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Can You Hear Me? Selective Listening and Your Hearing

Imagine you’re walking along a busy street with your friend. You’re having a lovely conversation about your upcoming summer plans when out of nowhere, someone taps you on the shoulder. You turn around and it’s one of your co-workers, breathing heavily after running to catch up with you.

“Didn’t you hear me?” they ask. “I’ve been calling your name for the last five minutes!”

Chances are, you’ve had this experience before, whether it was in a crowded cafe or on a city bus. Your brain has selectively tuned in to one source of noise while ignoring everything else.

Most of us are aware that we selectively listen to certain sounds throughout our day to day life – consider how you can listen to just the drumbeat in a rock song or pick up on just the announcement on a loudspeaker in the airport, but we don’t really know how it happens.

A team of researchers at Carnegie Mellon and the University of London have developed a new technique to understand how our brains selectively listen on a neurological level.

The Study

The study had participants to listen to a series of short tone melodies and another distracting noise at the same time while being hooked up to an fMRI machine. The participants were asked to respond when they heard the melody repeat while researchers studied their MRI scanner

When the participants listened to the melodies at different frequencies, the parts of the brain tuned into these frequencies were active. However, the most surprising result was that these frequencies activated not only a few select areas but much of the brain cortex where sound is generally processed.

To make sense of this, we first need to understand a bit of the inner workings of the brain. First, the brain surface, known of the cortex, contains ‘tonotopic’ maps of auditory frequencies. These maps are essentially locations where certain frequencies can be picked up by the brain.

Thus, if you look at the temporal lobes (the part of the brain that generally is associated with sound and the location of these maps) while someone listens to sound, you might identify which parts of their brain is currently being activated.

What’s even more interesting, however, is what happened when the researchers used a new high-resolution brain imaging technique, known as multiparameter mapping, to see how other brain features might affect this frequency mapping. In particular, the researchers were interested in how myelin – the electrical insulation of neurons – can affect a neuron’s affinity for a particular frequency.

Basically, myelin allows neurons to send electrical messages to each other without losing significant amounts of the electrical impulse along the way, much like insulation helps a copper wire conduct electricity. Myelin quantities are not equal across neurons, though, and some have significantly more myelin than others.

What the researchers found is that, in comparing the frequency maps with the myelin information, areas of increased myelin content had a stronger preference for a particular frequency. Or, in other words, an area of the brain with more myelin is better at honing in on a particular frequency than a part of the brain with less myelin.

This might seem like a small detail, but this research has the potential for far-reaching implications. For example, if we consider that myelin quantity and quality decreases with age, then perhaps we can use this new research to understand how this might affect older people’s ability to tune into a particular conversation in a busy room. The research could also prove helpful in better understanding how people learn languages or music.

Regardless, the future of this research is bright and its potential impacts for the medical and scientific communities are something to be excited about.

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