It is a standard practice in functional brain imaging studies to spatially "normalize" one's data to a common space such as Talairach space or MNI space. The reason for doing this is to allow data from multiple subjects to be averaged. (Here is a discussion of this process.) It is acknowledged that there is error in the process due to variation from one brain to the next. Yet at the same time the (also standard) practice of overlaying warped and group-averaged brain activations on a single high resolution structural MRI temps investigators and readers alike to interpret the location of the blob of activations very literally. More than once I have heard comments about this or that activation being on the upper or lower bank of a sulcus, for example. We simply don't have the spatial resolution to tell in this sort of fMRI analysis.
This situation is even worse though, at least for some brain areas. This issue came up in the comments to a previous post where the discussion with Raj Raizada turned to the localization of a particular activation focus in one of his papers (Raizada & Poldrack, 2007). They correctly localized the group activation to the supramarginal gyrus, which is a tad north of the Sylvian fissure. Here is the activation in question:
Although orthogonal to the main conclusions of the study, I suggested the "real" location may be inside the Sylvian fissure, and may reflect activity of a sensory-motor integration area, Spt (Hickok et al. 2009). The reason for this speculation is that we have found that simply normalizing a single subject's functional imaging data can cause the activation focus to jump from within the Sylvian fissure to above it, falling on the SMG. The figure below shows activations in native space (left side sagittal and coronal images), that is, non-normalized and on the subject's own MRI (also non-normalized) and after warping to a standard space (right side sagittal and coronal images). The activation of interest is in the crosshairs.
Notice that in native space the focus of activation is within the Sylvian fissure and after standardization it moves up above and is on the SMG. This happened consistently enough across subjects in two independent datasets that the group focus ended up in the SMG despite individual subjects showing activation in the Sylvian fissure. This bit of spatial error may not seem like much of a problem, but it is. This jump crosses cytoarchitectonic boundaries and could lead to dramatically different functional interpretations based on what is known about these areas.
So what do we do? If location matters, you can confirm your activations in native space in individual subjects, or you can try normalizing with sulcal based co-registration schemes rather than the standard methods.
Raizada, R., & Poldrack, R. (2007). Selective Amplification of Stimulus Differences during Categorical Processing of Speech Neuron, 56 (4), 726-740 DOI: 10.1016/j.neuron.2007.11.001
Hickok, G., Okada, K., & Serences, J. (2008). Area Spt in the Human Planum Temporale Supports Sensory-Motor Integration for Speech Processing Journal of Neurophysiology, 101 (5), 2725-2732 DOI: 10.1152/jn.91099.2008