| Abstract |
The functional activity of the intraparietal, agranular frontal and mesial motor cortex was mapped with the [14C]deoxyglucose method in one untrained control monkey and in six monkeys performing (i) fixation of a central visual target, (ii) saccades to visual targets, (iii) reaching in the light during fixation of a central visual target or without fixation, and (iv) acoustically triggered reaching in the dark while the eyes maintained a straight ahead direction. In the intraparietal sulcus different subregions of the intraparietal cortical area 7 were activated by fixation, saccades to visual targets, and acoustically triggered reaching in the dark. A region associated with fixation was revealed within the rostral part of the lateral bank of the intraparietal sulcus. The saccade related region covered almost the entire dorsoventral extent of the lateral bank in the rostral sector of intraparietal area 7. Subregions in the ventral part of the intraparietal cortex (around the fundus of the intraparietal sulcus) were activated only during reaching in the light, in which case visual information was available to guide the moving forelimb. In contrast, subregions in the dorsal part of the intraparietal cortical area 5 were activated during both reaching in the light and reaching in the dark, in which cases somatosensory information was the only one available in common. Activation in the dorsal intraparietal area 5 was enhanced in the monkey reaching in the dark probably acting as a compensatory mechanism to the absence of visual input to guide the forelimb. We conclude that within the intraparietal sulcus visual guidance of reaching is associated with the ventral intraparietal cortex, whereas somatosensory guidance, based on proprioceptive information about the current forelimb position, is associated with the dorsal intraparietal area 5. In the premotor cortex, dorsal premotor area F2 was activated contralateral to the moving forelimb in the reaching monkeys. The dorsocaudal-F2 subregion, below the superior precentral dimple, was activated in reaching under both visual and somatosensory guidance. A different subregion, the ventrorostral-F2, close to the arcuate sulcus and within its floor, was activated only in the monkeys reaching in the light. Thus, the dimple region of F2 is involved in reaching under somatosensory guidance whereas the arcuate sulcus region of F2 is involved in reaching under visual guidance. In the ventral premotor cortex, caudally located area F4 was activated in all visually stimulated and reaching monkeys. Subtractions of the maps corresponding to the different cases showed that area F4 is involved in visual processing and in reaching under somatosensory guidance. On the contrary, the rostrally located area F5 was not activated during visual processing in the absence of movement and showed only moderate activations (within the posterior bank of the arcuate sulcus) during reaching in the dark. However, area F5 displayed significant activations in the monkey reaching in the light in both the posterior bank of the arcuate sulcus and the lateral convexity close to the posterior crown of the arcuate sulcus. Thus, within the ventral premotor cortex area F4 is mainly involved in reaching under somatosensory guidance whereas area F5 is mainly involved in reaching under visual guidance. In the anterior bank of the arcuate sulcus, which is part of the frontal eye fields, the cortical center that controls eye movements, metabolic activity was increased in all visually stimulated monkeys. Moreover, a region around the fundus of the arcuate sulcus extending to both banks was activated in all hemispheres of fixating monkeys and is related to fixation of visual targets. Within the central sulcus bilateral activations of the trunk representation in both the primary somatosensory and motor cortex were observed in all reaching monkeys as a result of postural adjustment during the forelimb movement. Bilateral activation of the mouth representation in the central sulcus was related to licking during rewarded trials. The forelimb representation was symmetrically activated within the central sulcus in the hemisphere contralateral to the moving forelimb of the reaching monkeys. The activation of the primary somatosensory forelimb representation in the monkey reaching in the dark was higher as compared to the reaching in the light monkeys. This enhancement of somatosensory processing, when no visual information is available to guide the forelimb, could act as a compensatory mechanism to the absence of visual input. In the mesial wall the forelimb representation of area F3 was activated in the contralateral to the moving forelimb hemisphere in all reaching monkeys. Moreover, the dorsal bank of the cingulate sulcus corresponding to part of area 24 was bilaterally activated in the reaching monkeys. This result suggests a role of area 24 in reaching behavior. A circumscribed activation within area 23 was also observed in the ventral bank of the cingulate sulcus close to the fundus at the border with area 24. This activation was significant only in the reaching in the light monkeys. In summary, the parietal and frontal areas involved in reaching under visual and somatosensory guidance are revealed in the present study. The topographical organization of the neuronal populations involved in fixation, visual processing, execution of saccadic eye movements, reaching in the light and reaching in the dark is illustrated in functional maps of high resolution. Our results demonstrate that distinct parietofrontal networks are activated during reaching under visual and under somatosensory guidance.
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Xαρτογράφηση των βρεγματο-μετωπιαίων νευρωνικών κυκλωμάτων εγκεφάλου πιθήκου, τα οποία εμπλέκονται στην οπτικά και σωματαισθητικά καθοδηγούμενη κίνηση του άνω άκρου. Μελέτη με τη χρήση της ποσοτικής αυτοραδιογραφικής μεθόδου της [14C]-2-δεοξυγλυκόζης
