What's a White Matter 'Fiber'?

White matter anatomy is investigated using methods that operate at markedly different spatial scales and rely on distinct biological and physical principles. Nevertheless, the term "fiber" is routinely applied to the outputs of tract-tracing studies, diffusion MRI tractography, and postmortem fiber dissection, often as though these entities were directly interchangeable. This Viewpoint examines what a "fiber" represents in each methodological context. In tract tracing, the term refers to labeled axons or populations of axons at the cellular scale. In tractography, it denotes a mathematically reconstructed streamline based on local estimates of the preferential diffusion of water molecules across millimeter-scale voxels. In fiber dissection, it refers to a small but macroscopically identifiable fascicle or bundle comprising numerous axons ("anatomical fiber"). These entities differ substantially in scale, resolution, continuity, anatomical specificity, and inferential limitations. Failure to recognize these distinctions may generate apparent discrepancies between methods and lead to inaccurate interpretations of structural connectivity. A method- and scale-aware use of terminology is therefore essential for integrating findings across experimental neuroanatomy, neuroimaging, and anatomical dissection, particularly when white matter anatomy is translated into neurosurgical planning and clinical decision-making.