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October 21, 2011

A Mechanism of Auditory Processing

Ever wondered how you're able to distinguish between different sounds and words in conversation? In order to understand the world around you, you not only have to hear all of the sounds together, but you also have to be able to hear the silence between the sounds. But all of this has to occur very quickly, or else you would be stuck having people repeat themselves slowly every time they said something. So, how does it work? The answer is: rapid changes in concentration of ions from cells that are firing electrical signals and turning off.

Previous research has implicated two structures in the brain that are critical in recognizing sounds and silences, namely the superior paraolivary nucleus (SPN, sometimes spelled superior periolivary nucleus) and the medial nucleus of trapezoid body (MNTB), both of which are part of the superior olive in the brainstem. A more current research article ("The Sound of Silence: Ionic Mechanisms Encoding Sound Termination" by Kopp-Scheinpflug, et al.) looks at how these two structures connect to one another and what mechanisms they use for distinguishing sounds.

In general, when a neuron is not being activated, it sends electrical signals at a specific rate, called its basal firing rate. Stimulation can increase or decrease the neuron's firing, and when the stimulation is removed, the firing rate eventually returns to its basal level.

When a sound stimulus is presented, the MNTB neurons continuously fire for the entire stimulation, and then not only cease firing when the stimulation has ended, but also reduce firing to below their normal rate, and return back to normal after a short period of time. On the other hand, SPN neurons have little to no firing when a sound stimulus is presented, and when it stops, the neurons rapidly fire, corresponding to the intensity of the stimulus and then deplete the firing to their normal rate.

The signaling pathways for both SPN and MNTB also involve chloride ions (and possibly potassium ions). The flow of chloride ions into neurons inhibits firing, and is important for recognizing sound in the MNTB, but recognizing silence in the SPN.

The main idea here is that there are multiple mechanisms involved in how we process language and other sounds every day. Without these two brain regions and the chloride signaling between them, we wouldn't be able to communicate. It is necessary to have mechanisms in our brains not only for recognizing sound, but also for recognizing silence, both of which need to communicate with one another to be processed together. This is a very important finding for learning how we acquire language and learn to differentiate syllables and words so readily and easily in early childhood, and more research could possibly help with understanding different speech disorders.

Posted by      Anna G. at 10:54 PM MDT
  Christina Uhlir  says:

Thank you for spelling it out so simply, I understood that we process continuous sounds as discrete but I couldn't understand how that was accomplished, so thank you for elucidating that point for me.
Posted on Sun, 23 Oct 2011 7:44 PM MDT by Christina U.

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