Parallel organization of somatosensory cortical areas I and II for tactile processing

1. The two principal tactile processing areas in the cerebral cortex, somatosensory areas I and II, receive direct projections from the thalamus and, as well, are linked through intracortical reciprocal connections. Tactile information may therefore be conveyed to SII, for example, over either a direct path from the thalamus or an indirect, or serial, path from the thalamus via SI. 2. Reports in recent years that tactile responsiveness within the hand area of SII was abolished by surgical ablation of the hand area of the postcentral, or SI area of the cortex in the macaque and marmoset monkeys indicated that a serial processing scheme may operate at least in primates. However, as the surgical ablation is clearly irreversible and precludes examination of individual SII neurons in both the control and test circumstances, that is, when SI is intact and when it is inactivated, we have examined in the cat, the rabbit and the marmoset monkey the behaviour of SII neurons before, during and after the selective, rapidly-reversible inactivation of SI by means of localized cooling. 3. The results demonstrate that in the cat and rabbit, SII responsiveness is never abolished and infrequently affected by SI inactivation and that tactile inputs to SII therefore traverse a direct path from thalamus, organized in parallel with that to SI. In the marmoset (Callithrix jacchus), in contrast to earlier studies based on ablation of SI we found that with reversible inactivation of SI, SII responsiveness was unaffected in 25% of neurons and, although reduced in the remainder, was rarely abolished (< 10% of SII neurons). 4. The results indicate that there is substantial direct thalamic input to SII, even in this simian primate, and therefore necessitate revision of the hypothesis that tactile processing at the thalamocortical level in simian primates is based on a strict serial scheme in which tactile information is conveyed from the thalamus to SI and thence to SII.