Loudness Context Effects in Normal-Hearing Listeners and Cochlear-Implant Users

Abstract

Context effects in loudness have been observed in normal auditory perception and may reflect a general gain control of the auditory system. However, little is known about such effects in cochlear-implant (CI) users. Discovering whether and how CI users experience loudness context effects should help us better understand the underlying mechanisms. In the present study, we examined the effects of a long-duration (1-s) intense precursor on the loudness relations between shorter-duration (200-ms) target and comparison stimuli. The precursor and target were separated by a silent gap of 50 ms, and the target and comparison were separated by a silent gap of 2 s. For normal-hearing listeners, the stimuli were narrowband noises; for CI users, all stimuli were delivered as pulse trains directly to the implant. Significant changes in loudness were observed in normal-hearing listeners, in line with earlier studies. The CI users also experienced some loudness changes but, in contrast to the results from normal-hearing listeners, the effect did not increase with increasing level difference between precursor and target. A "dual-process" hypothesis, used to explain earlier data from normal-hearing listeners, may provide an account of the present data by assuming that one of the two mechanisms, involving "induced loudness reduction," was absent or reduced in CI users.

Induced Loudness Reduction and Enhancement in Acoustic and Electric Hearing

“…One way to provide such a comparison is to convert the amount of change in the matching stimulus into a proportion of the overall dynamic range (Wang et al 2015). We calculated these normalized values by considering the dynamic range of the CI users to be the difference between MCL and THS (in dB) for each subject individually, and then converting any changes in level into a proportion of the dynamic range.…”

“…First, the THS and MCL levels were determined for each subject individually using 200-ms pulse trains on each of the test electrodes (E2 and E8), as described in Wang et al (2015). Second, the different pairs of starting points for each of the four repetitions of the adaptive tracking procedure were 55/70, 60/75, 65/ 80, and 70/85 % DR. Third, the initial step size in the adaptive procedure was 5 % DR, which was reduced to 3 % DR after two reversals and to 2 % DR after four reversals.…”

“…When the precursor was more intense than the target, the listeners often adjusted the level of the comparison tone to be higher than that of the target at equal loudness, leading to the conclusion that the precursor had enhanced the loudness of the target tone (Zwislocki and Sokolich 1974;Elmasian and Galambos 1975). These effects declined as the spectral distance between the precursor and the target increased and tended to disappear completely at large spectral distances (Marks 1994;Wang et al 2015).…”

“…If the MOC system is the sole source of ILR, then ILR should not be observed in CI users. A recent study by Wang et al (2015) investigated loudness context effects in CI users using the traditional three-stimulus technique, with all three stimuli presented to the same electrode. Wang et al (2015) found both similarities and differences between the results from CI users and those from normal-hearing listeners.…”

“…A recent study by Wang et al (2015) investigated loudness context effects in CI users using the traditional three-stimulus technique, with all three stimuli presented to the same electrode. Wang et al (2015) found both similarities and differences between the results from CI users and those from normal-hearing listeners. In particular, in both normalhearing and CI groups, a more intense precursor resulted in the target sound being judged louder than the comparison signal when they were presented at equal levels, and frequency selectivity was observed in this effect.…”