Parallel thalamic activation of the first and second somatosensory areas in prosimian primates and tree shrews.

In Tupaia belangeri and Galago senegalensis, microelectrode recordings immediately after ablation of the representation of the forelimb in the midportion of the first somatosensory area, S-I, revealed that all parts of the second somatosensory area, S-II, remained highly responsive to cutaneous stimuli. In this way, prosimian primates, close relatives of simian primates, and tree shrews differ markedly from monkeys in which S-II is deactivated by comparable ablations, and resemble such mammals as cats and rabbits in which S-II also remains highly responsive following ablations in S-I. Thus, it appears that the generalized mammalian condition is that S-I and S-II are independently activated via parallel thalamocortical pathways. A dependence of S-II on serial connections from the thalamus to the S-I region and then to S-II apparently evolved with the advent of anthropoid primates, and may be present only in monkeys and perhaps other higher primates.