L'aire cérébrale de Broca remise en question

http://www.nature.com/news/2007/070423/full/446956b.html / Published online: 25 April 2007; | doi:10.1038/446956b

Brain's speech site is revisited and revised

Scans of pickled organs shed fresh light on Broca's area.

Kerri Smith

Analysis of two damaged brains, preserved in a museum since the nineteenth century, could force neuroscientists to rethink the area where language resides in the brain.

In 1861, the French surgeon and anatomist Paul Broca described two patients who had lost the ability to speak. One patient, Lelong, could produce only five words, and the second, Leborgne, could utter only one sound - "tan". After their deaths, Broca examined their brains and noticed that both had damage to a region in the frontal area on the left side. Broca's area, as it became known, is now thought to be the brain's speech-processing centre.

Broca kept the patients' brains for posterity, preserving them in alcohol and placing them in a Paris museum. And that's where Nina Dronkers, of the VA Northern California Health Care System in Martinez, and her colleagues picked them up, in order to reinspect the damage using magnetic resonance imaging.

Leborgne's brain had been scanned twice before, but not Lelong's. And neither had been compared with modern interpretations of Broca's area. After the team put the two brains through a scanner, they came up with a surprising finding: in both patients, the damaged area was much larger than the region that is now considered to be Broca's area.

The brain of Lelong, one of Broca's patients, about to be scanned. N. DRONKERS

"We were noticing that what people were calling Broca's area encompassed large areas of the frontal lobe," says Dronkers. The scans show that neither of the old brains had damage that affected the whole region now known as Broca's area. But damage also stretched far into other regions beyond this spot.

Broca realized this at the time, says Dronkers, and noted that the areas of damage were different in the two patients. But his conception of the area involved in speech processing has become simplified by others over time, the authors argue. They published their findings online earlier this month in the journal Brain (N. F. Dronkers, O. Plaisant, M. T. Iba-Zizen and E. A. Cabanis Brain doi:10.1093/brain/awm042; 2007).

This misplaced focus could lead to problems when diagnosing people with language impairments, says Dronkers. By assuming that only one small area of the brain is responsible for language, clinicians might overlook other regions involved in speech production. In other words, focusing too heavily on Broca's area could be missing the point, Dronkers argues.

Others agree. "There's a tendency for researchers to see activation in somewhere like Broca's area and to say 'oh well, we're tapping into a language area'," says Joseph Devlin, a neuroscientist at the University of Oxford, UK, who images language networks in the brain.

Newer imaging techniques may also help researchers to discover what Broca was unable to see. Dronkers and Devlin are both working on the use of alternative imaging techniques to investigate other regions of the brain that may be important in language processing but which are not detected by magnetic resonance imaging, such as the tracts of white matter that connect areas of grey matter.