Brains on Brains Week 7: The Auditory System

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The blog “A Plastic Journey: Introspection of Brains Learning About Brains” delves into the qualia, feelings and thoughts brought forth in the authors’ brains upon learning about the human brain and its functions. The blog will follow loosely the structure of the course “NBE-E4210 – Structure and operation of the human brain.” The blog is written in the form of a fact-filled, loopy and introspective story. [Logo: Oxford University Press (2018).]

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Like the eye and the visual system, auditory system was another major reason for me to join this course. I’m working with audiovisual content both in practice and in my research, so I was delighted to see how much information can actually be packed into one chapter. I’ve also studied music when I was younger, so there was a lot of relevant information that also resonated that part of me.

The book makes a good point about the nature of the sound. We often seem to think that the world around us is somehow filled with sounds that we can then pick up with our ears, while in reality sound is just audible variations in air pressure. It’s actually funny to think about for example a concerto: Highly trained musicians play instruments where vibrating strings cause the air to vibrate, and when it reached our ears, air density variations are converted into movement first in middle and then in the inner ear, and then the mechanical energy gets transduced into neural responses when stereocilia of hair cells bend. Later in the process (when neurons are involved, as hair cells are not neurons) most cells are frequency tuned, and some are also tuned to certain sound intensity. Some others don’t seem to be tuned almost at all, but the characteristics of this variation isn’t fully understood yet. But still that concerto can make us feel joy or sadness or visualize forests or mountain ranges in our minds. This is exactly why the brain keeps amazing me. Simple things like light or those variations in air density makes it possible for us to navigate in the world, understand other separate humans, and enjoy culture like music and visual arts.

I’m doing virtual reality productions as part of my research, so understanding mechanics of the sound localization is important. I had already learned about the interaural time delay when taking a course in ambisonics and had a personal experience of losing that ability when spending time underwater on a diving course, but I had never thought about it with this detail. I think the best example is still this virtual barbershop track: https://www.youtube.com/watch?v=IUDTlvagjJA

Make sure you are using headphones, though, because it won’t work with laptop speakers, and definitely not with a mobile phone speaker. I think this perfectly illustrates how our sense of space can be tricked just by controlling the interaural time delay and volume. It’s exactly the simplicity of this technique what makes it so fascinating.

For example, the fact that continuous sound is more difficult to localize, makes total sense and is very easy to understand, but it had just never occurred to me before. I also learned how the frequency plays a part in localization, when the distance covered with one cycle of the sound wave can vary so much, and that in the range of 2000 – 20 000 Hz we’re actually using another system (interaural intensity difference). This is definitely something that will help me in my thesis productions, but it’s also an interesting detail of human evolution. There’s a certain frequency range where most sounds we care about are, so our brain has developed into a very fast and accurate interpreter and localizer of those sounds. A roaring predator, howling wind, crying baby and many other sounds around us are too important to be missed.

The chapter also raised some questions. If we could change a human brain into some other animal’s body, would we able to understand the world at all? Would the differences in the ear, lack of pinna (which is possible for humans too, of course) and reduced or increased head size make the auditory signals too difficult to understand? Would we be able to sense the space we’re in or tell how far away some sound is coming from?

Posted by Free Willy

About Free Willy

The blog "A Plastic Journey: Introspection of Brains Learning About Brains" delves into the qualia, feelings and thoughts brought forth in the authors' brains upon learning about the human brain and its functions. The blog will follow loosely the structure of the course "NBE-E4210 - Structure and operation of the human brain." The blog is written in the form of a fact-filled, loopy and introspective story.
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