The Talairach coordinate of a point in the MNI space: how to interpret it

To perform group studies using functional imaging data, the individual brain images are usually transformed into a common coordinate space. The two most widely used spaces in the neuroscience community are the Talairach space and the Montreal Neurological Institute (MNI) space. The Talairach coordinate system has become the standard reference for reporting the brain locations in scientific publication, even when the data have been spatially transformed into different brain templates (e.g., MNI space). When expressed in terms of individual subjects, the mapping of a coordinate in MNI space to the Talairach space generates distinct coordinates for different subjects. In this paper, we describe two approaches to derive the Talairach coordinates from the MNI space, which is based on the ICBM152 template from the International Consortium of Brain Mapping. One approach is the Talairach Method of Piecewise Linear Scaling (TMPLS) as implemented in the AFNI software package; and the other is a template-matching approach using the linear transformation in SPM99. The uncertainty measurements of the mapping results are presented. This may allow researchers to better interpret results reporting in the Talairach coordinates obtained from the MNI space. This study also examines the discrepancy between the derived Talairach coordinates and those obtained from the mni2tal script, a tool commonly used by the neuroimaging community. Large discrepancies are found in the inferior regions, superior frontal and occipital regions.