Neuronal activities of the anterior part of the inferior parietal lobule (area 7b or PF) were investigated in five awake Japanese monkeys. There were neurons which had specific combinations of receptive field (RF) locations, most typically in both the face and hand; we refer to the seas Face-Hand neurons. The most interesting property of the Face-Hand neurons is that some of these neurons responded to specific behavior executed with synergism between the face (especially the mouth) and hand movements; namely, face-hand coordinated behavior (e.g., eating behavior). We call these cells Face-Hand coordination neurons (52% of all the Face-Hand neurons). These neurons discharged more strongly when the animal executed face-hand coordinated behavior, especially eating behavior, than when somatosensory stimuli were given to RFs passively, or when face movements and hand movements were executed separately. We thus propose that the neuronal activities of area 7b are related to the representation of face-hand coordination.
The anterior portion of the inferior parietal cortex possesses comprehensive representations of actions embedded in behavioural contexts. Mirror neurons, which respond to both self-executed and observed actions, exist in this brain region in addition to those originally found in the premotor cortex. We found that parietal mirror neurons responded differentially to identical actions embedded in different contexts. Another type of parietal mirror neuron represents an inverse and complementary property of responding equally to dissimilar actions made by itself and others for an identical purpose. Here, we propose a hypothesis that these sets of inferior parietal neurons constitute a neural basis for encoding the semantic equivalence of various actions across different agents and contexts. The neurons have mirror neuron properties, and they encoded generalization of agents, differentiation of outcomes, and categorization of actions that led to common functions. By integrating the activities of these mirror neurons with various codings, we further suggest that in the ancestral primates' brains, these various representations of meaningful action led to the gradual establishment of equivalence relations among the different types of actions, by sharing common action semantics. Such differential codings of the components of actions might represent precursors to the parts of protolanguage, such as gestural communication, which are shared among various members of a society. Finally, we suggest that the inferior parietal cortex serves as an interface between this action semantics system and other higher semantic systems, through common structures of action representation that mimic language syntax.
We can mentally calibrate the directions of our bodily movements into visual coordinate systems to achieve purposeful actions in space. Alternatively, we can apprehend characteristics of the peri-personal space through actions performed by our own body parts. Such interactions between representations of our body motions and extrinsic space should occur in the intraparietal cortices, where the hierarchically processed somatosensory information adjoins the information on spatial vision processed along the dorsal stream. In this brain area of monkeys, we analyzed the response properties of "bimodal joint neurones", which responded simultaneously to forearm joint displacements and visual stimuli moving in one direction in space. For the majority of these neurones, the directions of hand movements in space as a result of adequate joint displacements were congruent with the preferred directions of the moving visual stimuli. When the arm position was rotated, the preferred direction of the joint displacement became inverted so as to match the induced hand movement in space with the preferred visual stimulus direction. On the other hand, in some neurones the visual preferred direction became inverted when the joint was rotated, becoming to match the preferred direction of joint displacement. Hence, intraparietal neurones appear not only to represent mental recalibration of intrinsic movements into extrinsic coordinates, but also render delineation of extrinsic space through intrinsic actions.
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