The Autoradiographic Tracing of Axonal Connections in the Central Nervous System

Since the conception of the autoradiographic tracing method in the late 1960s (Weiss and Holland, 1967; Goldberg and Kotani, 1967; Lasek et al., 1968) and its popularization and first applications in the early 1970s (Cowan et al., 1972, Edwards, 1972), it has rapidly become one of the most widely used techniques for tracing neuroanatomical connections. Based on the process of axoplasmic transport and the technique of autoradiography, the autoradiographic tracing method represents a radical departure from past tracing methods that relied primarily on the visualization of degenerating axons. As such, it offers several new and significant advantages, the most important being the selective demonstration of pathways arising from neurons surrounded by passing fibers. This advantage alone has meant that the connections of whole new brain territories could be established for the first time and the connections of many previously examined areas clarified. But the technique also has definite limitations and certain disadvantages relative to other tracing methods, and, if naively used, it can produce misleading results.