Broca's aphasia an update using PET/fMRI

Bookheimer S. Functional MRI of language: new approaches to understanding the cortical organization of semantic processing. Annu Rev Neurosci 2002; 25:151-188. [review article]

The deficits traditionally included in Broca's aphasia involve deficits of articulation, sequential production of speech, sentence production, syntax, naming, and comprehension of complex syntactic structures. Alexander et al. (1990) found that damage to the white matter underlying Broca's area was necessary to produce the whole syndrome. Functional MRI has shown that "large module" theories are incorrect, but that the language model is organized into a large number of relatively small but tightly clustered and interconnected modules with unique contributions to language processing.

fMRI has identified functional heterogeneity in the inferior frontal lobule (IFG) corresponding to specific aspects of language. Three separate regions correspond to syntax (both productive and comprehension), semantics and phonology. It is involved in executive aspects of semantic processing, involving semantic working memory, semantic search, or drawing comparisons among semantic concepts in working memory. Petersen et al (1988) identified the anterior inferior portion of the IFG in semantic processing (area 47 of Brodmann), meaning processing semantic relationships between words and phrases, or retrieving semantic information. Posterior superior parts of IFG are important for attention and selection. Area 44 (as well as the temporal lobe) was activated during tasks involving Jabberwocky sentences, thought to be important for syntactic attentional tasks. Cortical stimulation of area 44 disrupts phoneme monitoring, accessing and sequencing (Ojemann &7 Mateer, 1979). f MRI suggested phoneme (v. tone) comprehension occurs in area 44/45 while word meaning tasks activate temporal lobes (demonet, 1992). Increased blood flow in posterior IFG occurs when subjects make judgments about embedded (ambiguous) phonemes. Zatorre (1992) using PET showed posterior IFG engaged when subjects made a phonological v. pitch discrimination judgment.

Non language tasks done by BA include tone discrimination (Muller, 2001) especially anterior inferior IFG (areas 45/47). Area 44 is important for imagery of motion