A new paper in PLoS Biology by Katrin Amunts and colleagues from Germany provides exciting and important new data on anatomical subdivisions of Broca's area, particularly from the perspective of receptor distribution. They document, using a multireceptor approach, that there are considerably more subdivisions in areas 44 and 45 and surrounding tissue (and some interesting lateralization patterns) than we had come to expect from more traditional cytoarchitectonic studies. This paper certainly raises the bar in terms of what we need to know about the neurobiology of Broca's region. Being aware of this type of more detailed analysis of this part of the brain should help us prevent to promulgate the rather naive and monolithic interpretations that we tend to entertain (and publish). The abstract is below.
There is a considerable contrast between the various functions assigned to Broca's region and its relatively simple subdivision into two cytoarchitectonic areas (44 and 45). Since the regional distribution of transmitter receptors in the cerebral cortex has been proven a powerful indicator of functional diversity, the subdivision of Broca's region was analyzed here using a multireceptor approach. The distribution patterns of six receptor types using in vitro receptor autoradiography revealed previously unknown areas: a ventral precentral transitional cortex 6r1, dorsal and ventral areas 44d and 44v, anterior and posterior areas 45a and 45p, and areas op8 and op9 in the frontal operculum. A significant lateralization of receptors was demonstrated with respect to the cholinergic M2 receptor, particularly in area 44v+d. We propose a new concept of the anterior language region, which elucidates the relation between premotor cortex, prefrontal cortex, and Broca's region. It offers human brain homologues to the recently described subdivision of area 45, and the segregation of the ventral premotor cortex in macaque brains. The results provide a novel structural basis of the organization of language regions in the brain.