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brain: Seeing is a hearing aid 


Hearing: the eyes and the ears have it.

When it comes to hearing, what you see is what you get. Which way people's eyes are pointing alters the brain signals generated by sounds, researchers at the Center for Cognitive Neuroscience in New Hampshire have found1.

"Combining what we see and hear seems to be a more profound process than we previously thought," says Jennifer Groh, leader of the study. Her results will help researchers to understand how the brain assembles a picture of the outside world.

When we hear a telephone ring, our brains compare the different noises arriving at each ear to figure out where the sound is coming from. The brain works out where the phone is based on the position of the ears and head.

"But the eyes can move independently of the head," Groh explains. "So our different senses are getting two different views of the world, based on eye and head position and our brains compare them to try to make a coherent picture." 

To investigate where in the brain the two versions merge, the researchers trained monkeys to look right, left or straight ahead while sitting in the centre of a semicircle of speakers. The team measured the signals coming from the inferior colliculus (IC), part of the brain that relays information from the ears to sound-processing centres. 

In a third of the nerve cells the same sound produced a different rate of firing depending on where the eyes were pointing. "We were astonished," says Groh. "The IC is not usually thought to be involved in advanced problems like coordinating hearing and vision." 

"This is the first report of eye position influencing hearing responses this early in the pathway," agrees Eric Knudsen, a neurobiologist working on sound localization at Stanford University in California. 

"It's long been known that what we see can influence what we hear," says Knudsen.

A classic example of this is ventriloquism. The brain is fooled into believing that the voice is coming from a puppet, because we see the puppet's mouth move. 

Movie sounds are another illusion. "The soundtrack comes from the speakers along the side of the theatre," says Groh, "but we perceive the voices as coming from the characters on the screen."

If the IC really puts sight and sound together, Knudsen questions why the other two-thirds of nerve cells aren't also affected by eye position. "The effect is clear, but puzzling," he says. The change in activity could be the result of a shift in attention rather than eye position, he points out.

  1. Groh, J.M., Trause, A.S., Underhill, A.M., Clark, K.R. & Inati, S. Eye position influences auditory responses in primate inferior colliculus. Neuron 29, 509518 (2001).
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