Broca's area, pars triangularis
Pars triangularis gyri frontalis inferioris
cortical
The anterior, triangular segment of Broca's area, bounded above by the inferior frontal sulcus, behind by the ascending ramus of the lateral fissure, and below by the anterior horizontal ramus of the lateral fissure that gives the gyrus its name. Brodmann area 45 has a more developed granular layer IV than the more posterior BA 44, marking it as a prefrontal-type rather than premotor-type cortex.
The pars triangularis is preferentially engaged by lexico-semantic retrieval and by syntactic unification operations, in which previously-stored lexical items are combined into novel phrasal structures. Functional imaging in language comprehension consistently dissociates BA 45 (semantic and integrative processing) from BA 44 (phonological-articulatory processing), although both are co-activated by most language tasks.
In Hagoort's MUC (Memory, Unification, Control) model the pars triangularis is the cortical home of *unification*, the on-line construction of multi-word meanings from lexical building blocks held in temporal-cortex stores. Lesions or transcranial-magnetic-stimulation virtual lesions of BA 45 selectively impair comprehension of syntactically complex sentences whose parse depends on integrating non-adjacent elements.
language
The two Broca’s sub-fields are structurally and functionally distinct despite their gross anatomical adjacency. The cytoarchitectural transition from BA 44 (dysgranular, premotor-type) to BA 45 (granular, prefrontal-type) tracks a more general posterior-to-anterior gradient across the frontal lobe, from motor cortex through premotor and prefrontal to the anterior pole.
The dorsal language stream that links Broca’s territory to Wernicke’s territory through the arcuate fasciculus is now understood to comprise two parallel sub-pathways with distinct cortical terminations: a direct fronto-temporal segment carrying phonological information, and an indirect fronto-parietal-temporal route through Geschwind’s territory in the inferior parietal lobule. The pars triangularis contributes preferentially to the indirect pathway, consistent with its role in higher-level integrative processing.