Following cochlear implantation (CI), auditory rehabilitation and training are crucial to optimizing a patient's adaption to electrical hearing. However, our understanding of neural plasticity after implantation remains limited.
Clinicians and researchers face several challenges in studying this critical period of implant-related central neural modulation. First, each CI user has a unique hearing history. Etiology of hearing loss and duration of deafness have differential effects on how well the patient adapts to CI hearing.
Second, cochlear implant devices, which are ferromagnetic, limit the use of functional magnetic resonance imaging (fMRI), the current gold standard for assessing patterns of brain activation linked to cognitive processing.
Neural Correlates of Adaptation in Freely-Moving Normal Hearing Subjects under Cochlear Implant Acoustic Simulations
Smalt CJ, Gonzalez-Castillo J, et al NeuroImage 2013;82:500-509 http://www.sciencedirect.com/science/article/pii/S1053811913006368
To address some of these challenges, Smalt et al turned to normal hearing listeners. These participants comprised a viable alternative study population for the use of acoustic CI-simulation models, eliminating the confounding factor of variable adjustment to deafness and enabling the use of fMRI.
Previous studies have examined the effect of short-term training on perception of vocoded, degraded speech using normal hearing listeners and acoustic CI simulation, but they were limited because participants had to remain in the artificial environment of the laboratory.
The current study by Smalt et al used a portable CI simulator on an iPod Touch that allowed for a “free-learning paradigm” in which users interacted with their environments during exposure to the degraded speech and heard their own voices through the simulator, the way CI users normally would.
In doing so, the authors strived to attain a more naturalistic understanding of neural adaptation to degraded speech.
Fifteen normal hearing individuals were outfitted with iPod Touch CI simulators to use two hours a day for two weeks in their everyday environments. Participants were asked to listen actively to speech or music during those two-hour periods.
Preexposure and postexposure behavioral testing and functional neuroimaging were conducted to evaluate the effect of exposure on tasks of degraded speech perception. Eight normal hearing controls had the same testing and imaging without the intervening exposure period to degraded speech.
By the end of the two weeks, participants who underwent the training period on the iPod Touch CI simulators had significantly improved in their ability to recognize degraded speech, both isolated words and sentences, but not in their ability to recognize normal speech. Control subjects did not improve in their perception of either degraded or normal speech.
Furthermore, fMRI analysis following the two-week training period revealed increased activation of the auditory cortex in response to vocoded speech, but not in response to normal sentences.
This increased activation was particularly prominent in the right hemisphere, which is associated with spectral processing. Wernicke's area, a classic perisylvian language region involved in semantic processing, also demonstrated higher activation.
Additional areas of high activity included the middle temporal gyrus and insula, which are associated with enhanced lexical processing and the utilization of working memory, as well as the left inferior frontal cortex (Broca's area), which is thought to be important for the processing of both speech and music.
The cingulate cortex, involved in attention and memory, and the supplemental motor area also demonstrated high levels of activity in the experimental group.
Interestingly, the secondary visual cortex showed enhanced activity after training in response to CI-simulated sentences. Previous positron emission tomography (PET) studies of CI users also demonstrated activation of the secondary visual cortex (Brain 2001;124:1307-1316 http://brain.oxfordjournals.org/content/124/7/1307.long ; NeuroImage 2004;22:1173-1181 http://www.sciencedirect.com/science/article/pii/S1053811904001430).
In this new study, Smalt and colleagues demonstrate that such recruitment of an additional sensory modality to aid in the context of degraded auditory information occurs even in normal hearing users within just two weeks.
Participants who underwent the training period demonstrated increased cortical activity in response to cochlear implant-simulated sentences not just in the primary and secondary auditory areas, but also in regions of the brain devoted to working memory, motor, and visual processing—areas not traditionally associated with speech perception.
COMMENDABLE AND NOVEL CONTRIBUTION
These results shed important light on our current understanding of the neural changes that may take place in the brain of cochlear implant users following implantation.
Implantation-associated neural plasticity leads to the recruitment of functional brain regions beyond those classically linked to speech processing, presumably in order to assist individuals in the task of interpreting impoverished auditory stimuli.
These findings imply that CI users’ brains undergo rapid adaptive functional changes to assist in implant-mediated listening. Further work is needed to determine whether or not this additional recruitment is observed with passive auditory stimulation alone, versus the more complex listening task employed here.
The mobile, user-friendly CI simulator used in this study has a broad range of potential uses that include the testing of new processing strategies, assisting in rehabilitation, and counseling patients and families preoperatively as to how auditory stimuli may sound following surgery.
From a research perspective, the ability of subjects to hear their own voices and experience auditory stimuli in their natural environments provides the opportunity to examine neural plasticity in an ecologically valid environment that is independent of the laboratory.
In this study, Smalt and colleagues have made a commendable and novel contribution to our understanding of adaptive mechanisms following cochlear implantation.
Turn to page 28 to read more about Charles J. Limb, MD, including what inspired him to pursue otolaryngology and the priority that keeps him engaged.