Reliability of categorical loudness scaling in the electrical domain

Abstract

Objective: In categorical loudness scaling (CLS), subjects rate the perceived loudness on a categorical scale with alternatives. ISO 16832 describes an internationally standardized CLS procedure for the acoustical domain. This study focuses on the reproducibility of CLS following the recommendations of ISO 16832 using electrical stimuli presented to cochlear implant (CI) users. Design: Repeated CLS measurements were done using single-electrode stimuli at four electrode positions. Loudness growth functions (LGFs) described loudness as a function of level (μA). LGF shapes were characterized with an exponential b parameter. The reproducibility of the b parameter and inter-session intra-subject differences in percentage dynamic range (DR) between ‘Very Soft’ and ‘Loud - Very Loud’ levels were analysed. Study sample: Ten CI users. Results: Inter-session differences did not significantly differ between loudness categories or electrode positions. Across loudness categories the standard deviation of inter-session differences equalled 7.2%DR. The reproducibility of LGF shapes was moderate (r = 0.63). The LGFs of 43% of the measured electrodes significantly deviated from linear (nonzero b parameter). Conclusions: The reproducibility was comparable to the reproducibility for acoustical stimulation in normal-hearing and hearing-impaired listeners. CLS data for electrical stimuli are preferably fitted with a model that is flexible in describing LGF shapes.

Key Words::

• Cochlear implant
• categorical loudness scaling
• loudness perception
• loudness growth
• reproducibility

Acknowledgements

We would like to thank the subjects for their time and effort. Also we would like to thank Cochlear^TM for their technical support. Furthermore we would like to thank Inge Brons for her comments on earlier versions of the article.

Declaration of interest: Financial support was received from Cochlear^TM.

Notes

1. It is not clear whether Robinson & Gatehouse used the SD of inter-session differences across subjects or the SD of inter- session differences per subject in the analysis.

2. Bimodal listeners use a hearing aid in one ear and a CI in their other ear, bilateral listeners use a CI in both ears.

3. For more information about the difference in loudness growth between single-electrode and multi-electrode pulse train stimuli we refer to the literature (e.g. McKay et al, 2003).

4. In the ISO document these numbers are referred to as Categorical Units. However, we use Loudness Units to prevent confusion with Current Units, the clinical unit for current used by Cochlear.

5. Notice that more iterative loops could have been used as an alternative for the first reason, but not for the second reason.

6. The HI listeners in the study of Al-Salim et al had at least one audiometric threshold > 15 dB HL, and the maximum test level was 105 dB SPL.

7. This SD is calculated for all inter-session differences in terms of %DR. The SD in terms of %DR based on the study of Al-Salim et al is calculated by converting the SD in absolute terms into %DR based on the estimated overall average DR. If in analogy with this procedure the SD of the inter-session differences found in this study in terms of μA is converted to %DR based on the overall average DR found in this study, the SD equals 7.8%DR.

8. It is not clear whether Robinson & Gatehouse used the SD of inter-session differences across subjects or the SD of inter- session differences per subject in the analysis.

9. The analyses were done for all levels in terms of CU, the clinical unit in which T-levels and C-levels are expressed for Cochlear devices.