Human short association fibers are thinner and less myelinated than long fibers

The size and complexity of the human brain requires optimally sized and myelinated fibers. White matter fibers facilitate fast communication between distant areas, but also connect adjacent cortical regions via short association fibers. The fundamental questions of i) how thick these fibers are and ii) how strongly they are myelinated, however, remain unanswered. We present a comprehensive analysis of ∼400,000 fibers of human white matter regions with long (corpus callosum) and short fibers (superficial white matter). We demonstrate a substantially smaller fiber diameter and lower myelination in superficial white matter than in the corpus callosum. Surprisingly, we do not find a difference in the ratio between axon diameter and myelin thickness (g-ratio), which is close to the theoretically optimal value of ∼0.6 in both areas. For the first time, to our knowledge, we shed light on a fundamental principle of brain organization that will be essential to understand the human brain.