Bilateral receptive field neurons in the hindlimb region of the postcentral somatosensory cortex in awake macaque monkeys

Single-neuron activities were recorded in the hindlimb region of the primar y somatosensory cortex and part of area 5 in awake Japanese monkeys. A tota l of 1050 units were isolated from five hemispheres of four animals. Recept ive fields (RFs) and submodalities were identified for 90% of isolated neur ons in areas 3a and 3b. The percentage decreased as the recoding site moved to the more caudal al-eas. Deep or skin submodality neurons were dominant in area 3a or al-ea 3b, respectively. Deep submodality neurons increased in more caudal areas and were the majority in areas 2 and 5. These observatio ns were consistent with those in the hand and/or digit or arm and/or trunk region. The identified neurons were classified by their RF positions into f our types: the foot, lee, foot leg, or hindlimb and other body parts type. Among 831 identified neurons, 33 neurons had bilateral RFs, 14 had ipsilate ral RFs, and the rest (N=784) had contralateral RFs. The relative incidence of neurons with bilateral or ipsilateral RFs among identified neurons was less than 1% in areas 3a, 3b, and 1, and 16% or 25% in areas 2 or 5, respec tively. Within areas 2 and 5, the percentage of neurons with bilateral or i psilateral RFs was significantly smaller in the foot type (5%) than in othe r RF types (24-57%). RFs of the foot type were on the sole or single toe bu t never on multiple toes. These observations contrasted with the previous f indings that neurons with bilateral RFs were more frequently seen in the ha nd and/or digit region and that RFs on multiple digit tips were dominant th ere. The present study thus demonstrated that neurons with bilateral RFs do exist in the hindlimb region. Similarly to the forelimb region, they were found mostly in areas 2 and 5, the caudalmost areas of the postcentral gyru s and hierarchically higher stages in information processing. The relative paucity of neurons with bilateral RFs on the foot, especially those with RF s on multiple toes, may reflect functional differences between the foot and the hand.