PARALLEL ORGANIZATION OF SOMATOSENSORY CORTICAL AREA-I AND AREA-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 thal amus and, as well, are linked through intracortical reciprocal connect ions. Tactile information may therefore be conveyed to SII, for exampl e, over either a direct path from the thalamus or an indirect, or seri al, path from the thalamus via SI. 2. Reports in recent years that tac tile responsiveness within the hand area of SII was abolished by surgi cal ablation of the hand area of the postcentral, or SI, area of corte x in the macaque and marmoset monkeys indicated that a serial processi ng scheme may operate, at least in primates. However, as the surgical ablation is clearly irreversible and precludes examination of individu al SII neurons in both the control and test circumstances, that is, wh en SI is intact and when it is inactivated, we have examined in the ca t, the rabbit and the marmoset monkey the behaviour of SII neurons bef ore, during and after the selective, rapidly-reversible inactivation o f SI by means of localized cooling. 3. The results demonstrate that in the cat and rabbit, SII responsiveness is never abolished and infrequ ently affected by SI inactivation and that tactile inputs to SII there fore traverse a direct path from thalamus, organized in parallel with that to SI. In the marmoset (Callithrix jacchus), in contrast to earli er studies based on ablation of SI we found that with reversible inact ivation 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 th alamic input to SII, even in this simian primate, and therefore necess itate revision of the hypothesis that tactile processing at the thalam ocortical level in simian primates is based on a strict serial scheme in which tactile information is conveyed from the thalamus to SI and t hence to SII.