Photonics meets connectomics: case of diffuse, long-range horizontal projections in rat cortex.

Recent years have seen progress in characterizing connections between different regions of the rodent brain to establish a "connectome." This effort involves systematically collected new data together with tools to characterize network relationships in new and preexisting data. The choices made during data collection, analysis, and display in order to generate these connectomes have emphasized dense, specific connections between cortical regions defined using a priori parcellation schemes that may obscure certain spatial relationships in the data. One example of a pattern of connectivity not clearly evident in these connectomes is a diffusely radiating, apparently nonspecific, border-crossing, long-range horizontal axonal projection that is related to horizontal spreads of evoked activity in the rat cortex. Here, we describe the horizontal projection system and explore evidence for this projection within the connectome data. We consider how the differences in optical and histological methodologies and analyses used for the connectome studies and our own studies lead to different emphases concerning this important horizontal projection pattern.