Hodology

Hodology is the study of the organization and pathways of neural connections within the nervous system, focusing on how information travels from one region to another. It seeks to describe the routes of neural signals, the directionality of connections, and the structure of circuits that underlie brain function.

The term derives from Greek hodós, road, and -logia, study. In its classical use, hodology referred to anatomical tracing of connections between brain regions, including the mapping of afferent and efferent projections and the distinction between monosynaptic and polysynaptic pathways.

Practically, hodology has employed a range of methods. Classical tract-tracing uses chemical, fluorescent, or immunohistochemical tracers injected into a target area to reveal outgoing and incoming connections, distinguishing anterograde versus retrograde labeling. Modern approaches include viral vectors for trans-synaptic tracing, alongside noninvasive imaging techniques such as diffusion MRI and tractography, which infer structural connectivity in vivo. Computational tools integrate these data to produce maps of brain networks.

Hodology sits within neuroanatomy and is closely related to connectomics and systems neuroscience. It emphasizes structure and pathways rather than function alone, though the two are studied together. Limitations include species differences, resolution constraints, and the invasive nature of many tract-tracing methods in animals, while noninvasive methods in humans infer connectivity indirectly.

Overall, hodology provides a framework for describing the architecture of neural circuits, contributing to our understanding of how brain regions communicate, organize processing, and contribute to behavior and disease.