The organization of somatosensory cortex in the short-tailed opossum (Monodelphis domestica)

The organization of neocortex in the short-tailed opossum (Monodelphis dome stica) was explored with multiunit microelectrode recordings from middle la yers of cortex. Microelectrode maps were subsequently related to the chemoa rchitecture of flattened cortical preparations, sectioned parallel to the c ortical surface and processed for either cytochrome oxidase (CO) or NADPH-d iaphorase (NADPHd) histochemistry. The recordings revealed the presence of at least two systematic representations of the contralateral body surface l ocated in a continuous strip of cortex running from the rhinal sulcus to th e medial wall. The primary somatosensory area (S1) was located medially whi le secondary somatosensory cortex (S2) formed a laterally located mirror im age of S1. Auditory cortex was located in lateral cortex at the caudal bord er of S2, and some electrode penetrations in this area responded to both au ditory and somatosensory stimulation. Auditory cortex was outlined by a dar k oval visible in flattened brain sections. A large primary visual cortex ( V1) was located at the caudal pole of cortex, and also consistently corresp onded to a large chemoarchitecturally visible oval. Cortex just rostral and lateral to V1 responded to Visual stimulation, while bimodal auditory/visu al responses were obtained in an area between V1 and somatosensory cortex. The results are compared with brain organization in other marsupials and wi th placentals and the evolution of cortical areas in mammals is discussed.