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TO BE…OR NOT TO BE

Neuroscientists feel they are much closer to an accepted unified theory about how the brain processes speech and language, according to a scientist at Georgetown University Medical Center who first laid the concepts a decade ago and who has now published a review article confirming the theory.

In the June issue of Nature Neuroscience, the investigator, Josef Rauschecker, PhD, and his co-author, Sophie Scott, PhD, a neuroscientist at University College, London, say that both human and non-human primate studies have confirmed that speech, one important facet of language, is processed in the brain along two parallel pathways, each of which run from lower- to higher-functioning neural regions.

These pathways are dubbed the “what” and “where” streams and are roughly analogous to how the brain processes sight, but are located in different regions, says Rauschecker, a professor in the department of physiology and biophysics and a member of the Georgetown Institute for Cognitive and Computational Sciences.

Both pathways begin with the processing of signals in the auditory cortex, located inside a deep fissure on the side of the brain underneath the temples – the so-called “temporal lobe.” Information processed by the “what” pathway then flows forward along the outside of the temporal lobe, and the job of that pathway is to recognize complex auditory signals, which include communication sounds and their meaning (semantics). The “where” pathway is mostly in the parietal lobe, above the temporal lobe, and it processes spatial aspects of a sound – its location and its motion in space – but is also involved in providing feedback during the act of speaking.

Auditory perception – the processing and interpretation of sound information – is tied to anatomical structures; signals move from lower to higher brain regions, Rauschecker says. “Sound as a whole enters the ear canal and is first broken down into single tone frequencies, then higher-up neurons respond only to more complex sounds, including those used in the recognition of speech, as the neural representation of the sound moves through the various brain regions,” he says.

Both human and nonhuman primate studies were examined in this review.

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