Inside Clinical Research | January 2017 Hearing Review
Those who follow the hearing healthcare research literature are certain to have encountered the fascinating work of Anu Sharma, PhD, who studies cross modal plasticity of the brain and cortical resource allocation as related to hearing loss, as well as other aspects of the impact of auditory deprivation on the human brain.
Beck: Hello Anu. It’s always a delight to speak with you. Thanks for your time this morning.
Sharma: Hi Doug. It’s my pleasure. Thanks for your interest in my work.
Beck: Anu, before we talk about your fascinating research, I should mention you earned your doctorate while working under Nina Kraus, PhD, at Northwestern University. Currently, you’re a professor and serving as Interim Chairperson at the Department of Speech, Language, & Hearing Science at the Institute of Cognitive Science and the Center for NeuroScience at the University of Colorado at Boulder. As such, for this year, you’re teaching neurodiagnostics and neuroscience to AuD students. Given that, please tell me about your recent paper1 that explains how even a mild sensorineural hearing loss (SNHL) causes changes in the brain.
Sharma: Fair enough, let’s jump right in! We’ve known for decades that brain changes occur secondary to substantial hearing loss and deafness. That is, when deaf lab animals have been studied, we’ve witnessed dramatic changes in the auditory cortex and other structures, and that’s been well known and accepted for many decades.
However, our recent work demonstrated brain changes and cortical re-organization occurs even with mild SNHL. That is, the brain changes and re-organizes based on a mild degradation of the stimulus or a mild lack of audibility.
Beck: So, to be clear, if a human brain is accustomed to the full complement of speech sounds, and these same sounds are missing or attenuated due to hearing loss, the human brain changes as a result of altered auditory stimulation?
Sharma: Yes, that’s correct. The first way this might happen is via compensatory mechanisms which engage to overcome the hearing impairment, as expected, such as the reliance on vision. So when a previously known and learned auditory task (such as listening in noise) becomes more effortful and challenging, the brain may endeavor to accomplish the same goal by emphasizing a greater reliance on vision to supplement what was previously accomplished via audition.
Beck: And in addition to reliance on vision as a supplement to audition, the brain physically changes?
Sharma: Yes. The higher-order areas of the auditory cortex get recruited by vision and they show functional changes. That is, after the brain compensates, when we stimulate the brain using exclusively visual input, some previously defined auditory areas respond to visual stimuli.
Beck: That is absolutely fascinating. And you suggested there might be another auditory compensatory mechanism?
Sharma: Yes, we also noticed the frontal and the pre-frontal areas of the brain become more active as auditory input is attenuated. This suggests that, given a mild SNHL, the brain extends more effort to listen, all of which seems to create a change in cortical resource allocation within the brain, as listening becomes increasingly effortful.
Beck: So, as hearing loss increases, the brain has to work harder to listen. If we define hearing as perceiving sound, and listening as the ability to make sense of sound, it seems correct to state that, as hearing loss increases, the brain expends increased effort to successfully listen, and very likely, this would be most distressing in background noise?
Sharma: Yes, that’s a reasonable interpretation. As listening becomes more effortful, it leads to the previously mentioned change in cortical resource allocation. We first witnessed these changes in deaf children who acquired cochlear implants (CIs), and as you know, we’ve recently reported the same type of change in an adult with sudden sensorineural hearing loss.2 However, the surprise to us was we found that these changes start much earlier than previously thought and can occur secondary to even a mild SNHL.
Beck: I’m curious to get your thoughts as to whether fitting hearing aids on people with mild SNHL, even those who experience minimal “pain” from their hearing loss, would be likely to slow down, reverse, or impact these brain changes?
Sharma: That’s a very important and thought-provoking question, and as you certainly guessed, Doug, we just don’t know yet! We don’t have well-designed, long-term studies on multiple populations of people with various types and degrees of hearing loss, to determine what’s what—let alone the impact of amplification in a controlled versus experimental group.
Beck: And, of course, once we know what happens with “plain old vanilla” amplification, we’d have to evaluate all types of amplification, such as wide dynamic range compression, linear fittings, long and short release times, various gain and output levels, frequency lowering and more. And each would have to be studied in depth to draw conclusions.
Sharma: Right, and until we have these long-term studies, which are designed at the front end to compare and contrast different technologies in different populations, we just don’t know. I know many of your readers will be interested to learn we are collecting data on these matters, and we are looking for other research centers and professionals who would like to collaborate on pre- and posthearing aid fittings to look for and examine brain changes.
Beck: Can you share any early observations related to hearing aid fittings and brain changes?
Sharma: We have seen in some case studies people wearing hearing aids appear to show reversal of cross-modal recruitment, and these are the same people who seem to do very well with speech in noise. And, again, we find that a brain which has not undergone changes in cortical resource allocation is more likely to be associated with better speech perception, and we find that underlying changes in neural networks are likely to be associated with more effortful listening. So it appears that a brain which has undergone fewer changes in cortical resource allocation is more likely to be associated with better speech perception.
Beck: Might we also suspect that, when someone is effectively fitted with appropriate amplification early on, less brain recruitment may occur, and that person may benefit more from well-fitted hearing aids?
