Attention tunes the mind’s ear

The brain’s power to focus can make a single voice seem like the only sound in a room full of chatter, a new study shows. The results help explain how people can pick out a speaker from a jumbled stream of incoming sounds.

A deeper understanding of this feat could help scientists better treat people who can’t sort out sound signals effectively, an ability that can decline with age.

“I think this is a truly outstanding study, which has deep implications for the way we think about the auditory brain,” says auditory neuroscientist Christophe Micheyl of the University of Minnesota, who was not involved in the new research.

For the project, engineer Nima Mesgarani and neurosurgeon Edward Chang, both of the University of California, San Francisco, studied what happens in the brains of people who are trying to follow one of two talkers, a scenario known to scientists as the cocktail party problem.

Electrodes placed under the skulls of three people for an epilepsy treatment picked up signs of brain signals called high gamma waves produced by groups of nerve cells. The pattern and strength of these signals reflect which sounds people are paying attention to. “We are able to assess what someone is actually hearing — not just what’s coming in through their ears,” Chang says.

Volunteers listened to two speakers, one female and one male, saying nonsense sentences such as “Ready tiger go to red two now.” The participants had to report the color and number spoken by the person who said one of two call signs (“ringo” or “tiger”). As soon as the male voice said “tiger,” for instance, the listener knew to focus on that speaker and ignore the other. Volunteers also performed the task while listening to each speaker solo.

All the while, the researchers were recording brain activity and sorting that activity into patterns that reflect voices and words. Before the call sign was uttered, these patterns were a mishmash. But as soon as the listener heard the right call sign, attention snapped to that voice, and brain activity shifted to a pattern similar to that seen when the listener heard a solo speaker, the team reports online April 18 in Nature.

“What we found was quite striking,” Chang says. “The brain was robustly encoding only the one we are attending to.”

In trials where the volunteers tracked the wrong voice, something different happened. The big attention switch happened before a call sign was uttered, suggesting that the listener had picked the wrong voice early and stuck with it. “Sometimes, for whatever reason, people start tracking the wrong one,” Chang says.

Scientists already knew that attention influences perception, says Micheyl, but the new results demonstrate the phenomenon in a clear way in the human brain. “This is a literal, direct reflection of auditory attention at the neural level.” 

The results are interesting, but offer only a snapshot of what’s going on in the brain, says neuroscientist David Poeppel of New York University. More work is needed to capture the richness of human acoustical behavior, he says. Other experiments have shown that people can still process acoustic information even without paying attention. And other kinds of brain signals that weren’t analyzed in this study also play a role in the cocktail party effect, he says.