Broca's region revisited: cytoarchitecture and intersubject variability

Area of Science:

• Neuroscience
• Cytoarchitecture
• Human Brain Anatomy

Background:

• Brodmann's areas 44 and 45, key components of Broca's speech region, exhibit variability in size and borders across cytoarchitectonic maps.
• This variability may stem from individual anatomical differences or subjective mapping techniques, impacting functional imaging interpretations.

Purpose of the Study:

• To objectively define the cytoarchitectonic borders of Brodmann's areas 44 and 45 using an observer-independent method.
• To quantify intersubject variability in the cytoarchitecture of these speech-related areas.
• To investigate potential morphological correlates of speech lateralization.

Main Methods:

• Analysis of laminar cell density distributions in serial coronal sections from 10 human brains.
• Observer-independent definition of cytoarchitectonic borders based on statistically significant changes in laminar patterns.
• 3D reconstruction of brains and areas to compare cytoarchitectonic borders with macroscopic landmarks.

Main Results:

• Cytoarchitectonic borders of areas 44 and 45 did not consistently align with sulcal contours, indicating macroscopic features are unreliable landmarks.
• Intersubject variability in cytoarchitecture was greater than differences between areas within individuals; area 44 volume varied up to tenfold.
• Area 44, but not area 45, demonstrated left-over-right asymmetry in cell density in all brains, with males showing stronger left-hemisphere dominance.

Conclusions:

• Macroscopic landmarks are unreliable for defining Brodmann's areas 44 and 45 borders.
• Significant intersubject variability exists in the cytoarchitecture of these speech areas.
• Morphological asymmetries in area 44 provide a potential neural basis for the functional lateralization of speech production.