Cytoarchitectonic and receptorarchitectonic organization in Broca's region and surrounding cortex

Broca's region was originally described as a cortical site in the inferior frontal gyrus, where speech impairment occurs after brain lesions. However, this clinico-pathological definition does not define the underlying microstructural correlates. Two cytoarchitectonic areas (BA44, BA45) were identified as correlates of Broca's region in the inferior frontal opercular and triangular parts, respectively. Recent microstructural analyses based on the distribution of transmitter receptors suggest a further segregation of BA44 and BA45 and the inclusion of neighboring areas in the adjacent inferior frontal and precentral sulci as well as in the frontal operculum as structural correlates of Broca's region. Here, we review these recent cytoarchitectonic and receptorarchitectonic findings in Broca's region, and compare them with functional imaging data containing sufficient topographical specifications. Finally, Broca's region of the human brain is compared with the homologue region in the brain of apes, baboon and macaque monkey.