Broca's area electrocorticography shows that the frontal speech area is silent during articulation.

Damage to Broca's area in the anterior perisylvian cortex of the frontal lobe produces hesitant, broken, incomplete speech, termed Broca's aphasia. Because of the tendency to attribute specific functions to the same areas that, when damaged, result in loss of that function, Broca's area has been felt to be directly responsible for speech production in humans. As usual in the brain, the story is more complicated, as Finker et al in the study below show.

Patients who, generally due to epilepsy, had implanted cortical electrode arrays directly on Broca's area and nearby motor cortex were studied during conversation. It was demonstrated that Broca's area is active just before speaking, but is relatively inactive while speech is being uttered. This suggests that when we are deciding what to say, Broca's area encodes a motor sequence, presumably of phonemes, that is subsequently executed by motor cortex as our actual speech. It's as if the message is first written by Broca's area and then, about a half second later, that message is "printed out" by motor cortex controlling our bodies to speak the sounds of the words. On the article's second page, in a sentence that revises textbook understanding of speech production in humans, the authors state:

This temporal window of activity constrains Broca's area processing to pre-articulatory stages rather than to the on-line coordination of the speech articulators.

In other words, speech production is highly modularized between semantics and phonemic "articulatory codes." Lingustics has tried to divide semantics and syntax, but the brain seems to divide language at the level of the phoneme, with semantics perhaps then including everything above that dividing line.

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ABSTRACT

PNAS Early Edition > Adeen Flinker, doi: 10.1073/pnas.1414491112

Redefining the role of Broca’s area in speech

Adeen Flinker, Anna Korzeniewska, Avgusta Y. Shestyuk, Piotr J. Franaszczuk, Nina F. Dronkers, Robert T. Knight,, and Nathan E. Crone

Edited by Mortimer Mishkin, National Institute for Mental Health, Bethesda, MD, and approved January 26, 2015 (received for review August 4, 2014)

Significance

Broca’s area is widely recognized to be important for speech production, but its specific role in the dynamics of cortical language networks is largely unknown. Using direct cortical recordings of these dynamics during vocal repetition of written and spoken words, we found that Broca’s area mediates a cascade of activation from sensory representations of words in temporal cortex to their corresponding articulatory gestures in motor cortex, but it is surprisingly quiescent during articulation. Contrary to classic notions of this area’s role in speech, our results indicate that Broca’s area does not participate in production of individual words, but coordinates the transformation of information processing across large-scale cortical networks involved in spoken word production, prior to articulation.

Abstract

For over a century neuroscientists have debated the dynamics by which human cortical language networks allow words to be spoken. Although it is widely accepted that Broca’s area in the left inferior frontal gyrus plays an important role in this process, it was not possible, until recently, to detail the timing of its recruitment relative to other language areas, nor how it interacts with these areas during word production. Using direct cortical surface recordings in neurosurgical patients, we studied the evolution of activity in cortical neuronal populations, as well as the Granger causal interactions between them. We found that, during the cued production of words, a temporal cascade of neural activity proceeds from sensory representations of words in temporal cortex to their corresponding articulatory gestures in motor cortex. Broca’s area mediates this cascade through reciprocal interactions with temporal and frontal motor regions. Contrary to classic notions of the role of Broca’s area in speech, while motor cortex is activated during spoken responses, Broca’s area is surprisingly silent. Moreover, when novel strings of articulatory gestures must be produced in response to nonword stimuli, neural activity is enhanced in Broca’s area, but not in motor cortex. These unique data provide evidence that Broca’s area coordinates the transformation of information across large-scale cortical networks involved in spoken word production. In this role, Broca’s area formulates an appropriate articulatory code to be implemented by motor cortex.