Knowledge of brain circuitry is critical for understanding the organization, function, and evolution of central nervous systems. Most commonly, brain connections have been elucidated using histological and experimental methods that require animal sacrifice. On the other hand, magnetic resonance diffusion tensor imaging and associated tractography have emerged as a preferred method to noninvasively visualize brain white matter tracts. However, existing studies have primarily examined large, heavily myelinated fiber tracts. Whether tractography can visualize fiber bundles that contain thin and poorly myelinated axons is uncertain. To address this question, the midbrain auditory pathway to the thalamus was investigated in Alligator. This species was chosen because of its evolutionary importance as it is the reptilian group most closely related to birds and because its brain contains many thin and poorly myelinated tracts. Furthermore, this auditory pathway is well documented in other reptiles, including a related crocodilian. Histological observations and experimental determination of anterograde connections confirmed this path in Alligator. Tractography identified these tracts in Alligator and provided a 3-dimensional picture that accurately identified the neural elements of this circuit. In addition, tractography identified one possible unrecognized pathway. These results demonstrate that tractography can visualize circuits containing thin, poorly myelinated fibers. These findings open the door for future studies to examine these types of pathways in other vertebrates.

脑电路知识对于理解中枢神经系统的组织，功能和进化至关重要。最常见的是，已经使用需要动物牺牲的组织学和实验方法阐明了大脑的连接。另一方面，磁共振扩散张量成像和相关的束线照相术已经成为非侵入性地可视化脑白质束的优选方法。但是，现有研究主要检查了大型，严重有髓的纤维束。束线照相术是否可以可视化包含纤细且髓鞘少的轴突的纤维束，尚不确定。为了解决这个问题，在鳄鱼中研究了通往丘脑的中脑听觉通路。选择该物种是因为其进化重要性，因为它是与鸟类关系最密切的爬虫类动物，并且其大脑含有许多稀疏且有髓鞘的区域。此外，这种听觉通路在其他爬行动物（包括相关的鳄鱼）中也有很好的记录。组织学观察和顺行性连接的实验确定在鳄鱼中证实了这条路径。断层扫描在短吻鳄中发现了这些道并提供了3维图片，可以准确识别该电路的神经元。此外，束线照相术还发现了一种可能的无法识别的途径。这些结果表明，束线照相术可以使包含细无髓鞘纤维的电路可视化。这些发现为今后研究其他脊椎动物中这些类型的途径打开了大门。