The brain's sound processing areas are split into two distinct regions — one which determines what a sound is, the other which tracks where it's coming from, according to research published online today (April 13) in Nature Neuroscience.

For decades, scientists have racked their brains to determine how the mammalian cerebral cortex handles different types of sensory information. In the visual system, two different brain areas are involved for determining what an object is and where it's located, but it was unclear whether this "what/where" hypothesis was true for the auditory system as well. Physiological recordings in animals, including humans, have been consistent with two pathways in auditory regions, but conclusive evidence was lacking. Now, researchers have separated the "what" and the "where" of hearing, and shown that dual-processing also exists in the auditory cortex.

"The brain breaks down information into two fundamental dimensions," said Stephen Lomber of the University of Western Ontario in London, Canada, who led the study. "This could be a defining principle for sensory information."

Lomber and his colleague, Shveta Malhotra of the University of Texas at Dallas, placed cooling coils on the surface of the brains of three cats to reversibly deactivate specific areas of the auditory cortex as the cats listened and responded to different sounds. They found that when they deactivated a region called the posterior auditory field, the cats failed to locate a sound; however, they could still discriminate between different sound patterns, indicating that only the "where" auditory pathway was affected. Conversely, when the nearby anterior auditory field was deactivated by cooling, cats could find where the sound was coming from, but they couldn't tell what type of sound it was, showing only the "what" pathway was affected.

David Moore of the MRC Institute of Hearing Research in Nottingham, England, who was not involved in the research, said the study shows a clear functional segregation of the hearing tasks tested, but he's not convinced by the conclusions that all "what" and "where" related auditory functions will be processed in the same regions. "What they've shown is that the areas they inactivated are indeed involved in those particular tasks," he told The Scientist. "There's a difference between the two particular sorts of test, but it doesn't necessarily give any more information that that."

Lomber has been using the cooling technique for deactivating specific brain areas for more than a decade, but he said: "This is the first time we've gotten results that were this awesome, to say the least." Jyrki Ahveninen of Harvard Medical School said the approach is a powerful way of causing temporary lesions in a healthy brain, but he worries that the technique may not be truly "reversible," and that longer term effects could interfere with the data. "If you cool part of the brain, it takes some time to heat back up," he said.

Also, it remains to be seen whether the dual pathway Lomber describes in cats also occurs in primates. Micah Murray of Vaudois University Hospital Center in Lausanne, Switzerland, described the cooling method as a "very interesting technique for emulating the behavioral consequences of stroke" in humans, but he cautioned that different functional regions of the brain might be involved in human hearing.