Topography of optic flow processing in olivo‐cerebellar pathways in zebra finches (Taeniopygia guttata)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are brainstem nuclei involved in the analysis of optic flow. A major projection site of both nBOR and LM is the medial column of the inferior olive (IO), which provides climbing fibers to the vestibulocerebellum. This pathway has been well documented in pigeons, but not other birds. Recent works have highlighted that zebra finches show specializations with respect to optic flow processing, which may be reflected in the organization of optic flow pathways to the IO. In this study, we characterized the organization of these pathways in zebra finches. We found that the medial column consists of at least eight subnuclei (i–viii) visible in Nissl‐stained tissue. Using anterograde traces we found that the projections from LM and nBOR to the IO were bilateral, but heavier to the ipsilateral side, and showed a complementary pattern: LM projected to subnucleus i, whereas nBOR projected to subnuclei ii and v. Using retrograde tracers, we found that these subnuclei (i, ii and v) projected to the vestibulocerebellum (folia IXcd and X), whereas the other subnuclei projected to IXab and the lateral margin of VII and VIII. The nBOR also projected ipsilaterally to the caudo‐medial dorsal lamella of the IO, which the retrograde experiments showed as projecting to the medial margin of VII and VIII. We compare these results with previous studies in other avian species. Optic flow is the pattern of visual motion across the retina that occurs during self‐motion. In birds two retino‐recipient nuclei in the brainstem are involved in the analysis of optic flow: the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (LM). Using anterograde tracing techniques, we show that the LM (green) and nBOR (magenta) in zebra finches project to a subset of subnuclei in the medial column (mc) of the inferior olive (IO). Moreover, the mc is highly elaborated in zebra finches, with multiple distinct subnuclei. Using retrograde techniques, we show that these subnuclei in turn project to folia IXcd and X in the cerebellum (red), whereas the other subnuclei in the mc project to IXab and the lateral margins of folia VI, VII, and VIII. In addition, the nBOR projects to a subnucleus in the dorsal lamella of the IO that projects to the medial region of folia VI‐VIII (light blue).