Pre Brocas Area

Semantic function in language, or analysis of word meaning, has been shown by functional imaging to involve the cortical areas immediately anterior to Broca's area proper, prefrontal BA 46/47. Figure 6

Figure 6 Activations in pre-Broca's area, BA 46/47, are shown in a meta-analysis of functional imaging data for covert speech paradigms. A set of mean coordinates was calculated from data reported from studies in Table II. The coordinates have been normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=1.2 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); □, Paus (1993); •, Braun (1997); A, Buckner (1995); ♦, Raichle (1994); O, Warburton (1999); ▼, Petersen (1988); W, Petrides (1993); V, Petrides (1995); □, Weiller (1995); }, Wise (1999). There is a relatively wide spread of data represented in the mean points; this is due to the fact that the data reported by authors were taken at face value, even in cases in which mean coordinates go outside the usual Brodmann areas associated with pre-Broca's area. The data reflect the fact that activation of pre-Broca's area is strongly left lateralized, especially when compared with the activation patterns in Broca's area proper.

Figure 6 Activations in pre-Broca's area, BA 46/47, are shown in a meta-analysis of functional imaging data for covert speech paradigms. A set of mean coordinates was calculated from data reported from studies in Table II. The coordinates have been normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=1.2 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); □, Paus (1993); •, Braun (1997); A, Buckner (1995); ♦, Raichle (1994); O, Warburton (1999); ▼, Petersen (1988); W, Petrides (1993); V, Petrides (1995); □, Weiller (1995); }, Wise (1999). There is a relatively wide spread of data represented in the mean points; this is due to the fact that the data reported by authors were taken at face value, even in cases in which mean coordinates go outside the usual Brodmann areas associated with pre-Broca's area. The data reflect the fact that activation of pre-Broca's area is strongly left lateralized, especially when compared with the activation patterns in Broca's area proper.

shows a meta-analysis of functional imaging data reporting activation in BA 46/47 in language paradigms. This region is often, but not always, activated when language tasks include semantic processing, but it does not become active in phonological tasks. Some studies that report activation in the left frontal lobe do not differentiate between activation in this area and in BA 6/44, and it has been hypothesized that as a result, some coordinates reported for what is traditionally considered Broca's area are shifted forward. However, language tasks are not the only tasks to activate BA 46/ 47. Episodic memory (i.e., memory of materials encountered in specific episodes) and implicit memory, (i.e., experience-induced changes in performance on indirect tests unrelated to the experience) have each been linked to BA 46/47. Like Broca's area proper, this area cannot be simply classified as a language processing area.

1. Overt Speech Activation

BA 46/47 is activated in tasks that involve narrative speech, verb generation, and stem completion, suggesting that the area plays a role in semantic processing. Specifically, verb generation in response to visual presentation of nouns activates BA 46/47, but repetition of words does not. In Petersen's study of single-word processing, activation from repetition of visually represented words is subtracted from that of generation of verbs: A left inferior frontal area was identified that almost certainly participates in processing for semantic association. Word repetition activates areas responsible for phonological processing, and subtracting those away from areas highlighted by generation of verbs reveals BA 46/47 as an area responsible for semantic processing. Weiler modified this subtraction by substituting pseudowords in the repetition condition, which might further reduce incidental activation of semantic areas. He drew a similar conclusion from the results: A modulatory, controlling function of nonautomatic, intrinsic generation processing can be attributed to the lateral prefrontal cortex. In a task generating spontaneous narrative speech and one constructing sentences from provided verbs, Braun demonstrated activation of BA 47. In the same study, a paced narrative task and an automatic speech task showed no activation in the area, which Braun attributed to lower semantic processing demands in the structured and automatic tasks. The multiple variations of semantic processing that are required in these tasks provide convincing evidence that BA 46/47 plays a key role in analysis of word meaning.

2. Working Memory Activation

Semantic working memory is another function attributed to the region of 46/47. The concept of semantic processing is basically retrieving word meanings; therefore, the line differentiating semantic working memory from semantic processing is uncertain. Gab-rieli offered a view of BA 46/47 activation based on his study of semantic versus phonological analysis of words: Left inferior prefrontal cortex is activated to the extent that semantic information must be held temporarily in working memory to answer a particular semantic question. He observed a decrease in activation in the area, caused by a decreased need for processing, during a semantic priming task in which subjects differentiated between nouns based on semantic categories (living/nonliving or abstract/concrete). Repetition priming refers to the increase in efficiency produced when a task is performed with a set of stimuli already presented versus performance with novel stimuli, and the improvement reflects implicit memory acquired the first time a stimulus is presented. Smith argued that the area is divisible into two sections: area 47, which is responsible for maintaining verbal information in working memory, and area 46, which regulates the manipulation of maintained information. Working memory tasks for different modalities, such as objects or faces, activate different areas of the prefrontal region, but those that deal with verbal, semantic information consistently utilize pre-Broca's.

3. Nonlanguage Activation

Pre-Broca's area is activated during imagined and executed movements when no language is produced, and one study suggests that it is not necessarily activated in semantic processing, suggesting more expanded functions. Actions unrelated to semantics and language, such as overt hand and arm movement and imagined movement, also activate BA 46. Stephan found that both preparation to move and motor imagery activated the area, and Parsons found similar activation in a task judging whether drawings were of left or right hands. Parsons credits the activation to demands on working memory. Bookheimer compared oral object naming, oral word reading, silent object naming, and silent word reading tasks with viewing meaningless line drawings. The oral object naming, silent object naming, and silent reading activated BA 47 significantly, but oral word reading did not. Book-heimer noted that this indicates that semantic processing does not occur in oral reading if BA 47 is responsible for semantics. She suggested an alternative: This region might reflect activation of an articu-latory code or motor plan corresponding to the recognized visual form. The variety of activation beyond semantic analysis makes it difficult to conclude that BA 46/47 acts strictly in a semantic capacity when it is engaged, and further evidence is needed to draw firm conclusions beyond the general finding that the area aids in processing and retrieving word meanings.

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