Organization of the S II Parietal Cortex

Previous physiological studies of the cortical location of responsiveness to somatic stimuli within and along the upper bank of the Sylvian fissure in primates demonstrated a single topographic representation in this part of the cortex (1, 76, 79, 80, 81). This somato-sensory representation was called the second somatic sensory area (S II) since it was discovered subsequent to the first somatic sensory area in most species that contained multiple cortical somatosensory representations. According to some investigators, the effective somatic stimuli for activating evoked potential or single neuron recording sites in SII in animals anesthetized with barbiturates were considerably different from the adequate stimulus parameters for eliciting activity in S I. Receptive fields in primates (1,76-79) and subprimates (4,45,58,60,81) were larger, often bilateral, poorly demarcated and extensively overlapping. Activity in the primate SII was also found to be more susceptible to the level of barbiturate anesthesia (77, 78). As a consequence, the topographic representation of the body in S II was found to be coarser and more difficult to define. In addition, the receptors associated with the sense of position and movement were said not to be represented in S II according to some (76), but not all investigators (41).