Absence of local retinotopy in the mouse optic tract.

Retinotopy is a fundamental organizational principle of the visual system, where neighboring neurons represent adjacent points in visual space. This spatial relationship is established by precise anatomical wiring across successive areas, e.g., from the retina to the lateral geniculate nucleus (LGN) to the visual cortex. To examine the precision of this topographic arrangement within the long-range projection axons themselves, we recorded retinal ganglion cell (RGC) axons in the mouse optic tract (OT) and mapped their receptive fields (RFs). As expected for a retinotopically organized area, we found that nearby LGN cell pairs had significantly smaller RF distances than distant pairs. In contrast, no such relationship was observed among RGC axons in the OT. Modelling analyses further confirmed that the observed RF distances in the OT were incompatible with any locally retinotopic arrangement. Instead, the OT retained only coarse topography, with ~18° RF deviations or ~40 μm axonal displacements from an ideal retinotopic organization. These results demonstrate that the mouse OT lacks fine-scale retinotopy and maintains only broad topographic structure.