6 December 2023 marked the launch of a new exciting Oticon initiative, the BrainHearing™ Network, with the aim of connecting scientists and hearing care professionals all around the world.
In this first webinar of a series, we had the pleasure of hosting Prof. Anu Sharma from the University of Colorado (Boulder) to talk about the consequences of hearing loss on the brain. As Prof. Sharma mentioned at the beginning of her talk: “There is this clear ear-brain connection that suggests that hearing loss not only impacts the ear but also the central auditory pathways”. Her talk was centered around the central consequences of hearing loss, and after briefly touching upon neuroplasticity in children, she then focused on neuroplasticity in age-related hearing loss.
Neuroplasticity in children with a hearing loss
So, what is neuroplasticity? “A basic tenet of neuroplasticity is that the brain will change or reorganise following sensory deprivation” explained Prof. Sharma. In order to explore and understand neuroplasticity in children, Prof. Sharma uses a robust biomarker of neuroplasticity, the P1 auditory cortical evoked response coming from the primary auditory cortex. This biomarker tells us about the maturation of the auditory cortex in children as they age and develop. Children with cochlear implants (CI) showed more typical auditory cortex development when they were implanted early (best by 9-12 months of age; Sharma et al., 2007). However, children that were implanted later in life often did not reach normal cortical development, which resulted in a struggle with oral language acquisition, despite being able to perform an audiogram – which can be explained by a partial decoupling of the primary auditory cortex and higher order cortical functions. This decoupling not only affects language acquisition, it also affects other cognitive functions like executive functions, attention, working memory, motor planning. Hence, hearing loss has cascading effects beyond the audiogram.
Now that we understand neuroplasticity in children with hearing loss, can we go a step further and use it to support clinical decision making? By following hundreds of children with a hearing loss, Sharma and Dorman (2006), Sharma et al. (2007), Sharma et al., (2015) could observe the change in P1 response after hearing-aid (HA) fitting or CI implantation. Can the hearing assistive device give enough auditory stimulation to restore normal cortical maturation? Today, Prof. Sharma uses the P1 response clinically to test children with hearing loss, auditory neuropathy spectrum disorder (ANSD) as well as children with multiple disabilities. Her conclusion is that cortical potentials can be particularly helpful for clinical management of children with hearing loss (and multiple disabilities) to understand how the maturation of the auditory cortex changes with HA or CI usage and whether the child gets enough benefits when aided or is a candidate for a CI.
Cross-modal plasticity
After talking about the consequences of hearing loss on the auditory system, Prof. Sharma talked about cross-modal compensatory plasticity, which also involves other systems. Campbell and Sharma (2016) showed evidence of cross-modal plasticity between the visual and auditory system. In particular, when a visual stimulus was presented, auditory areas of the brain were recruited in children with a CI in contrast to normal-hearing children recruiting the visual system only. What does this mean clinically? The more the children were struggling with speech perception, the more they activated this compensatory cross-modal plasticity to perform better. Sharma et al. (2015) additionally showed that the amount of cross-modal plasticity could be used to distinguish between a good and an average CI user. These CI users might show similar aided audiograms, but they have vastly different speech perception.
Interestingly, Prof. Anu Sharma explained that cross-modal plasticity was not only found in congenitally and profoundly deaf individuals, but also at much earlier stages of hearing loss (i.e., in mild to moderate hearing loss) as shown by Campbell and Sharma (2014). In other words, even a mild hearing loss can change and reorganise the brain. If we would only look at the audiograms of these people, we would typically not intervene. That’s why these results are so important because they highlight the need to look beyond the audiogram. Additionally, frontal and pre-frontal areas of the brain were also recruited, suggesting that listening has become effortful already in people with a mild hearing loss. “And if you are pulling in cognitive resources just to listen, how much cognitive reserve do you have left?” concluded Prof. Anu Sharma. To respond to this question, Glick and Sharma (2020) tested adults with an untreated mild to moderate hearing loss in several measures of cognitive functions and saw that they performed worse than normal-hearing individuals on all measures.
Reversal of cross-modal plasticity after HA fitting
These were all people without hearing aids, so the next question that Prof. Anu Sharma addressed in her talk was whether amplification after HA fitting can reverse these brain changes. Glick and Sharma (2020) showed that cross-modal reorganisation was completely reversed after 6 months of HA use and cognitive functions as well as speech-in-noise perception improved significantly. In other words, cross-modal plasticity is not permanent and can be reversed if it is addressed in time. Prof. Anu Sharma concluded that
“if you give early and appropriate treatment and restore the gain to the auditory cortex, then you likely reverse cross-modal plasticity, decrease cognitive compensation, reduce listening effort, and you may restore this needed balance between the senses.” However, Prof. Anu Sharma also highlights that the word “appropriate” is key here. Hearing aids need to be properly fitted to restore the gain to the auditory cortex. Prof. Anu Sharma is currently working on a project to look at the difference in brain reorganisation with over-the-counter hearing aids.
Some key takeaways from the talk of Prof. Anu Sharma:
- The audiogram does not fully describe the consequences of hearing loss.
- Even a mild to moderate hearing loss can lead to cross-modal plasticity, decreased performance in cognitive tests, and higher listening effort.
- Cross-modal re-organisation in mild to moderate hearing loss may be reversed by early and appropriate fitting of hearing aids.
For more information on Dr. Sharma’s research projects visit: https://www.colorado.edu/eeglab
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Sharma, A. & Dorman, M.F. 2006. Central auditory development inchildren with cochlear implants: Clinical implications. Adv Otorhinolaryngol, 64, 66-88.
Sharma, A., Gilley, P. M., Dorman, M. F., & Baldwin, R. (2007). Deprivation-induced cortical reorganization in children with cochlear implants. International journal of audiology, 46(9), 494-499.
Campbell, J., & Sharma, A. (2014). Cross-modal re-organization in adults with early stage hearing loss. PloS one, 9(2), e90594.
Sharma, A, Glick, H, Deeves, E, Duncan, E. (2015). The P1 biomarker for assessing cortical maturation in pediatric hearing loss: A review. Otorinolaringologia, 65(4): 103-114.
Sharma, A., Campbell, J., & Cardon, G. (2015). Developmental and cross-modal plasticity in deafness: Evidence from the P1 and N1 event related potentials in cochlear implanted children. International Journal of Psychophysiology, 95(2), 135-144.
Glick, H. A., & Sharma, A. (2020). Cortical neuroplasticity and cognitive function in early-stage, mild-moderate hearing loss: evidence of neurocognitive benefit from hearing aid use. Frontiers in neuroscience, 93.