Musical Sound Quality as a Function of the Number of Channels in Modern Cochlear Implant Recipients

Thursday, February 6, 2020

Our first journal club was a great success! We had students and postdocs in attendance from the departments of Physics & Astronomy, Applied Physics, Bioengineering, and Psychology.  The interdisciplinary mix of trainees sparked some good discussions.

Melia Bonomo presented on the following paper:

Katelyn Berg, Jack Noble, Benoit Dawant, Robert Dwyer, Robert Labadie, Virginia Richards, and René Gifford (2019) “Musical Sound Quality as a Function of the Number of Channels in Modern Cochlear Implant Recipients,” Frontiers in Neuroscience 13:999.

https://www.frontiersin.org/articles/10.3389/fnins.2019.00999/full

Cochlear implant users consistently report musical sound quality impairments following implantation.  Therefore, in this study, the objectives were to determine:

• Number of channels needed for high level of musical sound quality (a) overall, and (b) for different genres: Alternative, Hip Hop/Rap, Jazz, Pop, R&B, Rock
• Impact of device and patient factors on musical sound quality
• Relationship between musical sound quality, speech recognition, and speech sound quality 

The study contained 21 subjects who had been using cochlear implants for at least 6 months (cohort average usage was about 4 years).  There were six electrode conditions — each subject’s clinical mapping, and then several in which certain electrodes were deactivated such that there were only 4, 8, 10, 12, or 16 active electrode channels.  For each of these conditions, subjects listened to 15 song clips (30-seconds each) from 30 possible songs in the six different genres.

Musical quality was measured subjectively on a 1=very poor to 10=very good scale, for which subjects were asked to consider clarity, richness, and pleasantness and to not consider their familiarity or preference.

Overall, having at least 10 electrodes led to higher musical sound quality. For almost Alternative, Jazz, Pop, R&B, and Rock, there was a significant increase from 4 electrodes to 12 electrodes. A lack of significant increase during Hip Hop/Rap may be due to the musical complexity.

There was no significant impact from implant experience, the device manufacturer, electrode type, or the surgeon who performed the implantation. Patient’s musical sophistication was measured by the Ollen Index (which accounted for music knowledge and ability to play an instrument, sing, understand and create music, etc.) showed a positive correlation with music sound quality. However, when two musicians in the cohort were removed from the data, the correlation disappeared.

There was a positive correlation between speech recognition in both conditions and musical sound quality as well as between speech sound quality and music sound quality.

One of the limitations of the study was that it can be difficult to disentangle subjective musical sound quality from song preference.  One way of alleviating this concern would be to break down the three sound qualities (clarity, richness, pleasantness) and ask about each specifically, rather than one combined question, and additionally ask about familiarity and preference to control for these factors.

A major limitation of the study was that these subjective music quality ratings could have been affected by the unfamiliar electrode manipulation and frequency bandwidth that cells are being stimulated for.  One way of addressing this would be to look at speech quality as a function of the number of electrodes and used this to control for effect of lack of experience with the different electrode mappings.  A second way of addressing this could be to allow participant to become accustomed to hearing with the new electrode mapping (e.g., listening to different speech until speech quality and recognition become comparable to their norm), and then test for music quality.

To summarize: Musical sound quality significantly increased from 4 to 10 channels, and for all genres except Hip Hop/Rap. Higher musical sound quality was not correlated with musical experience (after controlling for two musicians in cohort), cochlear implant experience, or device specifications. Musical sound quality was positively correlated with speech recognition and sound quality.

Music perception is rarely measured in the clinic, but it’s an important factor to consider when developing electrode mapping to preserve music appreciation!