The white matter
(WM) consists of fiber tracts that connect different regions of the brain. Among these tracts, the intrahemispheric cortico-cortical connections are called association fibers. The U-fibers are short association fibers located in the superficial WM (SWM), and these U-fibers connect adjacent gyri. They were first described in the 19th century and were thought to work as part of the cortico-cortical networks to execute associative brain functions (Arnold, 1838
; Meynert, 1872
). There are several human studies that indicate a relationship between the involvement of U-fibers and neuropsychological impairments (Gootjes et al., 2007
; Lazeron et al., 2000
; Miki et al., 1998
; Rovaris et al., 2000
). These studies were based on the psychiatric observation of patients with brain impairments, such as ischemia and multiple sclerosis. However, most of the U-fibers have not been distinctively identified and their functions are elusive. This is understandable, because the SWM looks homogeneous, both in postmortem samples and in conventional magnetic resonance imaging, which makes it difficult to delineate these small fiber structures. In addition, the anatomical variability of the cortex and its gyral pattern make a comprehensive and systematic delineation difficult.
In these previous studies, diffusion tensor imaging (DTI) has been successfully used to investigate the anatomy of several human U-fibers (Anwander et al., 2007
; Gong et al., 2009
; Mori et al., 2002
). One of the advantages of a DTI-based study is that it enables a systematic population-based analysis of the locations and anatomy of the U-fibers, which would be difficult to perform by any other invasive or noninvasive approach. Such systematic studies have been reported to define intergyri connections. In group-based studies, the SWM was parcellated into nine blade-like structures that were commonly found in the population (Iwasaki et al., 1991
; Oishi et al., 2008
). An exhaustive search for connections between adjacent gyri identified 28
U-fibers, including 11
U-fibers that connected two adjacent blades (Zhang et al., 2010
). To the authors' best knowledge, this is the first atlas of the specific U-fibers defined in a common atlas space. Validation of the DTI-based finding is, however, not straightforward. The tensor estimation is known to oversimplify the underlying neuroanatomy and the connectivity search, which is performed by tractography, and has been criticized for possible false-positive and -negative results. On the other hand, histological validation would require invasive tracer studies, which cannot be applied to human subjects.
In this study, SWM and U-fiber anatomy were investigated based on high-resolution DTI of a macaque brain. There were two motivations for this study. First, the macaque is the most commonly used animal model for exploring cognitive functions (Nakahara et al., 2007
), and the macaque U-fibers have been identified by numerous in vivo
axonal tracing studies and are now publicly available through the website of the Collation of Connectivity data on the Macaque brain (CoCoMac; http://cocomac.org/home.asp
) (Kotter, 2004
; Stephan et al., 2001
). Thus, the macaque brain is an excellent model for validating the results of DTI-based tractography. Second, whether the anatomical unit of the SWM area called “blade,” which has been delineated by previous DTI studies, is consistent with that of the macaque brain can be investigated, assuming that such interspecies anatomical conservation provides support for the validity of the DTI-based findings in the human brain.
In terms of long fiber tracts, a high degree of cross-species structural conservation has been observed with histology and magnetic resonance imaging (Parker et al., 2002
; Schmahmann and Pandya, 2006
; Schmahmann et al., 2007
; Zhang et al., 2007
). First, whether the nine blades in the human brain are also present in the macaque brain was tested. Then, the U-fibers of the macaque brain were investigated using tractography, and the anatomy was compared with that of the human brain.