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?

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!

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.

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: 10.1016/j.neubiorev.2011.11.003

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 Kratochvil, 123rf, Wikimedia Commons, clker, and Grothe B., Pecka M. & McAlpine D. (2010). Mechanisms of Sound Localization in Mammals,Physiological Reviews, 90 (3) 983-1012. DOI: 10.1152/physrev.00026.2009.