Somatotopic organization and response properties of neurons of the ventrobasal complex in the opossum

Abstract Thalamic responses to somatic stimulation were studied in 47 specimens of the South American opossum, D. marsupialis aurita . Topographic representation of the body surface in the ventrobasal complex was determined by recording unit cluster responses to natural stimulation in sodium pentobarbital anesthetised animals. The pattern of representation observed was similar to that reported in the North American subspecies and comparable to the findings in eutherian mammals. The analysis of single units recorded with extra‐cellular microelectrodes showed that the great majority of neurons are place and modality specific. A predominance of skin (vs. deep) sensibility was observed. Units activated by displacement of hairs and vibrissae displayed receptive fields similar to those reported in higher mammals. Of the units isolated within the anatomical boundaries of the ventrobasal complex only 1% showed wide receptive fields, or bilateral representation. None of the units could be activated by auditory stimulation. Neurons displaying wide, discontinuous or bilateral receptive fields showing auditory‐somatic convergence were isolated in nucleus parafascicularis, subparafascicularis and in the Posterior group.