A histological and diceCT-derived 3D reconstruction of the avian visual thalamofugal pathway.

Amniotes feature two principal visual processing systems: the tectofugal and thalamofugal pathways. In most mammals, the thalamofugal pathway predominates, routing retinal afferents through the dorsolateral geniculate complex to the visual cortex. In most birds, the thalamofugal pathway often plays the lesser role with retinal afferents projecting to the principal optic thalami, a complex of several nuclei that resides in the dorsal thalamus. This thalamic complex sends projections to a forebrain structure called the Wulst, the terminus of the thalamofugal visual system. The thalamofugal pathway in birds serves many functions such as pattern discrimination, spatial memory, and navigation/migration. A comprehensive analysis of avian species has unveiled diverse subdivisions within the thalamic and forebrain structures, contingent on species, age, and techniques utilized. In this study, we documented the thalamofugal system in three dimensions by integrating histological and contrast-enhanced computed tomography imaging of the avian brain. Sections of two-week-old chick brains were cut in either coronal, sagittal, or horizontal planes and stained with Nissl and either Gallyas silver or Luxol Fast Blue. The thalamic principal optic complex and pallial Wulst were subdivided on the basis of cell and fiber density. Additionally, we utilized the technique of diffusible iodine-based contrast-enhanced computed tomography (diceCT) on a 5-week-old chick brain, and right eyeball. By merging diceCT data, stained histological sections, and information from the existing literature, a comprehensive three-dimensional model of the avian thalamofugal pathway was constructed. The use of a 3D model provides a clearer understanding of the structural and spatial organization of the thalamofugal system. The ability to integrate histochemical sections with diceCT 3D modeling is critical to better understanding the anatomical and physiologic organization of complex pathways such as the thalamofugal visual system.

Mapping the avian visual tectofugal pathway using 3D reconstruction.

Image processing in amniotes is usually accomplished by the thalamofugal and/or tectofugal visual systems. In laterally eyed birds, the tectofugal system dominates with functions such as color and motion processing, spatial orientation, stimulus identification, and localization. This makes it a critical system for complex avian behavior. Here, the brains of chicks, Gallus gallus, were used to produce serial brain sections in either coronal, sagittal, or horizontal planes and stained with either Nissl and Gallyas silver myelin or Luxol fast blue stain and cresyl echt violet (CEV). The emerging techniques of diffusible iodine-based contrast-enhanced computed tomography (diceCT) coupled with serial histochemistry in three planes were used to generate a comprehensive three-dimensional (3D) model of the avian tectofugal visual system. This enabled the 3D reconstruction of tectofugal circuits, including the three primary neuronal projections. Specifically, major components of the system included four regions of the retina, layers of the optic tectum, subdivisions of the nucleus rotundus in the thalamus, the entopallium in the forebrain, and supplementary components connecting into or out of this major avian visual sensory system. The resulting 3D model enabled a better understanding of the structural components and connectivity of this complex system by providing a complete spatial organization that occupied several distinct brain regions. We demonstrate how pairing diceCT with traditional histochemistry is an effective means to improve the understanding of, and thereby should generate insights into, anatomical and functional properties of complicated neural pathways, and we recommend this approach to clarify enigmatic properties of these pathways.