Stereotaxic localization, intersubject variability, and interhemispheric differences of the human auditory thalamocortical system.

Population (probability) maps of cytoarchitectonically defined cortical maps and myeloarchitectonically defined subcortical fiber tracts of the human brain became recently available. These maps can be used for the precise anatomical localization of activated foci as found in functional imaging studies. The aim of this study is to evaluate population maps of the human acoustic radiation (AR) for the purpose of brain mapping studies. Thus, a stereotaxic 3-D representation of the human AR was computed which includes myeloarchitectonic probabilistic maps of AR and its thalamic origin, i.e. the cytoarchitectonically defined medial geniculate nucleus (MGN). These maps were compared with earlier, mostly schematic drawings of AR and MGN. To generate these maps, the degree of intersubject variability of AR and MGN in each stereotaxic position was quantified for the standard axes (x, y, and z) of the reference space in a series of 10 adult human postmortem brains. The results show that the location and volumes of AR and MGN vary considerably between individuals and hemispheres. It is demonstrated that this striking degree of spatial variations may lead to structural-functional mismatch in brain mapping studies based on the Talairach atlas (discrepancy maps). Furthermore, the volumetric measures showed no hemispheric asymmetry for AR and MGN. In contrast to earlier maps, our approach provides microanatomically defined data as a probabilistic reference system for research in auditory structure-function relationships.