Sparse local excitation from LR cells
Our study shows that while these LR cells are the most prevalent cell type in the superficial layers of the RSG, they rarely synapse onto their neighboring FS, RS and LR neurons. We have confirmed that the most likely reason for this sparse connectivity is because the axons of LR cells rarely ramify within L2/3, but instead travel into deeper layers and the corpus callosum. Similar axonal trajectories for RSG neurons are likely to exist in both mice and rats (Kurotani et al., 2013). In additional related work, when pyramidal neurons in the superficial layers of one RSC hemisphere were labeled, their axons very often terminated in the contralateral RSC (Geijo-barrientos et al., 2019; Sempere-Ferràndez et al., 2018). These studies did not identify whether LR or RS neurons were the more likely source of these projections, but combined with our results, suggest a circuit in which LR cells of one hemisphere might serve to influence the activity of the contralateral hemisphere. However, it is also possible that LR axons continue onto other brain regions, as the RSC projects to several areas involved in memory and spatial navigation processing (van Groen & Wyss, 1990). A major challenge in the field will be to identify precise molecular markers for LR cells to help in identifying their cell-type-specific projections.