Researchers have used a rare surgical procedure to map the part of the human brain associated with processing language, including grammar and speech.

This X-ray shows the electrodes that surgeons use to find and remove areas of the brain that cause seizures. (Ned Sahin) Three patients who were getting brain surgery to treat epilepsy agreed to have the electrical activity in their brains measured while they were asked questions about language.

The patients all had a network of electrodes placed into their brains to determine which regions were healthy and which were causing the patients' seizures. Surgeons use the technique to pinpoint which areas of the brain to remove to stop the seizures.

The researchers asked the patients questions after the electrodes were inserted and before the surgery was scheduled to begin.

The patients, while awake and responsive, were asked to repeat a given word, then to give derived forms of the word, such as plurals for nouns or past tense forms for verbs.

The patients were also asked to mouth the words without speaking.

The researchers used a procedure called intra-cranial electrophysiology to measure brain activity while the patients were being quizzed on grammar. The technique gave the scientists information on changes in activity accurate down to the millimetre and the millisecond.

Because opening a person's skull and inserting electrodes into his brain is something surgeons would only do when medically necessary, the study gave researchers a rare look into human brain function.

A three-dimensional scan showing the positions of electrodes used in the study. (Ned Sahin) Researchers in other fields of neurology can use animals to study brain activity, but complex language is unique to humans.

Neurologists studying language do use MRI scans of the brain to map where language is processed, but the resolution of such scans is much lower than the intra-cranial technique.

The research focused on a portion of the brain called Broca's area, known to be important in language and speech.

French doctor Pierre Paul Broca found in 1865 that two of his patients who were unable to produce more than a few words had lesions on their brains in the same spot.

"In the 150 years since this discovery, progress in the understanding what precisely Broca's area contributes to language has been disappointing," said Eric Halgren of the University of California at San Diego.

The research found that the different language processes the scientists tested were in separate but partly overlapping locations within Broca's area. They found that the brain's computation of vocabulary, grammar and articulation took place very quickly: roughly 200, 320 and 450 milliseconds, respectively.

"We showed that distinct linguistic processes are computed within small regions of Broca's area, separated in time and partially overlapping in space," said Ned Sahin of UCSD, first author on the study, which appeared this week in the journal Science.

Sahin said the research dispels some notions that are still taught in medical textbooks, such as the idea that Broca's area is only involved in speaking, while reading and hearing are handled in a different part of the brain.

"Our task involved both reading and speaking, and we found that aspects of word identity, grammar and pronunciation are all computed within Broca's area," he said.