The brain’s ability to reorganize itself by forming new neural connections throughout life is called Neuroplasticity. The neuroplasticity process allows the neurons (nerve cells) in the brain to compensate for injury and disease by adjusting their activities in response to new situations or to changes in their environment.  Medicine.Net (2015) describes the process as brain reorganizing by “axonal sprouting” where undamaged axons grow new nerve endings to reconnect neurons whose links were injured or severed.

Undamaged axons can also sprout nerve endings and connect with other undamaged nerve cells, forming new neural pathways to accomplish a needed function.  For example, if one hemisphere of the brain is damaged, the intact hemisphere may take over some of its functions. The brain compensates for damage in effect by reorganizing and forming these new connections between intact neurons.

In order to reconnect, the neurons need to be stimulated through activity.  Audiologists see this routinely in vestibular rehabilitation as this is how labyrinthine exercise works to reduce vestibular symptoms over time. Neuroplasticity sometimes may also contribute to impairment. For example, people who are deaf may suffer from a continual ringing in their ears (tinnitus), the result of the rewiring of brain cells starved for sound. For neurons to form beneficial connections, they must be correctly stimulated. Neuroplasticity is also called brain plasticity or brain malleability.  

How Does Neuroplasticity Work in Hearing?

Carlucci (2012) posits that three primary types of anatomical changes that may occur in hearing-related neuroplasticiticity. First, synaptogenesis and pruning are the processes of developing or removing whole synapses or synaptic groups that modify connections between neurons. Pruning occurs when a stimulus is inhibited or a new experience is deemed to be more functional. A good example is a patient who becomes hearing impaired, and his auditory skills diminish when experience is not reinforced. Carlucci says “that his is where the old adage ‘use it or lose it’ becomes anatomical”. Connections can, in a matter of weeks, develop and reorganize when functional hearing is restored. 

Secondly, neuronal migration allocates neurons as they extend across the brain to connect processing areas. This function is especially important to the developing brain because maturational changes will set up how the brain cells should operate together. This is a specialized process, and research has shown that specific proteins influence tonotopic organization throughout the auditory system. 

Finally, neurogenesis, as its name implies, generates new neurons from fetal through early adult development but not at the same rate after that. Infants with cochlear implants are a great example of how neurogenesis works. Early implantation continues new cell development in the auditory system and related connections. It is clear that the deleterious effects of sensory deprivation are for the most part avoided by providing a foundation for stimulus-driven learning.  Carlucci’s observed that most audiological neuroplastic measures were subjective, whereas functional MRI, EEG, or other physical measures of neuroplasticity would enable clinicians to maximize the benefits of amplification and therapy to patients with hearing loss.

New Research in Neuroplasticity & Deafness

Recently, at the University of Colorado, Dr. Anu Sharma has done just that. She used EEG to document neuroplasticity and hearing, presenting her research at the recent meeting of the Acoustical Society of America in Pittsburgh (May, 2015).   The work of Dr. Sharma’s group centers on electroencephalographic (EEG) recordings of adults and children with deafness and lesser hearing loss, to gain insights into the ways their brains respond differently from those of people with normal hearing.

The EEG recordings involve placing as many as 128 tiny sensors (electrodes) on the scalp, allowing the researchers to measure brain activity in response to sound simulation.  In Science Daily, Dr. Sharma states that, “sound simulation, such as recorded speech syllables, is delivered via speakers, to elicit a response in the form of ‘brain waves’ that originate in the auditory cortex — the most important center for processing speech and language — and other areas of the brain … We can examine certain biomarkers of cortical functioning, which tell us how the hearing portion of a deaf person’s brain is functioning compared to a person with normal hearing,”  

Dr. Sharma ‘s group and other researchers have recently discovered that the areas of the brain responsible for processing vision or touch can recruit, or take over, areas in which hearing is normally processed, but which receive little or no stimulation in deafness. This is called “cross-modal” cortical reorganization and reflects a fundamental property of the brain to compensate in response to its environment.  According to Dr. Sharma, “We find that this kind of compensatory adaptation may significantly decrease the brain’s available resources for processing sound and can affect a deaf patient’s ability to effectively perceive speech with their cochlear implants.” 

Cochlear implants are implanted devices that bypass damaged portions of the ear and directly stimulate the auditory nerve. Signals generated by the implant are sent by way of the auditory nerve to the brain, which recognizes the signals as sound, according to the National Institutes of Health, which funds Dr Sharma’s research.  Next, Sharma and colleagues will continue to explore fundamental aspects of neuroplasticity in deafness that may help improve outcomes for children and adults with hearing loss and deafness. Their goal is to develop user-friendly EEG technologies, allowing clinicians to easily ‘image’ the brains of individual patients with hearing loss to determine whether and to what degree their brains have become reorganized.  ”  According to Dr. Sharma, “in this way, the blueprint of brain reorganization can guide clinical intervention for patients with hearing loss.”

References:

Carlucci, D., HEaring Matters:  Neuroplastcity The New Frontier in Audiology,  Retrieved June 15, 2015:  http://journals.lww.com/thehearingjournal/Fulltext/2012/10000/Hearing_Matters___Neuroplasticity___The_New.11.aspx

Acoustical Society of America (ASA). “How does the brain respond to hearing loss?.” ScienceDaily. 19 May 2015:   http://www.sciencedaily.com/releases/2015/05/150519104604.htm