Symmetry and asymmetry in the human posterior thalamus. I. Cytoarchitectonic analysis in normal persons.

We parceled the posterior thalami of nine normal human brains according to cytoarchitectonic criteria, measured relevant nuclear volumes, and sought left-right asymmetries. We found that thalamic zones with multiple projections to the cerebral cortex, using the centromedian-parafascicular nucleus as a prototype, were mostly symmetric. This group includes the medial, lateral, and inferior pulvinar nuclei. Thalamic zones that project discretely to a few, clearly defined cortical receptor fields (using the medial geniculate nucleus [MG] as a prototype) closely reflected the asymmetry of the cortical fields to which they project. Hence, the MG showed a slight right-sided bias, and the lateralis posterior nucleus (related to the grossly asymmetric inferior parietal lobule) showed a significant leftsided bias in eight of the nine brains measured. This asymmetry may partially explain the apparent language specialization of the dominant thalamus.