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Brain has center for detecting sound motion

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16.09.2004

Close your eyes and acutely listen to the sounds around you, and you’ll find you’re able not only to accurately place the location of sounds in space, but their motion. Imagine then that, strangely, you suddenly became unable to distinguish the motion of sounds, even while you retained the ability to pinpoint their location. That’s exactly the experience of a patient reported by Christine Ducommun and her colleagues, who used studies of the patient to demonstrate conclusively for the first time that the brain has a specialized region for processing sound motion.

While it was known that the visual system has a specialized region for perceiving motion, it wasn’t known whether the auditory system has such a region--or whether sound location and motion are processed by the same circuitry. Previous studies of the capabilities of brain-damaged patients had found only that both their location and motion processing abilities were impaired, and animal and human neuroimaging studies had not been able to conclusively tease apart the two abilities.

Ducommun and her colleagues discovered the region by studying a woman who was to be operated on to alleviate intractable temporal lobe epilepsy. The operation would involve the removal of the affected regions of the right anterior temporal lobe and the right posterior superior temporal gyrus (STG).

Perceptual tests before the operation showed her ability to perceive sound location and motion to be normal. In those tests, the researchers played white noise through headphones and simulated movement of the noise source, asking the patient to detect the noise location and how it seemed to be moving. The patient could also identify sounds such as a piano, dog, water, sneezing, and a saw.

Also, the researchers electrically stimulated the brain region to be removed--using electrodes already in place to monitor epileptic activity. They found that such stimulation of the right posterior STG led the patient to report that "there was a humming sound coming toward [her] face." When the researchers presented moving sounds to the patient while they recorded from the electrodes, they also detected the strongest activity in the right posterior STG.

After the operation, however, the patient showed what the researchers dubbed "cortical motion deafness." During perceptual tests, she stated "I do not perceive the sounds as moving at all, but rather as being completely stable." In contrast, she could still perceive the location of sounds and their identity. Also, all her visual abilities remained normal.

While her ability to detect sound motion improved over the next three years, she still remained deficient. According to the researchers, such improvement likely was due to partial recovery because of adaptive changes in her cortex.

According to the researchers, their findings "provide evidence of an auditory motion module within the right posterior STG" and that by analogy with the motion-detection region of the visual system this region "may constitute the core region for auditory motion analysis."

Heidi Hardman | Source: EurekAlert!
Further information: www.cell.com

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