High-Field Magnetic Resonance Mapping of the Border Between Primary Motor (Area 4) and Somatosensory (Area 3a) Cortex in Ex-Vivo and In-Vivo Human Brains

Abstract

Unraveling the functional properties of structural elements in the brain is one of the fundamental goals of neuroscientific research. In the cerebral cortex this is not so easy to accomplish, since cortical areas are defined microstructurally in post-mortem brains but functionally in living brains with electrophysiological or neuroimaging techniques – and cortical areas vary in their topographical properties across individual brains. To map both microstructure and function in the same brains noninvasively in vivo would represent a huge leap forward. In this chapter, we show our approach, based on a MP2RAGE sequence run on a 7 T Siemens MR scanner to produce in living human subjects quantitative T1 maps that reflect local microanatomy. On inflated surface maps of individual subjects, cortical areas known from post-mortem studies to be heavily myelinated are easily discernible from surrounding less myelinated regions. Classically, cortical areas (and their precise borders that are indispensable for a valid correlation with functional data) are defined by their myelo- and cytoarchitectonic pattern ex vivo, i.e., in post-mortem brains. Hence, with the same MP2RAGE sequence at 7 T we scan fixed tissue blocks of the human cortex, section them with a microtome, stain the sections for myelin sheaths or cell bodies, and correlate the MR architecture directly with myelo- and cytoarchitecture. This “triple jump” approach allows to (i) define a cortical area based on myelo- and cytoarchitecture, (ii) extract the “MR fingerprint” of this area ex vivo, and (iii) transfer this “fingerprint” to living brains and define this area in vivo. With this technique we mapped in living subjects the functionally important border between primary motor (Brodmann Area 4) and somatosensory (Brodmann Area 3a) cortex. This technology sets the stage for the development of an in vivo myeloarchitectonic brain map, with the enormous potential to make direct correlations between microstructure and function in living human brains.