How the Brain processes speech

[Guest guest]

Scientists reaching consensus on how brain processes speech

May 26, 2009

Source: town University Medical Center

Neuroscientists feel they are much closer to an accepted unified theory

about how the brain processes speech and language, according to a

scientist at town 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, f

Rauschecker, PhD, and his co-author, Sophie , 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 town 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.

In humans, researchers use functional magnetic resonance imaging (fMRI)

to " watch " activity move between brain regions in experiments testing

speech " (re)cognition,

" Rauschecker says. In non-human primates, investigators use a technique

known as single-cell recording, which can measure changes within a

single neuron. To do this, anesthetized animals are equipped with

microelectrodes that can pick up activity in pinpointed brain areas, a

technique that can be used only rarely in human patients 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 although speech and

language are considered to be uniquely human abilities, the emerging

picture of brain processing of language suggests " 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. "

While this research is basic science trying to solve fundamental

questions about the brain, it may ultimately yield some valuable

insights into disorders that involve problems in comprehending auditory

signals, such as autism and schizophrenia, he says.

" Understanding speech is one of the major problems seen in autism, and a

person with schizophrenia hears sounds that are just hallucinations, "

Rauschecker says. " Eventually, this area of research will lead us to

better treatment for these issues.

" But mostly, we are fascinated by the fact that humans can make such

exquisite sense of the slight variation in sound waves that reach our

ears, and only lately have we been able to model how the brain knows how

to attach meaning to these sounds in terms of communication. " he says.