When the subjects successfully focused their attention, their brain activity showed that they were processing only that voice. It was “as if the other one was not even heard”, says Chang.

When listeners failed to pick a voice out of the mix, their brain activity was smudged and unfocused. When they attended to the wrong voice, the data showed a clean representation of that voice.

The finding fits with a growing body of evidence that sound is processed as soon as it enters the brain. “The early areas of the cortex don’t just represent a stimulus, but what the subject does with it,” says Andrew King, an auditory neuroscientist at the University of Oxford, UK.

King would like to see the experiment run in more realistic surroundings, with voices coming from different directions. “It would be interesting to do the same study by presenting two sources, one to each ear, or from different locations in space,” says King.

It would also be intriguing, he says, to see how the brains of hearing-impaired people direct their attention when handling multiple voices.