What did you say? The science behind hearing loss.

 

Have you ever wondered why your grandparents cannot hear you when you talk to them in a noisy room? However, they often can hear you in a quiet room? Well, in a paper Cochlear neural degeneration disrupts hearing in background noise by increasing auditory cortex internal noise by Dr. Jennifer Resnik, and Daniel B. Polley shows it much more complicated than just bad ears.

Normally we think that damage to the Cochlea (the organ in the ear responsible for turning sound into neural signals) is responsible for hearing loss. While this is true to some extent, the brain tries to compensate for this lack of hearing by adjusting the synaptic connections in your primary auditory cortex (A1). 

In normal conditions, the brain has excitatory pyramidal cells that are responsible for encoding information about auditory stimuli. However, in a noisy environment, there is a lot of extraneous information, that the human brain is excellent at filtering out. To prevent this noise from exciting the pyramidal neurons inhibitory interneurons called PV neurons adapt to the noise, and inhibit (or stop) the pyramidal neuron from firing in the noise. That way, whenever you hear an important signal, such as your friend talking, then your pyramidal cells will fire.  This sort of inhibition that removes noisy signals is a key for being able to listen in a noisy environment, and it relies on a balance between excitation and inhibition, so called (E/I) balance.

Interestingly, this balance is broken as your grandparents cochlea gets old. In order to compensate for the weaker signals coming from the cochlea, the brain increases the excitatory strength of the pyramidal neurons and decreases the inhibitory strength of the PV neurons. This helps boost the signal in quiet rooms, allowing your grandparents to make out our voice. However, it has the unfortunate consequence of causing  erroneous synchronization in noise. That is, the pyramidal neurons spontaneously fire randomly. This has the effect of their brain being unable to distinguish between the noisy room and your voice. Therefore, while the increase in the E/I ratio boosts the gain in quiet, while making matters worse in the noisy room.

Interestingly, by taking advantage of the neuroplasticity of the human brain, we may be able to help older folk recover lost hearing.  This suggests that by practicing hearing in a noisy room may actually improve the situation by training the brain to increase the inhibition in the PV neurons.  In the meantime, now you can understand why your grandparents have trouble hearing you, and if you have the patience, you can help your grandparents hear better by repeating yourself.

by: Alex White