Anatomical and Functional Variability

The traditional "anatomical space'' taught in medical and graduate schools is feature based. The brain is named and divided in terms of visual landmarks (sulci and gyri, nuclei and tracts). The assumption is that these landmarks bear reliable relationships to underlying functional architecture. This assumption is tenuous, at best. The caudate and putamen are so similar in cell structure, connectivity, and function as to be considered a single functional entity [47], yet they are divided by a broad white matter tract, and therefore separately named. Subnuclei within the thalamus are extraordinarily diverse in their afferent and efferent connectivity, yet they form a visually cohesive mass, even in high-resolution MR images. Only with the microscope and advanced histochemical techniques can functional areas be accurately discerned. Anatomical/functional anomalies are even more diverse in the cortex. The issue dates back at least to Brodmann, who in his 1909 monograph on cytoarchitecture (translated by Garey [27]) repeatedly cautioned that many functional zones bear no consistent relationship to visible landmarks. For example, concerning primary visual cortex (Brodmann area 17), Brodmann warned: ''The borders of this area, especially laterally, are extraordinarily variable, which is particularly important for pathology. But even medially there are no regular and constant relationships to any 'limiting sulci'...'' ([27], p. 120). Thus, the notion of "limiting sulci,'' upon which a "gross anatomical space'' rests, is inherently flawed for mapping many functional areas. Subsequent studies of human cytoarchitecture continue to confirm the wisdom of Brodmann's warnings [28,58]. This observation has been echoed and amplified by neurosurgeons who observe a remarkable diversity of sulcal patterns and tremendous variability in the functional organization of the brain, as determined by intraoperative cortical electrical stimulation [39]. Finally, neuroimaging studies have observed that functional areas as fundamental as primary motor cortex for mouth may "bear no relation to any detectable sulcal or gyral feature'' [29,45,52]. This lack of correspondence between functional areas and gross anatomical features is a clear indication that an alternative to the "gross anatomical space'' is needed for functional mapping ofthe human brain. For those mapping cortical areas in the macaque monkey and other non human primates, the alternative that has been developed and adopted is a surface-flattened, planar reporting "space" [51]. For a large and rapidly growing segment of the human neuroscience community, the alternative to the "gross anatomical space'' is a 3D volumetric space, the Talairach space [19].

Talairach was the first to use this space as a modeling construct within which structural and functional probability contours could be created [48]. Using intraoperative cortical stimulation to localize functional areas and pneumoencepha-lography to localize sulcal and gyral landmarks, Talairach created probability distributions — probabilistic models — for functions and structures. Although his subjects were not normal and his sample sizes were never large (10-25 per area), the spatial distributions appeared normal and the variance was not large.

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