FIG. 1.

Nissl stain of primary visual cortex (reproduced from ref. #77), illustrating the cytoarchitecture of cortical gray matter. The use of 1 mm isotropic voxels shown in (A) inevitably leads to substantial mixing of signals from different cortical layers, as well as a high likelihood of partial-volume contamination from the adjacent white matter and CSF. On the other hand, the use of highly anisotropic voxels, as shown in (B) for the example of 0.25×3×3 mm voxels, can greatly reduce these partial-volume effects without sacrificing SNR, as long as the voxels are suitably positioned and oriented with respect to the radial and tangential directions of the cortical surface, indicated in (A). Although the points above are illustrated on primary visual cortex (V1) in this figure, they are applicable to any cortical area, e.g., primary somatosensory cortex (S1) and primary motor cortex (M1), which are the focus of this study.