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 somat osensory 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 (Trichosu rus :vulpecula) to determine which organizational scheme operates in marsup ials, which have taken a quite different evolutionary path from the placent al species studied in this respect. The effect of rapid reversible inactiva tion of SI, achieved by localized cortical cooling, was examined on both ev oked potential and single neuron responses in SII. SI inactivation was with out effect on the amplitude, latency, and time course of SII-evoked potenti als, indicating that the transient inputs responsible for the SII-evoked po tential reach SII directly from the thalamus rather than traversing an indi rect serial route via SI, Tactile responsiveness was examined quantitativel y before, during, and after SI inactivation-in 16 SII neurons. Fourteen wer e unchanged in their responsiveness, and two showed some reduction, an effe ct probably attributable to the loss of a facilitatory influence exerted by SI on a small proportion of Sn: neurons. The temporal precision and patter n of SII responses to dynamic forms of mechanical stimuli were unaffected, and temporal dispersion in the SII response bursts was unchanged in associa tion with SI inactivation. In conclusion, the results establish that, withi n this marsupial species, tactile inputs can reach SII directly from the th alamus and are not dependent on a serially organized path through SI. A pre dominantly parallel organizational scheme for SI and SII operates in this r epresentative of the marsupial order, as it does in a range of placental ma mmals including the cat and rabbit, the tree shrew and prosimian galago, an d at least one primate representative, the marmoset monkey.