Introducing ... Broca's area

So far I’ve looked at general cortical circuits, not specifically at language regions. Today I’ll take a closer look at one of more specifically language related areas: the Broca’s area. Broca’s area deserves a special attention not only because of historical reasons but also because of the clear left-right asymmetry. The Broca’s area expands Brodmann’s cytoarchitectural Areas (BA) 45 (anterior) and 44 (posterior). This 45/44 division roughly corresponds to Economo’s cytoarchitectural division FDG/FCBm and neuroanatomical regions PTr/POp (Pars Triangularis / Pars Opercularis).

Both BA45 and BA44 have leftward volumetric asymmetry, and the asymmetry is affected by handedness (Foundas et al. 1998). The volume fraction of cell bodies in areas 44 and 45 (Amunts et al. 2003) and the size of layer III pyramidal neurons in area 45 (Hayes and Lewis 1995) are shown to be greater on the left side. There are extensive horizontal connections in supragranular layers (I-III) and to a lesser extent in infragranular layers (Tardif et al. 2007). Compared to the visual cortex, there are less connections from infragranular layers to supragranular layers (Tardif et al. 2007).  These two facts (horizontal and inter-layer) may suggest extensive topological processing in the feature space [according to my imagination].

BA45 is characterized by a larger and more uniformly granular layer IV compared to BA44, which is characterized by a dysgranular layer IV invaded by numerous pyramidal neurons (Brodmann 1909). Since BA45 is more granular than BA44, it is predicted that feed forward connection tends to originate more from supragranular neurons in BA45 and terminates in infragranular layers in BA44. It is also predicted that feedback connection tends to originate more from infragranular neurons in BA44 and terminates in supragranular layers in BA45.

Adams RA, Shipp S, Friston KJ. Predictions not commands: active inference in the motor system. Brain Structure and Function. 2013 May 1;218(3):611-43.

Amunts K, Schleicher A, Ditterich A, Zilles K. Broca's region: cytoarchitectonic asymmetry and developmental changes. Journal of Comparative Neurology. 2003 Oct 6;465(1):72-89.

Barrett LF, Simmons WK. Interoceptive predictions in the brain. Nature Reviews Neuroscience. 2015 Jul 1;16(7):419-29. -> Barbas et al. diagram “cortical layer infragranular supragranular M2 M1 connection flow”

Brodmann K. Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues. Barth; 1909.

Foundas AL, Eure KF, Luevano LF, Weinberger DR. MRI asymmetries of Broca's area: the pars triangularis and pars opercularis. Brain and language. 1998 Oct 1;64(3):282-96.

Hayes TL, Lewis DA. Anatomical specialization of the anterior motor speech area: hemispheric differences in magnopyramidal neurons. Brain and language. 1995 Jun 30;49(3):289-308.

Tardif E, Probst A, Clarke S. Laminar specificity of intrinsic connections in Broca's area. Cerebral Cortex. 2007 Dec 1;17(12):2949-60.