| |
Flat Maps
18
3.5 What are Flat Maps?
Cortical gray matter of each hemisphere is like a folded sheet. Gray matter can be visualized
in three-space by creating a surface model. A disadvantage of this visualization technique is
that one can mainly see the areas on the exterior surface because much of the gray matter lies
within the sulci and is not easily visible from the outside. To get a better view, the cortex can
be flattened computationally and fMRI measurements can then be overlaid on the flattened
representation of the cortical surface. The resulting functional flat maps have many advan-
tages; some of them are given in the next section. To computationally flatten the cortex from
anatomical MR images, several tasks have to be performed. They are described after the fol-
lowing section.
3.6 Advantages of Flat Maps
The cortical surface is hard to see without significant processing. For example, most of the
primary visual cortex lies between the two hemispheres and is therefore hidden from view
[Gazzaniga, et al., 1998 #9]. To simplify the task of identifying visual areas, and to improve
the visualization of the cortex, an unfolding algorithm can be used to transform the cortex to a
non-convoluted surface, in our case to a flat 2-dimensional plane. A sphere or an ellipsoid can
also be used [Fischl, et al., 1999 #36]. [Van Essen, et al., 2001 #37] mapped the visual cortex
by using surface-based atlases.
Functional data can be projected onto the flat map. Functional flat maps can improve the
interpretation of functional data because relations between different functional loci can be
seen easier as in the case where one is looking at the folded cortex. In particular, the spatial
organization of the visual areas can be understood easier by looking at a flat map than by ex-
amining the folded cortex in three-space [Wandell, 1999 #16]. This is because functional flat
maps make it possible to view cortical activity across a large region of cortex within a single
image [Teo, et al., 1997 #8]. Therefore, flattened representations of the cortex can help re-
searchers to learn more about the functional organization of the cortex. They can also be used
to compare functional data between different subjects.
|  |
|
| |
|
|
