Making sense of sounds.

Byline: Greg Bolt The Register-Guard

Even the sound of a single note sets off a chorus in your brain.

That's what researchers at the University of Oregon have discovered in some of the first experiments to reveal how the brain is able to process sound and determine where it's coming from. Their work could ultimately speed the development of new techniques or devices to help the elderly or hearing impaired.

In a study published in the current issue of Nature magazine, UO researchers Avinash Bala and Matt Spitzer, working with biology professor Terry Takahashi, found that hearing even simple sounds involves the work of a whole suite of neurons in the brain. Using barn owls to understand how the brain is able to detect a sound and locate its source in the environment, the pair discounted earlier assumptions that the brain only "listens" to the very small number of neurons that show the highest level of activity in response to sound.

In essence, what they found is that instead of paying attention to a neural soloist, the brain hears an entire chorus of neurons and takes an average to figure out where a sound is coming from.

"What this research gives us is an easier way to understand how sounds are separated in the brain and how one sound is compared to all the other sounds," said Bala, who is doing post-doctoral research at the UO.

He and Spitzer focused on a portion of the brain called the inferior colliculus, which is part of the area responsible for spatial perception. It contains a kind of three-dimensional map of the space around us that helps coordinate vision, hearing and other sensory information.

Extremely thin wires were inserted into an owl's inferior colliculus to find the neurons in the brain's "space map" that correspond to a particular point in space. A sound originating at that point will cause a sharp spike in the activity of those corresponding neurons, while surrounding brain cells show progressively weaker responses.

Owls have particularly good hearing and are able to locate prey in the dark using very faint sound, such as a mouse skittering along the forest floor. They're able to pick out those slight sounds even when they're mixed in with background noises.

In this phase of their research, Bala and Spitzer wanted to know how the owls could pick out one sound from another nearby sound. Scientists know how sound is converted to an electrical signal that is sent to the brain and they can see the reaction once that signal is processed, but up until now, they didn't know what happened in between.

The tests measuring neural activity in the brain's space map confirmed that a sound in a particular location excited very specific neurons at the point on the space map that correspond to the point in space where the sound originates. That's akin to a blip on a radar screen.

But unlike radar, the brain takes all the blips - the strong ones and weak ones - to locate the sound. By measuring the owl's physical responses to a test sound and comparing them to the neural responses, the researchers were able to determine that the brain averages the neural signals rather than focusing only on the strongest ones to detect location.

That's important because it protects an animal's ability to locate sound even if a few neurons in the space map are disabled. Otherwise, damage to a few neurons in the space map would leave a blank space where sound couldn't be located.

The discovery also could lead to a better understanding of a common hearing problem that often develops in older people, the inability to pick out one sound when it's competing against a lot of background sound. It's a condition that allows people to hear well in a quiet room but not in a noisy restaurant.

Bala said it could be that the neurons that should be showing the sharpest blips on the space map's radar screen instead have degenerated to weak blobs. That would throw off the brain's averaging mechanism that helps locate sounds.

"It's possible in some kinds of hearing loss those blips are converted to fuzzy blobs and it's harder to tell sounds apart," he said.

Bala and Spitzer are continuing their work with owls to shed more light on that aspect of hearing.

They want to know how the brain is able to pick out one particular sound from a cacophony of background sound.

Think of a cocktail party. Everyone is talking at the same time at about the same volume, and you're stuck in a boring conversation. Soon you find yourself tuning out the conversation you're in and eavesdropping on people talking off to the side or behind you.

"The amazing thing about that is you don't even need to move your head," Bala said. "That's amazing, and we don't know how the brain does that."

CAPTION(S):

Matt Spitzer, Terry Takahashi and Avinash Bala are using owls such as this one to research how the brain is able to distinguish particular sounds from among many. Their findings could lead to better understanding of various types of hearing impairments including those associated with aging. "What this research gives us is an easier way to understand how sounds are separated in the brain ..." - AVINASH BALA, UO RESEARCHER