Scientists Able to Eavesdrop on Brain, Map ‘Trails’
Imagine a voyage through a structure scientists call the most complex in the known universe, a trip to the wellspring of poetry, quantum mechanics, symphonies and crime.
The first step is over a bridge, the corpus callosum, through a fissure much like any of the geological fractures that define Africa’s Great Rift Valley.
But you’re in a living brain.
Destination: regions of language processing.
This is a tour for which brain cartographers have begun to produce highly detailed maps targeting sites involved in understanding words that are spoken, read or heard.
Scientists are just scratching the surface of language processing, one of the functions that set humans apart from all other animals. They are charting language processing regions by testing responses to single words or syllables while watching a living brain process the information.
“We can see these areas light up,” said neurophysiologist Steven Petersen of the Washington University School of Medicine in St. Louis, referring to high-tech computerized studies of the brain.
He and a team of scientists are eavesdropping on the three-pound organ while it does some of the things it does best: thinking, seeing, hearing and producing speech.
Using the imaging technique known as positron emission tomography (PET), which converts digital information from the brain into computer-generated pictures, scientists are recording brain activity in real time. PET studies such as these permit researchers to freeze-frame the types of brain activity that underlie a lawyer’s ability to argue a case in court or enable an infant to utter mommy or daddy for the first time.
The brain responds strikingly to language, the new studies confirm, bathing specific sites involved with hearing, sight, speech and language in a surge of blood at the sight or sound of the simplest word.
As a result, the St. Louis team has produced an unprecedented set of “maps” that chart how a single word is perceived and understood by measuring blood flow in regions associated with sight, hearing, speech and motor activity.
When test subjects repeat a word they have have heard, nerve cells in certain of these regions fire, blood flow increases, and areas that trigger muscle activity power the lips, tongue and voice to initiate the response. Still other brain regions are seen to flash with nerve cell activity and a rush of blood when a word is read and a response to it spoken.
“This is just part of the story,” said neurologist Dr. Marcus Raichle, who led the investigation.
“We have known for 100 years that when neurons (nerve cells) change their activity because of something we do, blood flow in that area of the brain goes up immediately. So measurement of blood flow is a good indicator of what’s going on in a particular region of the brain.”
He said these investigations, which are to be followed by many more, support prevailing scientific wisdom that the human brain is the master “parallel processing” machine, which means that several of its regions respond at once to perform a single task.
To see how the brain processes a single word, Raichle and his team injected a group of 17 healthy volunteers with a short-lived radioactive tracer. Up to 10 PET scans were taken of each subject.
“In one test, we would show them a visual word, like cake, “ explained Petersen, “and they would have to respond with an associated word like eat or bake. “
Blood Flow Mapped
This process instantly called on the brain’s visual cortex area located at the rear of the brain. But the maps of blood flow showed that nerve cells zap a representation of the word to the left frontal area of the brain where it apparently was decoded, because blood flow increased there.
This is contrary to current scientific dogma suggesting that the message would first have to land in the brain’s mid-region associated with semantics before moving toward the front. Nevertheless, the map is completed with an indication of blood flow increase toward the top of the brain in an area associated with control of motor activity.
It took less than a second for subjects to speak the response, Petersen said.
In the test where subjects repeated a word, blood flow increase was detected in the mid-brain area associated with perception of sound before increasing in the left frontal area which controls speech, Petersen said.
But even with the new results, scientists say they are in the dark when it comes to producing a precise description of how language is processed.
The St. Louis illustrations indicating the path of blood flow are intriguing, said USC neuroscientist Richard Thompson, but the results of such studies have not been accepted as scientific fact.
Even the St. Louis researchers are baffled by some of the results, because when brain responses of the subjects were tested with nonsensical sounds, such as buzzes, beeps and monotones, none reacted with an instant surge of blood in language processing regions. Raichle says this suggests that positive responses to actual words were a result of the brain’s ability to instantly call upon its “dictionary” to initiate a response.
Complicated Process
Thompson argues that word storage is a complicated process involving memory that may store words for certain categories of things in specific sites of the brain.
“Many individuals have postulated that memories for words are all over hell and gone,” he said. “But I think we’ll eventually find that the brain is much more specialized. In one stroke patient, the only thing the illness abolished was the man’s memory for the names of fruits and vegetables. So there probably are regions in the brain that store specific types of words.”
Thompson laments that the bulk of scientific literature involving language processing is derived from cases where something in the brain went awry, primarily as a result of stroke or an accident that disrupted normal function.
“The study of human brain damage is an inexact science because you primarily study it during autopsy,” Thompson said. “And the major problem with autopsies are the changes that occur in the brain after death. You never get a good representation of what it was like before.”
He noted that famous 19th-Century French physician and researcher Pierre Paul Broca discovered the area of the brain (which now bears his name) as the center of speech after autopsies on former patients who had lost their ability to speak.
Broca believed that a section in the brain’s lower left hemisphere controlled speech, a fact proved time and again in subsequent autopsies.
The other major region of the brain associated with language processing, Wernicke’s area, located a short distance behind Broca’s, controls semantics or meaning of language. That area, also in the left hemisphere, was discovered by 19th-Century German neurologist Carl Wernicke, who found that a patient spoke articulately but in a flow of words seemingly without thought.
An autopsy later revealed a lesion in the semantics area scientists also associate with comprehension of reading and writing.
Century-Old Studies
Raichle, who reported his study in a recent issue of the British scientific journal Nature, said his investigations of blood flow and language processing grow out of a series of studies dating back nearly a century.
He cited work by British scientists in 1890 that correlated the hyperactivity of brain cells in epileptic seizures to an immediate rush of blood that caused the brain to swell, Raichle said.
This observation led to the hypothesis that blood supply in any portion of the organ occurs in response to the neurochemical conditions underlying the function in that part of the brain at a specific time.
Because epileptic seizures cause a diffusion of activity in nerve cells throughout the organ, blood flow increases dramatically in all regions, leading to swelling of the brain, Raichle explained.
By the 1960s, he said, scientists were using gamma ray cameras to investigate cerebral blood flow, but pictures of the precision of mental activity were not always clear, leaving much about the neurophysiology of language processing to conjecture.
Petersen said, however, that with the PET examinations, scientists obtained the clearest images to date of actual mental processing in defined regions of both the left and right hemispheres of the brain.
He said even though the language centers of the brain are on the left side, the organ uses both of its hemispheres to process written and spoken language. In a sense, he said, the PET investigations permit scientists to literally “read” someone’s thoughts in the form of patterns that blood forms as it courses through the brain.
