Cochlear Implants, Brain Reorganization, and Hearing Loss: Interview with Anu Sharma, PhD
Douglas L. Beck, AuD, spoke with Dr. Sharma about cochlear implants, brain reorganization after hearing loss, and early stage hearing loss (mild hearing loss) correlating with brain changes.
Academy: Good morning, Anu. Always great to chat with you!
Sharma: Hi, Doug. Nice to speak with you, too.
Academy: Anu, if I recall, you earned your doctorate from Northwestern University?
Sharma: Yes, that's right. I graduated in 1995. I worked with Nina Kraus, PhD, on topics relating to neural correlates of pitch and place of articulation perception.
Academy: And your latest research addresses brain reorganization following hearing loss. Can you tell me a little bit about that?
Sharma: Absolutely. My areas of interest include children and adults who are deaf, as well as cochlear implants, and, of course, brains! Consistent with what others have previously published, we too, noticed that when a person is deaf, the areas of the brain in which sounds are normally processed (i.e., the temporal lobe) change, as they are not being used to process or understand sound, and indeed, other areas of the brain recruit the available areas from the temporal lobe, for their own purposes. This is called cross-modal plasticity.
Academy: And you've noticed this activity in deaf children who wear cochlear implants?
Sharma: Yes, that's correct. Other modalities, such as vision and somatosensory, recruit auditory areas. Further, it seems there is a correlation between the child's ability to process speech-in-noise through a cochlear implant, and how much cross-modal recruitment has occurred.
Academy: So the recruitment phenomena you just mentioned is reasonably well known and accepted in neuroscience, and isn't that more or less the "use it or lose it" issue?
Sharma: Yes. If the ears cannot deliver sound to the brain, the area of the brain which normally processes sound gets assigned to another function, as the brain is highly adaptable, or "plastic."
Academy: And you also looked at this issue with respect to adults with mild-to-moderate hearing loss. What did you find?
Sharma: Glad you asked! Most of the adults we examined had normal hearing to 2000 Hz and then had a mild-moderate sloping sensorineural hearing loss after that, consistent with presbycusis. However, we found that even in these adults, these same types of brain cross-modal changes had occurred, secondary to their mild hearing loss.
Academy: That's pretty interesting. And can you tell me how you went about studying this?
Sharma: Sure. You see, if we gave them a visual motion stimulus we know which part of the brain normally responds to it. However, in our patients with deafness and even people with hearing loss, we noticed the auditory areas responding to visual motion—and that doesn't happen in normal hearing people.
Academy: And the really amazing thing was that these brain changes occurred with mild-moderate high-frequency hearing loss.
Sharma: Yes, that surprised all of us, too.
Academy: And you also found that it wasn't just visual areas and visual processing that usurped auditory areas, but the somatosensory modality areas also recruited available auditory areas?
Sharma: Exactly. And not only do we see auditory processing areas being recruited by visual and somatosensory modalities, we also noticed a negative correlation with speech perception in noise. And so when we gave these patients the QuickSin, we found that as there was more apparent recruitment by other modalities, the worse the patients performed with regard to speech in noise.
Academy: And so the upshot of this work is that for people with mild-to-moderate hearing loss, listening in noise may be more difficult than indicated by the degree of hearing loss, because the temporal portion of the brain which normally detangles speech in noise and helps to make sense of speech in noise, may not be completely available to perform these auditory processing functions?
Sharma: Exactly…that's what we're thinking….and in other studies we found that frontal regions and the pre-frontal cortex are also being activated to help process sounds. So although speculative, it appears that for people with hearing loss, less of the auditory cortex is stimulated, and so the areas not stimulated by sound, they may be re-assigned…and when sounds come in more listening effort is required which activates the frontal and pre-frontal areas to help with sound processing.
Academy: Amazing….and in retrospect, it makes total sense. And so where is this leading you over the next few years?
Sharma: We need to examine compensatory changes and see if it is linked to cognitive decline secondary to hearing loss. Perhaps more important than figuring out exactly what is happening, is the issue of aural rehabilitation (AR). That is, we need to better understand what we can do to help the patient who has undergone these brain changes, and how we might help them to better process difficult sounds, perhaps through some kind of brain training or re-training.
Academy: Fantastic work, Anu. I appreciate your taking the time to chat about these issues. Can you list a few of your articles which address these same issues for readers who'd like to dig deeper?
Sharma: Absolutely…I recommend:
Campbell J, Sharma A. (2014) Cross-modal re-organization in adults with early stage hearing loss. PLoS ONE 9(2).
Campbell J, Sharma A. (2013) Compensatory changes in cortical resource allocation in adults with hearing loss. Frontiers in Systems Neuroscience 7:71.
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. Intl Journal of Psychophysiology 95(2):135-44.
Academy: Thanks, Anu. It's been a pleasure speaking with you.
Sharma: Thanks, Doug. I appreciate your interest in our work.
Anu Sharma, PhD, is a professor in the Speech, Language, and Hearing Science Department at the Institute of Cognitive Science and Center for Neuroscience, University of Colorado at Boulder.
Douglas L. Beck, AuD, Board Certified in Audiology, is the Web content editor for the American Academy of Audiology and the director of public relations with Oticon, Inc.