Scientists often test auditory processing in artificial, silent settings, but real life usually comes with a background of sounds like clacking keyboards, chattering voices and car horns. Recently researchers set out to study such processing in the presence of ambient sound—specifically the even, staticlike hiss of white noise.
Their result is counterintuitive, says Tania Rinaldi Barkat, a neuroscientist at the University of Basel: instead of impairing hearing, a background of white noise made it easier for mice to differentiate between similar tones. Barkat is senior author of the new study, published last November in Cell Reports.
It is easy to distinguish notes on opposite ends of a piano keyboard. But play two side by side, and even the sharpest ears might have trouble telling them apart. This is because of how the auditory pathway processes the simplest sounds, called pure frequency tones: neurons close together respond to similar tones, but each neuron responds better to one particular frequency. The degree to which a neuron responds to a certain frequency is called its tuning curve.
The researchers found that playing white noise narrowed neurons’ frequency tuning curves in mouse brains. “In a simplified way, white noise background—played continuously and at a certain sound level—decreases the response of neurons to a tone played on top of that white noise,” Barkat says. And by reducing the number of neurons responding to the same frequency at the same time, the brain can better distinguish between similar sounds.
To determine whether the mice could differentiate between tones, the researchers used a behavioral test in which the rodents had to react to a specific frequency. Like humans, the mice easily recognized very different tones and struggled with similar ones. But with white noise added, the mice could better tell similar tones apart. The researchers investigated further by measuring neural activity in the mice’s auditory cortexes as white noise played, and they also stimulated particular neurons directly to induce the curve-suppressing effect.
Future research should address how this mechanism works, says Kishore Kuchibhotla, a brain scientist at Johns Hopkins University, who was not involved in the study. And “the jury remains out on whether and how this relates to human perception,” he adds.
It is possible that understanding this effect could eventually help people hear better. “Adding noise into the ear will not help someone with hearing loss,” says Daniel Polley, who studies auditory neuroscience at Harvard University and also was not involved in the new study. “But learning how to turn down the hyperexcitability in the brain of someone with hearing loss could be helpful for hearing sounds in noise—as well as other related conditions, such as tinnitus and hyperacusis,” hypersensitivity to loud sounds.
