Brain’s Speech Processing Center Broader than Realized

This was previously discussed on the blog here, but more information is also below.

March 8, 2012 – The brain’s speech processing center is much larger than previously reported, a new study shows.

The newly identified zone spans beyond Wernicke area and is closer to the front of the brain.

Scientists have long believed human speech is processed toward the back of the brain’s cerebral cortex, behind the auditory cortex where sounds are received. This region is known as the Wernicke area. However, mounting evidence suggests that this zone may be much larger.

New results, published in the February 21 Proceedings of the National Academy of Sciences, may prompt textbook rewrites on the location of the brain’s speech processing center, the researchers say.

It is a controversial issue spurred on by other researchers as well, who are questioning the location of the Wernicke area.

“This study provides a definitive, irrefutable answer,” senior investigator Josef Rauschecker, PhD, from Georgetown University Medical Center in Washington, DC, said in a news release.

Dr. Rauschecker’s team analyzed more than 100 imaging studies from several laboratories and identified an area about 3 cm closer to the front of the brain and on the other side of auditory cortex – miles away from the Wernicke area in terms of brain architecture and function.

“Functional magnetic resonance imaging and positron emission tomography brain imaging implicate a much broader portion of the superior temporal gyrus and sulcus in speech comprehension than has been previously appreciated,” Iain DeWitt, a PhD candidate at Georgetown, told Medscape Medical News. He pointed out that the region near the anteriolateral aspect of the Heschl gyrus is robustly activated in auditory word processing experiments.

The investigators suspect greater similarity in organization between the visual and auditory cortices than was previously appreciated.

“Clinically, it suggests we may have been missing something in analysis of lesion data, both from temporal lobe resections and from stroke,” DeWitt said. “Our work suggests we ought to take a closer look at the effect of anterior superior temporal gyrus and sulcus lesions on auditory comprehension.”

The prevailing sentiment in the field was that word recognition occurred in the posterior superior temporal gyrus and sulcus or Wernicke area, DeWitt said. “Therefore, it was an open question about just where the totality of the evidence from functional imaging stood on the matter of anterior superior temporal gyrus involvement in word recognition. When I began a systematic review of the literature, the surprising thing was just how many papers actually found anterior superior temporal gyrus activation in auditory word processing, but made little notice of the result. Many papers even went out of their way not to see what was clearly in the data.”

These new findings support the anterior-directed hierarchical account of word recognition by Binder and colleagues ( Cereb Cortex. 2000;10:512-528). They also back the Cohen et al hypothesis of an auditory word-form area in the left anterior superior temporal gyrus (( Neuroimage. 2004;23:1256-1270).

“If you Google ‘language organization’ in the brain,” DeWitt said, “probably every cartoon illustration out there is wrong.”

A lot of evidence from brain imaging and primate electrophysiology and neuroanatomy support their conclusions, DeWitt pointed out.

“The evidence from lesion studies is sparser and, perhaps, more equivocal. We really just don’t have high-resolution, well-controlled studies using modern methods, like lesion symptom mapping, to estimate acute auditory comprehension deficits following anterior superior temporal gyrus and sulcus lesions,” he noted.

“There is probably 1 study of high quality that I am aware of and it appears to support our findings,” he concludes. “Critics and myself alike will want to see more work done in this particular area to show consistency between neurological data and functional imaging data.”

The researchers have disclosed no relevant financial relationships.

Proc Natl Acad Sci. 2012;109:E505-E514. Abstract


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