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Figure 3 Activations in Broca's area proper, BA 44/6, are shown in a meta-analysis of functional imaging data for overt and covert speech paradigms (see Table II). Mean coordinates are normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=1.6 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); &,Bookheimer(1995); ®,Braun(1997); A, Fox (1999); ♦, Jennings (1998); 0,Paulesu(1993); ▼, Petersen (1988); W, Petrides (1993); V, Petrides (1995); □, Weiller (1995); }, Wise (1991). The points are distributed in both hemispheres but are predominantly found on the left, demonstrating the low degree of lateralization of BA 44/6. For all meta-analyses, data were taken at face value, such that those points reported by the author to be within a given subarea are displayed there, although some points may appear to go beyond the Brodmann areas typically included in that subarea.

Figure 3 Activations in Broca's area proper, BA 44/6, are shown in a meta-analysis of functional imaging data for overt and covert speech paradigms (see Table II). Mean coordinates are normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=1.6 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); &,Bookheimer(1995); ®,Braun(1997); A, Fox (1999); ♦, Jennings (1998); 0,Paulesu(1993); ▼, Petersen (1988); W, Petrides (1993); V, Petrides (1995); □, Weiller (1995); }, Wise (1991). The points are distributed in both hemispheres but are predominantly found on the left, demonstrating the low degree of lateralization of BA 44/6. For all meta-analyses, data were taken at face value, such that those points reported by the author to be within a given subarea are displayed there, although some points may appear to go beyond the Brodmann areas typically included in that subarea.

1. Overt Speech Activation

Posterior Broca's area is activated in fMRI and PET studies when overt speech is produced, specifically in repetition of words presented visually or aurally or generation of verbs or sentences in response to presented nouns. In trying to elucidate the exact reason for its activation, paradigms have been created in a hierarchical scheme. The most complex tasks, such as semantic processing, are performed along with phonological processing and a baseline rest condition so that subtracting one from another will isolate the activation area responsible for the semantic or pho nological portions of the task. Often in studies in which Broca's area proper is activated, activation is explained simply by stating that the area is involved in spoken language production. It is credited with a phonological rather than a semantic aspect of language production, but even the nature of that phonological language role is questioned. Petersen found that areas near Broca's area proper are activated bilaterally during movement of the mouth and tongue, and he credits Broca's area with general motor rather than language-specific output planning. In addition to this broadening of the function of Broca's area, one study by Petrides stated that speech can occur without i i

Figure 4 The pattern of activation for overt speech in the left hemisphere is shown through fMRI. The areas most active are Broca's area proper (B) and M1-mouth (A), whereas the prefrontal BA 46/47 shows no significant activation. The motor mouth area can be distinguished from Broca's area proper because the motor area extends well above BA 44/6.

Figure 4 The pattern of activation for overt speech in the left hemisphere is shown through fMRI. The areas most active are Broca's area proper (B) and M1-mouth (A), whereas the prefrontal BA 46/47 shows no significant activation. The motor mouth area can be distinguished from Broca's area proper because the motor area extends well above BA 44/6.

activation of BA 44: It is possible that Broca's area may not necessarily be activated in all speech production tasks. In a study by Braun that compared stuttering to normal speech, a paced speech task and an overlearned speech task showed decreased activation of BA 44 relative to narrative, self-paced speech activation in normal subjects. Braun proposed that in slow or overlearned conditions, it is easier to process and produce language plans. Broca's area, which may be crucial in initial word production or monitoring, is not needed once the given words are learned: Phonological or semantic monitoring may be less critical to the degree that significant engagement of the neocor-tical language areas is no longer essential. Figure 4 demonstrates the pattern of activation in the production of overt speech, concentrating in BA 44/6 and the Ml-mouth motor region.

2. Covert Speech Activation

The description of the function of Broca's area proper expanded from overt speech production when tasks involving inner, or covert, speech revealed activation and complicated the classical description. BA 44/6 is activated under conditions of covert verb generation in response to heard nouns presented at a slow rate. Wise postulated that Broca's area shows activation in an automatic, internal speech response to remembering words even though vocalization is not the goal: The act of retrieving words from semantic memory activates networks concerned with the production of speech sounds. Another aspect of covert speech is subvocal rehearsal, or internal repetition of speech often coupled with phonological working memory. Paulesu described an articulatory loop that he tested with a short-term memory task for letters and rhyming judgment for letters. Both tasks required subvocal rehearsal, and both showed strong activation in Broca's area, whereas only the memory task activated the left supramarginal gyrus. He concluded that Broca's area is crucial to the subvocal rehearsal system. Assuming a connection of Broca's area to subvocal rehearsal, Caplan set out to prove that Broca's area is involved in syntactic processing when subvocal rehearsal is prevented. Subjects judged the goodness of sentences that represented two distinct levels of syntactic difficulty, and as they made judgments they repeated the word "double" timed with a metronome. The task contrast showed increases in rCBF in Broca's area in judging the more difficult type

Shoulder Muscle Axial

Figure 5 Covert speech, or thinking about words without vocalization, shows heightened activity in left pre-Broca's area, BA 46/47. This indicates that motor aspects of speech are not necessary for this area to be activated, and pre-Broca's is clearly defined in an area forward of Broca's area proper that is probably linked more closely to semantic processing of words than speech or phonological processing.

Figure 5 Covert speech, or thinking about words without vocalization, shows heightened activity in left pre-Broca's area, BA 46/47. This indicates that motor aspects of speech are not necessary for this area to be activated, and pre-Broca's is clearly defined in an area forward of Broca's area proper that is probably linked more closely to semantic processing of words than speech or phonological processing.

of sentences: The increase in rCBF in Broca's area reflects the demands of processing the more complex syntactic structure. The possible explanations given are maintenance of the complex structure in working memory or need for more intermediate constructs in computation of the more complex sentences; either explanation supports the addition of syntactic processing to the list of Broca's area functions. Figure 5 shows activation produced when thinking about words, including activation in BA 44/6.

3. Nonlanguage Activation

Movement in imaging tasks that does not incorporate speech shows activation in Broca's area. Both overt and imagined movements of the arm and hand cause activation in BA 44. This activation might be explained by subjects' unintentional performance of covert speech during the movement tasks. Parsons used a task that involved judging whether a presented drawing of a hand was of a right or a left hand, thus engaging the brain to shift the mental representations of its own hands to try to imitate the presented hands. Parsons suggested that since the activation was in the limb-contralateral hemisphere, it was correlated with the actions of the hand covertly moving: The activated BA 44 site subserves the mental simulation of reaching a specific limb into a specific target posture. Also, in a study of saccadic eye movements, activation was found in ventral BA 6, on the edge of Broca's area, when saccades were performed in the dark. This overt movement was performed in proprioceptive space, as opposed to a similar task of saccades to visual targets, performed in visual space, that activated the dorsal premotor area BA 8. This distinction between dorsal and ventral premotor activation suggests that BA 44/6 may be responsible for motor planning in somatic space for movement that cannot be calculated within the visual field. Saccadic eye movements in the dark as well as oral movement must employ somatic planning, and BA 44/6 may be the ventral, somatic counterpart to dorsal premotor areas.

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