Somatosensory cells in area PEc of macaque posterior parietal cortex.

PEc is an area defined on cytoarchitectural grounds, located in the posterior part of the superior parietal lobule of macaque brain (Pandya and Seltzer, 1982). The aim of this work was to assess whether passive somatosensory stimulation elicited responses in PEc neurons. Extracellular recordings were performed in three awake Macaca fascicularis. Passive somatosensory stimulation was performed in darkness, and eye movements were monitored continuously. Recording sites were assigned to different areas according to the cytoarchitectonic criteria described by Pandya and Seltzer (1982) and Luppino et al. (2005). Only recording sites within the limits of the cytoarchitecturally defined area PEc were taken into account in this work. Of 147 PEc cells, 83 (56%) were modulated by passive somatosensory stimulation. The majority of them (73%) responded to joint rotations, and 24% responded to tactile skin stimulation. The majority of PEc somatosensory responses (90%) were evoked by contralateral stimulation. Joint-modulated cells were mostly activated by the upper limbs (82%). The majority of tactile receptive fields (61%) were located on the arms, and a minority was located on the legs and trunk. One-half of PEc somatosensory cells were polysensory, because they were sensitive to visual stimulation. The majority of PEc somatosensory cells were activated by active reaching movements. Somatosensory cells, somatosensory submodalities, and body part representations were not clustered in PEc subregions; in other words, PEc does not show a somatotopic organization. Although the caudal sector of the superior parietal lobule has been traditionally considered as a somatosensory area, this is the first demonstration of the presence of somatosensory cells in this cortical region.