Fig. 1.

Evolution of cortical commissures. (A) Cladogram of birds and mammals showing the origin of the neocortex (NCx) and the corpus callosum (cc). (B) Coronal brain schematics. The anterior commissure (ac) of pigeons contains mostly unidirectional projections from the ventral arcopallium (Av) to contralateral homotopic and heterotopic targets, including the mesopallium (M), nidopallium (N), and hyperpallium intercalatum (HI). In eutherians, callosal axons are topographically segregated and connect mostly homotopic regions, whereas the ac carries axons from the piriform cortex (Pir) and lateral NCx. Homologous circuits in noneutherians are carried exclusively through the ac.