Brain images reveal key language areas.

Brain images reveal key language areas

When a person looks at a string of letters, a specific area at the back of the brain quickly determines whether those letters meet learned criteria for a word, report researchers who have conducted a new brain-imaging study. If the string of letters passes muster, it then rapidly activates a section of the left frontal lobe, they say, suggesting that this brain region helps assign meaning to words. The twopart evaluation process apparently occurs without conscious awareness.

These finding, described in the Aug. 31 SCIENCE, support the recet psychological theory that some aspects of vocabulary and semantic processing automatically commence without conscious effort whenever a word is read.

Neurologist Steven E. Petersen of Washington University in St. Louis and his colleagues mapped blood-flow changes in the brain with positron emission tomography (PET). Increased blood flow in a particular area reflects greater activity there. The scientist injected minute amoutns of a radioactively labeled oxygen compound into eight healthy, right-handed adults with good reading skills. This tracer remains active in the body for only a few minutes, allowing for a rapid series of PET scans. The scans record gamma rays emitted as the radioactive isotopes decay; a computer then generates color-coded images of blood flow.

The team scanned participants looking at four lists, each presenting a different type of stimulus: common nouns; "pseudowords" that followed English spelling rules (such as FLOOP); consonant letter strings (such as JVJFC); and strings of curved and straight lines not corresponding to any alphabetical letters. Each list consisted of 256 words or stimuli. One stimulus per second appeared on a computer screen, remaining visible for 150 milliseconds.

To pinpoint brain activity specific to each task, computer software compared individual PET scans taken while participants concentrated on a blank screen with scans taken during each of the four trials.

Petersen's team found that only the real words andpseudowords activated the left medial extrastriate visual cortex, located in the rear brain near regions that handle visual information. Right-handers process most basic language tasks in the left brain. The left-brain area detected by the scans appears to distiguish between letter strings that do or do not conform to learned spelling rules, and it does so just as visual processig gets underway, the researchers assert.

To identify brain activity prompted by general visual features, the team used PET scans showing volunteers' responses to nonalphabetical symbols. When compared with scans taken during other tasks, there revealed an important difference between brain activity sparked by real words and by pseudowords: Only real words elevated blood flow in the left frontal lobe. In a previous PET study, Petersen and his associates chartered increased blood flow in the same brain region among people who read a list of 40 nouns and verbally reported a use for each item (SN: 4/30/88, p. 281).

The left frontal lobe area must handle some yet-unspecified, automatc aspect of assigning meaning to individual words, they propose.