Sharma: That’s speculative, but yes, it seems to be where we’re heading, and that’s one of the areas we’d like to study in-depth. Further, we have seen a negative correlation in many patients between speech perception and cross-modal recruitment; as speech perception in noise decreases, cross-modal recruitment increases.
As Julia Campbell and I said in a recent publication, “Due to the strong relationship between hearing loss (HL) and cognitive deficits, such as dementia, that arise later in life, it is important that clinical evaluation of cognitive reserve (in the presence of HL) be included as part of intervention services. Future research should focus on better understanding the relationship between the severity of cognitive re-allocation in relation to severity of HL as well as reversibility of re-organization as a result of intervention with amplification.”3
Beck: And as you know, along with my colleagues Barbara Weinstein and Michael Harvey,4 we’ve been advocating the potential need for audiologists to screen basic cognitive function. However, you’ve gone above and beyond our application, and that’s very exciting! Before I let you go, let me ask your thoughts on a few related topics, with regard to auditory neuroscience.
Sharma: OK, sure. What did you have in mind?
Beck: Given the discussion above, please tell me your thoughts as to strategies for managing people with single-sided deafness (SSD)?
Sharma: Well, this too, is a controversial area in audiology, and all of this needs more research. That said, we published one case of a child with SSD,5 who I believe was the first child with SSD in Colorado to receive a CI. She had a progressive SNHL. By age 9, she demonstrated cross-modal recruitment from visual and somatosensory systems, and her auditory pathway was not organized or set up as we would have expected in a bilaterally normal-hearing child. That is, the only normal auditory findings were those contralateral to her normal-hearing ear.
Beck: I recall reading about this child, and she was even more fascinating because of what happened after she got her CI.
Sharma: Yes, that’s right. After her CI, her cross-modal recruitment reversed. She had a partial reversal with regard to vision and a complete reversal in her somatosensory system. In addition, it was exciting to see that the auditory pathway contralateral to the CI ear started to become normalized.
Beck: So, given a child or an adult with SSD, there are a number of considerations to be addressed before we jump to CROS and BiCROS and similar solutions. For adults who specifically would like to be able to hear better when the “bad ear” is closest to the talker, a CROS or BiCROS might be fine, as long as the adult with SSD is knowledgeable about what’s going on and how things work.
However, for a child with SSD, if we want to enable maximal brain growth and maturation, we may have to consider a CI. Specifically, a CI is the only technology which will engage both sides of the brain and is likely to allow the brain to benefit from interaural loudness differences (ILDs) and interaural timing differences (ITDs), allowing the brain to compare and contrast sounds from both sides of the head, thus facilitating the use of spatial cues and more.
Sharma: That’s what I’m thinking, too. But again, we don’t want to get ahead of the research. I can speculate that, if we fit a child with a CROS or BiCROS, we seem to be less likely to see meaningful changes in the auditory cortex. But again, that’s speculative, and each case is different.
That’s what makes research so incredibly exciting and important. As we find or prove one thing, it sometimes leads to a domino effect in thinking and application, and leads to new ideas and interpretations.
Beck: I agree. That’s what keeps me reading and writing, too. Every time I read a new article or participate in writing a new article, I find new knowledge which forces me to re-think everything. Of course, that’s not only the best part of research, it’s often the worst!
Sharma: Good point!
Beck: Thank you so much for your amazing research and your time this morning, Anu. Perhaps we can get together in the spring and discuss one of your other passions, like auditory neuropathy spectrum disorder (ANSD)?
Sharma: Absolutely Doug. I’ll look forward to it!
Glick H, Sharma A. Cross-modal plasticity in developmental and age-related hearing loss: Clinical implications. Hearing Research. 2016; pii:S0378-5955(16)30139-3. Available at: http://dx.doi.org/10.1016/j.heares.2016.08.012 or https://www.ncbi.nlm.nih.gov/pubmed/27613397
Campbell J, Sharma A. Cross-modal re-organization in adults with early stage hearing loss. PLoS ONE. 2014; 9(2): e90594. doi:10.1371/journal.pone.0090594. Available at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0090594
Campbell J, Sharma A. Compensatory changes in cortical resource allocation in adults with hearing loss. Frontiers Systems Neurosci. 2013; October, Volume 7, Article 71. Available at: http://journal.frontiersin.org/article/10.3389/fnsys.2013.00071/full
Beck DL, Weinstein BE, Harvey M. Issues in cognitive screenings by audiologists. Hearing Review. 2016; January. Available at: http://www.hearingreview.com/2016/01/issues-cognitive-screenings-audiologists/
Sharma A, Glick H, Campbell J, Torres J, Dorman M, Zeitler DM. Cortical plasticity and reorganization in pediatric single-sided deafness, pre and post cochlear implantation. Otology & Neurotology. 2016;37:e26-e34. Available at: https://www.ncbi.nlm.nih.gov/pubmed/26756152
Correspondence to Dr Beck at: [email protected]
Original citation for this article: Beck DL. Inside Clinical Research: Auditory Deprivation, Brain Changes Secondary to Hearing Loss, and More: An Interview with Anu Sharma, PhD. Hearing Review. 2017;24(1):40.