Single neuronal activities were recorded in the arm/trunk region of the postcentral gyrus in awake Japanese monkeys. A total of 1608 units were isolated from four hemispheres of two animals, and receptive fields (RFs) and submodalities were identified in 1162 units. Deep or skin submodality neurons were dominant in area 3a or area 3b, respectively. The deep/skin ratio increased as the recording site moved from area 3b to the more caudal areas. In areas 3a and 3b, neuronal RFs were almost exclusively on either the arm or trunk. In areas 2 and 5, neurons with RFs on the trunk decreased and those with RFs on the hand or covering more than one body part, etc. increased. We found a total of 107 neurons with bilateral RFs and 56 with ipsilateral RFs, while the rest (n = 999) were with contralateral RFs. Bilateral or ipsilateral neurons of skin submodality (n = 37) were found in areas 1, 2, and 5. Twenty six (70%) had RFs on the trunk and/or occiput, five on the forelimb, and the rest (n = 6) on both the trunk and forelimb (the combined type). Among 33 skin bilateral neurons, 90% (n = 30) had RFs across the midline. Bilateral or ipsilateral neurons responding to joint manipulation (n = 104) were found in areas 2 and 5. Most of them were activated by manipulation of the shoulder and/or elbow (the proximal type, n = 72, 69%). There were 25 neurons of the combined type (both the proximal and distal joints were effective, 24%). Bilateral or ipsilateral neurons of deep-others submodality (n = 20) were found in areas 1, 2, and 5. The forelimb type (n = 12, 60%) was dominant in this category. The combined-type neurons in both the skin- and joint-manipulation categories were found only or mostly in area 5. These results indicate the presence of hierarchical processing for bilateral as well as contralateral information within the arm/trunk region of the postcentral gyrus.
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