How does the brain locate sound sources?

The brain has an amazing ability to identify the source of sounds around you. When driving, you can tell where an approaching fire truck is coming from and pull over accordingly. In the classic swimming pool game of “Marco Polo,” the player who is “it” swims toward the players who says “Polo.” In the field of neuroscience, this ability is called sound localization. Humans can locate the source of a sound with extreme precision (within 2 degrees of space)! This remarkable feat is accomplished by the brain’s ability to interpret the information from both ears. So how does your brain do it?

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Neuroscientists have been working to understand the mechanisms of sound localization for many years, and they have identified two cues that are essential for sound localization in the horizontal dimension. Imagine there is a circle that makes a perfectly flat plane around your head, as shown below. When a sound comes from the speaker, how can you identify its location so accurately? In the 1790s, Venturi played a flute around people and asked them to point in his direction. He proposed that the sound amplitude (loudness) difference between the two ears was the cue used for sound localization. Much later in 1908, Malloch proposed that the time difference of the sound reaching each ear was the cue used for sound localization. Years later, neuroscientists found neurons in the auditory centers of the brain that are specially tuned to each cue: intensity and timing differences between the two ears. So, the brain is using both cues to localize sound sources. For example, sound coming from the speaker would reach your left ear faster and be louder than the sound that reaches your right ear. Your brain compares these differences and tells you where the sound is coming from!

fig 1

But what happens when a sound comes from anywhere along the midline of your head? It could be directly in the front of you, behind you, or above you. In any of these cases, there would be no difference in sound loudness or delay between your two ears! It turns out that your brain uses a third cue to locate sounds in the vertical dimension: the different frequency profile of sound caused by the size of your head and your external ear, called the pinna. The pinnae are exquisitely shaped not only to collect sound, but also to change the frequency profile of a sound. Depending on its origin, certain frequencies get enhanced, while others get attenuated. As shown in the picture below, freqnency changes in colors are tied to their locations. This cue is unique to each pinna and therefore monoaural. Neuroscientists have found neurons in the lower level of auditory brain that are tuned to these frequency notches as well.

fig 2
Horizontal location will change the frequency profile of each sound. Each location has a certain notch represented by correasponding color.

So, what happens when sounds are moving? Obviously, sounds become louder as they near us and softer as the move away, but the perceived frequencies of sound also change. For example, the frequency of the siren from a fire truck sounds higher as it moves toward us and lower as it moves away. This phenomenon was first discovered by the Austrian Physicist Christian Doppler, and is thus named the Doppler effect. The Doppler effect may be a cue for the perception of distance changes. Additionally, the brain tracks the vertical and horizontal angle by the binaural and monaural cues such as the three cues mentioned above.

Overall, the brain uses a variety of cues to determine the location of a sound. Our current understanding of the mechanisms of sound localization is mostly limited to the cues themselves and how the lower levels of the brain’s auditory pathway process these cues. It is a really exciting time to explore how the higher level auditory brain uses those signals from lower levels to form the perception of the sound location!

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Written by Xiaorui “Ray” Xiong

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References:

Phillips D.P., Quinlan C.K. & Dingle R.N. (2012). Stability of central binaural sound localization mechanisms in mammals, and the Heffner hypothesis, Neuroscience & Biobehavioral Reviews, 36 (2) 889-900. DOI: 

Letowski T.R. and Letowski S.T. (2012) Auditory Spatial Perception: Auditory Localization, Army Research Laboratories ARL-TR-6016 

Images adapted from Crowd At Busy Street by Petr Kratochvil123rf, Wikimedia Commons, clker, and Grothe B., Pecka M. & McAlpine D. (2010). Mechanisms of Sound Localization in Mammals,Physiological Reviews, 90 (3) 983-1012. DOI: 

knowingneurons

knowingneurons

Knowing Neurons is an award-winning neuroscience education and outreach website that was created by young neuroscientists. The global team members at Knowing Neurons explain complicated ideas about the brain and mind clearly and accurately using powerful images, infographics, and animations to enhance written content. With an extensive social media presence, Knowing Neurons has become an important science communication outlet and resource for both students and teachers.
knowingneurons

knowingneurons

Knowing Neurons is an award-winning neuroscience education and outreach website that was created by young neuroscientists. The global team members at Knowing Neurons explain complicated ideas about the brain and mind clearly and accurately using powerful images, infographics, and animations to enhance written content. With an extensive social media presence, Knowing Neurons has become an important science communication outlet and resource for both students and teachers.

8 thoughts on “How does the brain locate sound sources?

  • Pingback:HOW DOES THE BRAIN LOCATE SOUND SOURCES? | Ray Xiong

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  • May 7, 2016 at 11:42 am
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    Thanx a lot. Should find the availability of a digital system to do what the brain n ears do

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  • November 2, 2016 at 1:11 am
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    sound waves in are ears is what sets off the love chemicals for romance what makes us fall in love adding i have metal plates magnets and electricitys produced in my brain which makes a magnetic fild like a radio station is made what makes prince harrys voice herd in my ears and acnoligment ways herd
    How does the brain locate sound sources? – Knowing Neurons
    knowingneurons.com/…/how-does-the-brain-locate-sound-sourc…/
    15 Mar 2013 – The brain has an amazing ability to identify the source of sounds around you. … Imagine there is a circle that makes a perfectly flat plane around your head, as shown below. … difference between the two ears was the cue used for sound localization. …. Why Prairie Voles Fall in Love: A Chemical Romance.

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  • November 2, 2016 at 1:21 am
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    is they anything i can do to prove this any exsperament or science or reasearch that can be done i have electricity in my brains and metal plates and magnets in my face i have a voice of prince harrys where i fallen in love with him he feels the same im just a common girl i know a lot about what im going throue how its made with my common nollige of how to understand allso read up on papaer books and google allso facebook i can connect the pattern of names that all link together in a worldly puzzel of names houses birthdays and faces of peoples same and sur names they all link to me and prince harrys family names famouse sur names after harry and every sur name i know being named after famouse names after me if thats a way to understand why it leads to me i see the pattern verry well i see coqincidences a lot and can prove them in this pattern this is made from the satterlite signals and brains electricitys we produce in are brains all day making a pattern and puzzels of a chain could anyone help me please

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    • November 12, 2016 at 6:55 am
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      I think you may be beyond help Stacey but I am sure someone may be prepared to try if you make it clearer how you want to be helped. Do you want to prove your powers? Understand them? Cure them? Marry Harry? Some people see colours when they hear sounds and it turns out that their brains are wired differently. Maybe you have a similar condition.

      Reply
  • May 14, 2017 at 2:38 am
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    Interesting. Since we have two ears, which are connected to systems which can detecy both amplitude difference and time lapse, we end up with two possible locations which both satisfy the conditions. One is ahead of us and one behind us, on the same side of our heads, equidistant. I think we are able to resolve this problem because our external ears change the amplitude, and maybe even slightly change the timing, of the incoming sound. Your thoughts?

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  • Pingback:Sound localisation and spatial audio

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