Histological studies also showed a stripe pattern in V2 of post-mortem brain specimens when techniques for the staining of myelin were used ( Tootell et al., 1983 Krubitzer and Kaas, 1989 Horton and Hocking, 1997). In humans, these widths are approximately doubled in size ( Hockfield et al., 1990 Tootell and Taylor, 1995 Adams et al., 2007).
In macaques, stripes of the same type have a center-to-center distance of around 4.0 mm and a width ranging from 0.7 to 1.3 mm ( Shipp and Zeki, 1985 Tootell and Hamilton, 1989). Using cytochrome oxidase (CO) staining, these stripes were found first in squirrel monkeys and macaques as dark and pale patches organized in repeating pale-thin-pale-thick cycles, running through V2 and oriented approximately perpendicular to the V1/V2 border ( Livingstone and Hubel, 1982 Tootell et al., 1983). For example, thin stripes are sensitive to color content and project to functional area V4, whereas thick stripes are more sensitive to binocular disparity and project to area MT (V5) ( Shipp and Zeki, 1985 Hubel and Livingstone, 1987 Livingstone and Hubel, 1987 Ts’o et al., 2001). Functional properties include the sensitivity to different visual features like color, orientation, binocular disparity, and motion, which are largely processed in different stripe types and sent to distinct cortical areas. These stripes form a columnar system in the sense that their functional properties extend roughly through cortical depth ( Tootell and Hamilton, 1989). In primates, visual information sent from the primary visual cortex (V1) to the secondary visual cortex (V2) is segregated into distinct modules known as thin, thick, and pale stripes ( Hubel and Livingstone, 1987 Livingstone and Hubel, 1987). The study demonstrates the feasibility to investigate structure-function relationships in living humans within one cortical area at the level of columnar systems using qMRI. No consistent differences were found for effective transverse relaxation rates ( R 2*). Thereby, we found lower longitudinal relaxation rates ( R 1) of thin and thick stripes compared to surrounding gray matter in the order of 1–2%, indicating higher myelination of pale stripes. Resulting functional activation maps showed robust stripe patterns in V2 which enabled further comparison of quantitative relaxation parameters between stripe types. Thin and thick stripes were functionally localized by exploiting their sensitivity to color and binocular disparity, respectively.
We used quantitative magnetic resonance imaging (qMRI) in conjunction with functional magnetic resonance imaging (fMRI) at ultra-high field strength (7 T) to localize and study myelination of stripes in four human participants at sub-millimeter resolution in vivo. The repeating pattern of pale-thin-pale-thick stripes of cytochrome oxidase (CO) activity in the primate secondary visual cortex (V2) is a prominent columnar system, in which histology also indicates different myelination of thin/thick versus pale stripes. However, knowledge about cortical myelination is largely based on post-mortem histology, which generally renders direct comparison to function impossible. The characterization of cortical myelination is essential for the study of structure-function relationships in the human brain.
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