A major milestone in the understanding of the inter-areal network of the cerebral cortex was achieved in 1991 with the publication of the first comprehensive database describing the distributed hierarchical processing across defined pathways of the primate cerebral cortex (Felleman and Van Essen, 1991).
While this study is hugely influential but it is based on collated data, and because different labs often used different atlases, planes of section and tracers, the database is not consistent and importantly lacks proper quantification. One consequence of this inconsistency was that the numerous studies developing large-scale models of the cortex underestimated the density of cortical graph. The Kennedy lab had previously shown a large-category of previously unsuspected long-distance connections linking primary sensory areas across different modalities (Falchier et al., 2002). The lab then undertook a systematic evaluation of the connectivity of the cortex using retrograde tract tracing.