| Research Abstract |
When we use a tool, it becomes an extension of the hand both physically and perceptually. Thus, during tool-use, our body image is altered in accordance with the characteristics of the tool at hand. Thus, we can mentally manipulate our body image. Integration of somatosensory and visual information is indispensable for creating such an image. This most likely takes place in the intraparietal cortex, where the hierarchical processing of somatosensory information adjoin the information on spatial vision processed along the dorsal stream. In this cortical region of monkeys, we found groups of bimodal neurons coding various aspects of the body image which are subject to mental manipulation as follws :
Visual receptive fields (V-RFs) of bimodal neurons encompassed, and were anchored to, their somatosensory receptive fields (S-RFs) as if coding visual image of the hand in the space. V-RFs of other bimodal neurons coded the territory of accessible space. During tool-use, those V-RFs were modified so as to include the entire length of the tool or to cover the expanded accessible space. When watching hand movement through a video monitor instead of seeing it directly, these bimodal neurons seemed to code the image in the video screen as a projection of monkey's own body. These modification was observed only in well-trained monkeys intending to use tools.
We can move arms purposefully in the dark without visual feedback of the arm movement, perhaps using mentally produced image of the arm. Visual stimuli near the S-RFs activated a group of bimodal neurons even after the hand was hidden invisible ; when the arm was moved invisibly, the V-RE followed invisible S-RE.This would imply that these bimodal neurons code mentally maintained and updated visual images of the arm postures by referring to somatosensory information.
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