There was an interesting article in the January 2017 issue of Hearing Review in which Doug Beck interviews Anu Sharma about her work on auditory deprivation. Anu has done extraordinary work on this topic. We have know for a long time, from the work of Anu Sharma and others that significant hearing loss causes changes in brain activity, the auditory cortex and other brain structures.
Mild hearing loss
Anu is now showing that even a mild hearing loss can have significant brain changes and cortical reorganization. She is demonstrating that the brain changes and re-organizes based on mild degradation of the stimulus or a mild lack of audibility.
If the brain is not receiving the full complement of sound, compensatory mechanisms engage to overcome the hearing loss. This might be more reliance on vision to supplement what was previously accomplished with hearing. Higher-order areas of the auditory cortex get recruited by vision and, slowly, they demonstrate functional changes. After the brain has made these compensations, Sharma has seen the pre-frontal areas of the brain become more active as auditory input is decreased.
What does this mean?
According to Sharma, this suggests that even with a mild sensorineural hearing loss, the brain has to extend more effort to listen, which changes cortical resource allocation within the brain. As hearing loss increases, the brain has to work harder to listen. The surprise for Sharma, (and the rest of us) is that these changes start much earlier than previously thought and can occur even with mild hearing loss.
We know that brain changes are not easily reversed so how should we interpret this data? There have been some case studies showing that early use of hearing aids for those with mild hearing loss slows down changes. Brains that do not show changes demonstrate better speech perception, and changes in neural networks are more likely associated with effortful listening.
Single sided deafness
Sharma reports on one child with single sided deafness who received a cochlear implant. Prior to receiving the implant, she had demonstrated cross modal recruitment from visual and somatosensory systems and her auditory pathway was not well organized. After receiving the implant, her cross-modal recruitment reversed and the auditory pathway contralateral to the CI ear started to normalize. For adults who can control where to look to listen to a person who is talking, a CROS hearing aid may provide sufficient help, but children will not have this skill and for them, a CI for the poor ear may offer the best result.
What should we do clinically?
Anyone who has worked in the field of hearing loss knows how hard it is to convince both adults and parents that mild hearing loss needs to be managed. How many times have I heard “He hears me most of the time. I’ll just stand near him. Hearing aids are so much trouble and I don’t want people to think of him as ‘different’”.
First, I give sympathy. I know this is difficult. It is not easy to have a child who has any issues. It is not what we planned for. But then I have to talk about what happens if we do not deal with the issues.
- We know that auditory deprivation leads to brain changes which may not be reversible.
- We know that even a mild hearing loss leads to academic difficulties. (Here is one of the good places to talk to families about what their goal is for their child is. “Where do you want your child to be at age 5, 10, 15, 20?” What does it take to get there?”
- What message are we sending to a child if we say that we do not want them to wear hearing aids? What are we saying about how we accept that child and his hearing loss?
These are questions we can discuss with adults too. I, for one, am not great about wearing my hearing aids all the time. I have excuses. I have RICD hearing aids so when I want to listen to books on tape as I often do when I a walking or on the subway I have to take the hearing aids out and put in earbuds. And of course “I manage.” I wear them when I need to listen. Well, that has to stop. I will wear my hearing aid more often. I promise. I cannot afford to lose any more brain activity.