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How can we tell where the sound is coming from? Experiments with ferrets revealed the weird mechanism

How do we know where sound is coming from? Generally, we turn our head just slightly, allowing our brain to determine how this movement changes the sound volume in each ear. However, a new study revealed that the mechanism is quite a bit more complicated – while most neurons in the brain’s auditory cortex detect where a sound is coming from relative to the head, some are tuned to a sound source’s actual position in the world.

Scientists say that the way we locate the source of the sound is either egocentric or allocentric. Using the first method we can determine from which side sound is coming from and using the second – where is the source in the world.  In other words, neuron response will only change when you move your head in egocentric way of locating the source of the sound. In the systems that work allocentricly head position does not matter – phone is on the table wherever your head is pointing to.

There are two main ways of locating the source of sound – egocentric and allocentric. Image credit:  Shizhao, Wikimedia (CC BY-SA 1.0)

For example, if your auditory location system is more egocentric, after hearing a bird sing you will be able to tell “it’s behind me”. If your system is more allocentric, you will be able to say “Oh, I can hear a bird in that tree”. This is quite an interesting phenomenon, but, as you might imagine, it is not easy to research – scientists had to use animal models.

Interestingly, scientists distinguished these two ways by researching ferrets. Brain activity of these animals while they were moved around a small arena surrounded by speakers that emitted clicking sounds. Most ferrets displayed egocentric orientations by tracking where a sound source was relative to their head, but some (around 20 %) traced the actual location in the world. Previous researches relied on using people with fixed head position, which is not ideal.

Scientists say that their findings can find a pretty unexpected application – in the development of augmented or virtual reality games and programs. Dr Stephen Town, first author of the study, said: “We often hear sounds presented though earphones as being inside our heads, but our findings suggest sound sources could be created to appear externally, in the world, if designers incorporate information about body and head movements”.

Augmented and virtual reality games are trying to replicate real senses. Knowing that it is not all in the stereo or 3D sound technologies is helpful, because production teams have to have in mind that some people do not distinguish sound location by comparing sound levels in each of their ears. However, it is unlikely that the results of this research are going to make their way into gaming industry any time soon.

 

Source: UCL

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