Washington, DC – Leading neuroscientists say they are close to an accepted unified theory about how the brain processes speech and language - and animals have some of these mechanisms too.
In the June issue of Nature Neuroscience, Josef Rauschecker of the Georgetown University Medical Center and Sophie Scott of University College, London say that both human and non-human primate studies have confirmed that speech 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.
Both pathways begin with the processing of signals in the auditory cortex. Information processed by the 'what' pathway then flows along the outside of the temporal lobe, allowing the recognition of communication sounds and their meaning.
The 'where' pathway is mostly in the parietal lobe, above the temporal lobe. It processes the location and motion in space of a sound, but is also involved in providing feedback during speech.
In humans, researchers use functional magnetic resonance imaging to observe activity move between brain regions in experiments testing speech recognition. In non-human primates, investigators use a technique known as single-cell recording, which can measure changes within a single neuron. This can be used only rarely in humans, but provides much better resolution.
"In both species, we are using species-specific communication sounds for stimulation, such as speech in humans and rhesus-specific calls in rhesus monkeys," Rauschecker says. "We find that the structure of these communication sounds is similar across species."
What is so interesting to Rauschecker is that the findings suggest that "in evolution, language must have emerged from neural mechanisms at least partially available in animals," he says.
"Speech, or the early process of language, is well modeled by animal communication systems, and these studies now demonstrate that primate auditory cortex, across species, displays the same patterns of hierarchical structure, topographic mapping, and streams of functional processing," Rauschecker says. "There appears to be a conservation of certain processing pathways through evolution in humans and nonhuman primates."