Organization of somatosensory areas I and II in marsupial cerebral cortex: parallel processing in the possum sensory cortex. Controversy exists over the organization of mammalian thalamocortical somatosensory networks. An issue of particular contention is whether the primary and secondary somatosensory areas of cortex (SI and SII) are organized in a parallel or serial scheme for processing tactile information. The current experiments were conducted in the anesthetized brush-tail possum (Trichosurus vulpecula) to determine which organizational scheme operates in marsupials, which have taken a quite different evolutionary path from the placental species studied in this respect. The effect of rapid reversible inactivation of SI, achieved by localized cortical cooling, was examined on both evoked potential and single neuron responses in SII. SI inactivation was without effect on the amplitude, latency, and time course of SII-evoked potentials, indicating that the transient inputs responsible for the SII-evoked potential reach SII directly from the thalamus rather than traversing an indirect serial route via SI. Tactile responsiveness was examined quantitatively before, during, and after SI inactivation in 16 SII neurons. Fourteen were unchanged in their responsiveness, and two showed some reduction, an effect probably attributable to the loss of a facilitatory influence exerted by SI on a small proportion of SII neurons. The temporal precision and pattern of SII responses to dynamic forms of mechanical stimuli were unaffected, and temporal dispersion in the SII response bursts was unchanged in association with SI inactivation. In conclusion, the results establish that, within this marsupial species, tactile inputs can reach SII directly from the thalamus and are not dependent on a serially organized path through SI. A predominantly parallel organizational scheme for SI and SII operates in this representative of the marsupial order, as it does in a range of placental mammals including the cat and rabbit, the tree shrew and prosimian galago, and at least one primate representative, the marmoset monkey.