Where’s that noise coming from?

Dogs, cats, deer and a myriad of other mammalian species can do something we can’t –swivel their ears to direct their hearing towards what particularly interests them at the time. Humans or our ancestors are thought to have forfeited this ability some 25 million years ago, which is possibly a tad unfortunate as it may have been an interesting sight to behold.

And today, how the brain implements spatial hearing is still not widely understood. The theory currently goes that the brain works out sound direction by comparing the times when sound reaches the left compared to the right ear. This is known as the interaural time difference, or ITD for short. But how exactly the brain decodes this information is still something of a conundrum. Hearing has even been coined the “forgotten sense”.

Now, researchers at the University of Wurzburg in Germany are revealing how our brains may have developed the ability to ‘swivel’ in search of ambient sounds to compensate for our shell-likes’ now diminished mobility. The researchers used electroencephalography (EEG) to monitor the electrical activity of the brain while volunteers were on the move around them.

Lighter, smaller and particularly wireless EEG devices now enable the measurement of brain activity to start to associate brain function with behaviour while on the move. It is becoming ever-clearer that movement has significant effects on how our brains work. We were already aware of how walking influences our processing of visual information so we better observe objects in our peripheral field of vision that we may not have noticed when stationary. Now researchers are showing that something similar applies to sound, with the brain constantly adjusting the direction it most keenly needs to listen in.

In an experiment involving 35 volunteers wearing with EEG and motion sensors, the subjects were asked to walk in a figure of eight while listening to continuous sound played through in-ear headphones. The EEG showed a significant boost in the brain’s sound processing function when the subjects started to move. When the volunteers changed direction the brain adapted to concentrate on sounds coming from that direction, with the focus of the brain continuing to switch with each turn while walking.

The studies also have potential to help improve navigation aids for the visually impaired by helping support technology that filters background noise based on the walking direction.

One train of thought currently being given an attentive ear is that this internal ear swivelling has evolved to help keep us safe, enabling better navigation and shorter reaction times in unknown dynamic environments. The research may also shed more light on why exercising outdoors appears to have more benefits for brain health and cognition than that gained from exercising on treadmills or stationary bikes in the confines of the home or gym. Active exploration does indeed hone visual and auditory perception, while integrating multisensory experiences for a minor cognition workout.

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