI 1.

Figure 3

Psychometric functions describing percent correct as a function of Δr for the five NH listeners who took part in experiment 2. Mean data are shown in the bottom right panel. Performance in the diotic and dichotic conditions is shown by circles and triangles, respectively. Error bars show +/− one standard error, which, for the individual listeners' data, were estimated from the normal approximation to the binomial distribution. Dashed lines in bold type show the mean data with listener NH 3 excluded.